William D Hopkins

Oliver Coulon

Adrien Meguerditchian

Nicky Staes

Chet C Sherwood

Steven J Schapiro

Jean-Francois Mangin

Brenda Bradley

Genetic Determinants of Individual Variation in the Superior Temporal Sulcus of Chimpanzees (Pan troglodytes)

The superior temporal sulcus (STS) is a conserved fold that divides the middle and superior temporal gyri. In humans, there is considerable variation in the shape, folding pattern, lateralization and depth of the STS that have been reported to be associated with social cognition and linguistic functions. We examined the role that genetic factors play on individual variation in STS morphology in chimpanzees. The surface area and depth of the STS were quantified in sample of 292 captive chimpanzees comprised of two genetically isolated population of individuals. The chimpanzees had been previously genotyped for AVPR1A and KIAA0319, two genes that play a role in social cognition and communication in humans. Single nucleotide polymorphisms in the KIAA0319 and AVPR1A genes were associated with average depth as well as asymmetries in the STS. By contrast, we found no significant effects of these KIA0319 and AVPR1A polymorphism on surface area and depth measures for the central sulcus. The overall findings indicate that genetic factors account for a small to moderate amount of variation in STS morphology in chimpanzees. These findings are discussed in the context of the role of the STS in social cognition and language in humans and their potential evolutionary origins.

The superior temporal sulcus (STS) is a highly conserved fold of the cerebral cortex in primate brains that divides the superior and middle gyri of the temporal lobe [START_REF] Connolly | The fissural pattern of the primate brain[END_REF][START_REF] Connolly | External morphology of the primate brain[END_REF]) (see Figure 1). In cercopithecid and platyrrhine monkeys, the STS is largely a smooth fold that begins at the temporal pole and extends posteriorly beyond the most posterior point of the Sylvian fissure and terminates in the parietal lobe. In great apes and humans, there is considerably more variation in the shape and trajectory of the STS, particularly in the posterior region where, posterior and dorsal to the terminal point of the Sylvian fissure, the sulcus can bifurcate into descending and ascending limbs that may or may not be connected to the primary fold (see Figure 2). Further, in humans, and to a lesser extent in chimpanzees, the STS may be split into multiple sections or pieces due to the presence of "pli-de-passages" or buried gyri [START_REF] Bodin | Plis de passage in the superior temporal sulcus: Morphology and local connectivity[END_REF], which has been reported to be more frequently found in the left compared to the right hemisphere in humans (Le [START_REF] Guen | The chaotic morphology of the left superior temporal sulcus is genetically constrained[END_REF]). Recently, it has been suggested that asymmetries in the central region of the STS, referred to as the STAP (Superior Temporal Asymmetric Pit), are a human-specific anatomical landmark that has been hypothesized to be linked to the evolution of language. Specifically, [START_REF] Leroy | New human-specific brain landmark: The depth asymmetry of superior temporal sulcus[END_REF] quantified the depth of the STS in regions anterior and posterior to Heschl's gyrus in MRI scans of humans and chimpanzees and reported robust, rightward asymmetries in the STAP region in human subjects, whereas chimpanzees showed a relatively small, and only marginally significant rightward bias.

These findings are particularly interesting because previous studies in chimpanzees have reported population-level asymmetries in the perisylvian region, including the posterior STS and planum temporale; results that are quite similar to those in humans [START_REF] Hopkins | Planum temporale surface area and grey matter asymmetries in chimpanzees (Pan troglodytes): The effect of handedness and comparison within findings in humans[END_REF][START_REF] Kong | Personality in chimpanzees (Pan troglodytes): Exploring the hierarchical structure and associations with the vasopressin V1A receptor gene[END_REF][START_REF] Spocter | Reproducibility of leftward planum temporale asymmetries in two genetically isolated populations of chimpanzees (Pan troglodytes)[END_REF]. Thus, given the role of the posterior STS and superior temporal gyrus (STG) in a variety of linguistic, sociocommunicative and cognitive functions (i.e., such as the processing of faces, auditory stimuli and biological motion) [START_REF] Redcay | The superior temporal sulcus performs a common function for social and speech perception: implications for the emergence of autism[END_REF][START_REF] Deen | Functional Organization of Social Perception and Cognition in the Superior Temporal Sulcus[END_REF][START_REF] Specht | The functional and structural asymmetries of the superior temporal sulcus[END_REF] and the purported human uniqueness of asymmetries in the STAP, further studies on the morphology of the STS in other primate species are warranted as a means of characterizing the evolution of this cortical fold, as well as factors that influence individual variation in its expression. To this end, we examined the contribution of genetic and non-genetic (environmental) factors to individual variation in the surface area, mean depth and the depth profile along the length of the STS in chimpanzees. In the initial set of analyses, we divided the entire STS into anterior and posterior regions and quantified the overall surface area and mean depth. We next quantified the depth profile of the anterior STS, which included the STAP region, in the left and right hemispheres in the chimpanzee sample. From these data, in one analysis, we tested for population-level asymmetries as a means of evaluating their uniqueness as hypothesized by [START_REF] Leroy | New human-specific brain landmark: The depth asymmetry of superior temporal sulcus[END_REF]. In a second set of analyses, following methods used in previous studies with human and nonhuman primates [START_REF] Rogers | Heritability of brain volume, surface area and shape: An MRI study in an extended pedigree of baboons[END_REF][START_REF] Fears | Identifying heritable brain phenotypes in an extended pedigree of vervet monkeys[END_REF][START_REF] Rogers | On the genetic architecture of cortical folding and brain volume in primates[END_REF][START_REF] Mckay | Sulcus depth-position profile is a genetically mediated neuroscientific trait; Description and characterization in the central sulcus[END_REF][START_REF] Hopkins | Genetic factors and orofacial motor learning selectivelyinfluence variability in central sulcus morphology in chimpanzees (Pan troglodytes)[END_REF][START_REF] Hopkins | Heritability of Gray Matter Structural Covariation and Tool Use Skills in Chimpanzees (Pan troglodytes): A Source-Based Morphometry and Quantitative Genetic Analysis[END_REF][START_REF] Hopkins | Further evidence of left hemisphere dominance in motor skill by chimpanzees on a tool use task[END_REF], we used quantitative genetic analyses based on the available pedigree information for the chimpanzees to estimate heritability in the STS regions, as well as the contribution of non-genetic factors including their sex, age, rearing history and the magnet strength of the scanner used to obtain the images.

One unique aspect of the chimpanzee sample was that it was comprised of two cohorts of apes that were founded by separate wild-born animals and their descendants were raised at two different research facilities with no transfer of individuals between them.

Thus, the two cohorts were genetically isolated from each other and this allowed us to evaluate the repeatability of the heritability results.

We also examined whether polymorphisms in two different genes were implicated in individual variation in STS morphology. In one set of analyses, we tested whether single nucleotide polymorphisms (SNPs) in the KIAA0319 gene potentially explain individual variation in the STS and, specifically in the STAP region. KIAA0319 is a gene strongly implicated in language functions and polymorphisms in this gene have been repeatedly implicated in developmental dyslexia (DD) in humans, a reading disorder that is primarily manifest in difficulties learning associations between sounds with letters and/or words [START_REF] Cope | Strong evidence that KIAA0319 on chromosome 6p Is a susceptibility gene for developmental dyslexia[END_REF][START_REF] Dennis | A common variant associated with dyslexia deduces expression of the KIAA0319 gene[END_REF][START_REF] Scerrl | DCDC2, KIAA0319 and CMIP are associated with reading-related traits[END_REF].

Meta-analyses have shown that individuals with DD show atypical patterns of gray matter volume, cortical thickness and white matter connectivity in brain regions implicated in language [START_REF] Eckert | Gray Matter Features of Reading Disability: A Combined Meta-Analytic and Direct Analysis Approach[END_REF][START_REF] Eicher | Dyslexia and language impairment associated genetic markers influence cortical thickness and white matter in typically developing children[END_REF]. Studies in humans have also shown that SNPs in the KIAA0319 gene are associated with the prevalence of DD and are linked to differences in grey and white matter cortical organization, particularly within the posterior superior-temporal lobe and parietal junction within the left hemisphere [START_REF] Robichon | Developmental dyslexia: atypical cortical asymmetries and functional significance[END_REF][START_REF] Darki | Three dyslexia susceptibility genes,DYX1C1,DCDC2,and KIAA0319, affect temporo-parietalwhite matter structure[END_REF][START_REF] Pinel | Genetic variants of FOXP2 and KIAA0319/TTRAP/THEM2 locus are associated with altered brain activation in distinct language-related regions[END_REF]. Similarly, in chimpanzees, a recent study identified two SNPs in the KIAA0319 gene that were associated with individual variation in gray matter covariation in the posterior superior temporal gyrus (Hopkins et al. in press). For this reason, we hypothesized that these same polymorphisms might also be associated with variation in the surface area and depth of the chimpanzee STS.

We also tested for associations between polymorphisms in the gene coding for arginine vasopressin receptor 1A, AVPR1A, and variation in STS morphology. AVPR1A has been shown to play a role in social behavior and cognition in mammals [START_REF] Goodson | Social behavior functions and related anatomical characteristics of vasotocin/vasopressin systems in vertebrates[END_REF]Donaldson and Young 2008;[START_REF] French | Neuropeptide diversity and the regulation of social behavior in New World primates[END_REF][START_REF] Hopkins | Role of Oxytocin and Vasopressin V1a Receptor Variation on Personality, Social Behavior, Social Cognition, and the Brain in Nonhuman Primates with a Specific Emphasis in Chimpanzees[END_REF] and has been hypothesized to be a candidate gene for psychological disorders in which deficits in social behavior are prominent, such as autism spectrum disorder (ASD) [START_REF] Hammock | Oxytocin, vasopressin and pair bonding: implications for autism[END_REF][START_REF] Yirmiya | Association between the arginine vasopressin 1a receptor (AVPR1a) gene and autism in a family-based study: mediation by socialization skills[END_REF][START_REF] Carter | Sex differences in oxytocin and vasopressin: Implications for autism spectrum disorders?[END_REF]Donaldson and Young 2008;[START_REF] Melke | Autism: Which genes are involved?[END_REF][START_REF] Francis | Variants in Adjacent Oxytocin/Vasopressin Gene Region and Associations with ASD Diagnosis and Other Autism Related Endophenotypes[END_REF][START_REF] Parker | Arginine vasopressin in cerebrospinal fluid is a marker of sociality in nonhuman primates[END_REF]. Consistent with this view, neuroimaging studies have shown that gray matter volume and white matter connectivity in the STS differ between individuals with and without a diagnosis of ASD [START_REF] Zilbovicius | Autism, the superior temporal sulcus and social perception[END_REF]) and other neuropsychiatric disorders [START_REF] Plaze | Where do auditory hallucinations come from?"--a brain morphometry study of schizophrenia patients with inner or outer space hallucinations[END_REF].

Previous studies have shown that chimpanzees possess an indel deletion in the AVPR1A microsatellite, resulting in a bimodal distribution of subjects with (DupB+/-) or without (DupB-/-) the RS3 duplication [START_REF] Hammock | Microsatellite instability generates diversity in brain and sociobehavioral traits[END_REF]Donaldson et al. 2008).

Behaviorally, DupB+/-(which can include a small cohort of DupB+/+ individuals) and DupB-/-chimpanzees have been found to differ significantly in dimensions of personality and social behavior [START_REF] Hopkins | A polymorphic indel containing the RS3 microsatellitein the 5 ' flanking region of the vasopressin V1a receptor gene is associated with chimpanzee (Pan troglodytes) personality[END_REF][START_REF] Anestis | AVPR1A variation in chimpanzees (Pan troglodytes): Population differences and association with behavioral style[END_REF]Latzman et al. 2014;[START_REF] Staes | Oxytocin and vasopressin receptor gene variation as a proximate base for Inter-and intraspecific behavioral bifferences in bonobos and chimpanzees[END_REF][START_REF] Wilson | Chimpanzee Personality and the Arginine Vasopressin Receptor 1A Genotype[END_REF] . For example, [START_REF] Mahovetz | The influence of AVPR1A genotype on individual differences in behaviors during a mirror self-recognition task in chimpanzees (Pan troglodytes)[END_REF] recorded the social behavior of chimpanzees when presented with a mirror and found that DupB+/-males showed higher rates of scratching (reflecting anxiety) and agonistic behaviors compared to DupB-/-males and both DupB+/-and DupB-/-females.

Polymorphisms in AVPR1A are also associated with a composite score reflecting ASD phenotypes [START_REF] Weiss | Early social rearing, the V1A arginine vasopressin receptor genotype, and autistic traits in chimpanzees[END_REF] as well as social cognition measures linked to ASD, notably receptive joint attention (RJA) (Hopkins, Keebaugh, et al. 2014). Previous studies have also found that both humans and chimpanzees with poor joint attention skills differ in gray matter volume of the posterior, but not anterior, portion of the superior temporal gyrus [START_REF] Hopkins | Poor receptive joint attention skills are associated with atypical grey matter asymmetry in the posterior superior temporal gyrus of chimpanzees (Pan troglodytes)[END_REF][START_REF] Mundy | A review of joint attention and social-cognitive brain systems in typical development and autism spectrum disorder[END_REF]. Finally, [START_REF] Mulholland | AVPR1A variation is linked to gray matter covariation in the social brain network of chimpanzees[END_REF] using source-based morphometry, found that DupB-/chimpanzees had significantly higher covariation in gray matter within the superior temporal gyrus compared to DupB+/-apes. In light of the significant differences found between DupB+/-and DupB-/-apes in behavior and gray matter covariation within the superior temporal gyrus, we hypothesized that chimpanzees with these two genotypes would also differ on measures of STS surface area and depth.

Finally, we tested whether the influence of polymorphisms in the KIAA0319 and AVPR1A were specific to the STS. To test this hypothesis, we examined the influence of polymorphisms in KIAA0319 and AVPR1A on depth and surface area measures of the central sulcus (CS). The surface area and depth of the CS in chimpanzees has been previously reported to be significantly heritable and associated with manual and oro-facial motor skills in this same chimpanzee sample (Hopkins, Coulon, Meguerditchian, Autrey, Davidek, Mahovetz, S., et al. 2017). If the influence of polymorphisms in the KIAA0319 and AVPR1A genes were specific to the STS, then we hypothesized that no significant effects would be found for the surface area and depth of the CS.

Methods

Subjects

For the quantitative genetic analyses on STS heritability, magnetic resonance images (MRI) were obtained from 292 captive chimpanzees housed at either the Yerkes National Primate Research Center (YNPRC, n = 133) or the National Center for Chimpanzee Care (NCCC, n = 157) of The University of Texas MD Anderson Cancer Center. There were 170 females and 122 males ranging from 6 to 54 years of age (Mean = 27.99 years, SD = 11.11). Within this sample, there were 136 mother-reared, 92 nursery-reared and 64 wild-born individuals. A breakdown in the composition of relatedness of the two chimpanzee samples has been previously reported in [START_REF] Hopkins | Genetic basis for motor skill and hand preference for tool use in chimpanzees (Pan troglodytes)[END_REF]. With the exception of two individuals, nursery-reared (NR) chimpanzees were separated from their mothers within the first 30 days of life, due to unresponsive care, sickness, or injury, and leaving them in their circumstance at the time would have increased the risk of death [START_REF] Bard | Orientation to social and nonsocial stimuli in neonatal chimpanzees and humans[END_REF][START_REF] Bard | Evolutionary roots of intuitive parenting: Maternal competence in chimpanzees[END_REF]. The NR chimpanzees were placed in incubators, fed standard human infant formula, and cared for by humans until they could sufficiently care for themselves, at which time they were placed with other infants of the same age until they were 3 years of age [START_REF] Bard | Orientation to social and nonsocial stimuli in neonatal chimpanzees and humans[END_REF][START_REF] Bard | Evolutionary roots of intuitive parenting: Maternal competence in chimpanzees[END_REF]. At 3 years of age, NR chimpanzees were integrated into larger social groups of adult and sub-adult chimpanzees. Mother-reared (MR) chimpanzees were not separated from their mother for at least the first 2.5 years of life and were raised in nuclear family groups ranging from 4 to 20 individuals. The two exceptions of NR individuals were Lana and Panzee, chimpanzees that were born at the YNPRC but raised in a rich social-linguistic environment by humans at the Language Research Center (LRC) as part of an initiative to investigate the linguistic skills of chimpanzees [START_REF] Rumbaugh | Language learning by a chimpanzee: The Lana project[END_REF]Brakke andSavage-Rumbaugh 1995, 1996). Wild-born (WB) chimpanzees were individuals who had been captured in the wild and subsequently brought to research facilities within the United States prior to 1974, when the importation of chimpanzees was banned. All scanning procedures performed with the chimpanzees were approved by the local Institutional Animal Care and Use Committees and followed all recommendations by the Institute of Medicine for the ethical treatment of chimpanzees in research.

KIAA0319 and AVPR1A Genotypes

All genotyping was performed on DNA extracted from banked blood samples obtained from the chimpanzees during their annual physical examination within each facility prior to 2015. DNA samples were not available on the entire sample of 292 chimpanzees; thus, all analyses examining the influence of genotype on the STS involved fewer individuals. The details for DNA extraction, primer sequences and analyses have been provided in detail in previous publications [START_REF] Hopkins | A polymorphic indel containing the RS3 microsatellitein the 5 ' flanking region of the vasopressin V1a receptor gene is associated with chimpanzee (Pan troglodytes) personality[END_REF]Hopkins et al. 2021). For KIAA0319, one SNP, RS_P1, had allele frequencies of 99 (AA), 104 (AT) and 46 (TT). For the second SNP, RS_63, the allele frequencies within our population were 78 (AA), 116 (AC) and 54 (CC). For AVPR1A, there were 85 DupB+/-and 153 DupB-/-within the sample.

MRI Image Collection

Both in vivo (n = 228) and ex vivo (n = 64) post-mortem MRI scan data were used in this study. In vivo scans were obtained at the time the chimpanzees were being surveyed for their annual physical examinations beginning in 1998 and continuing until 2014. Subjects were first immobilized by ketamine (10 mg/kg) or telazol (3-5mg/kg) and subsequently anaesthetized with propofol (40-60 mg/(kg/h)) following standard procedures at the YNPRC and NCCC facilities. YNPRC subjects were then transported to the MRI facility, while NCCC subjects were wheeled to a mobile imaging unit. The subjects remained anaesthetized for the duration of the scans, as well as the time needed to transport them between their home cage and the imaging facility (between 5 and 10 minutes) or mobile imaging unit (total time ~ 5 minutes). Subjects were placed in the scanner chamber in a supine position with their head fitted inside the human-head coil. Scan duration ranged between 40 and 60 minutes as a function of brain size.

Seventy-seven chimpanzees were scanned using a 3.0 Tesla scanner (Siemens Trio, Siemens Medical Solutions USA, Inc., Malvern, Pennsylvania, USA) at YNPRC. T1-weighted images were collected using a three-dimensional gradient echo sequence (pulse repetition= 2300 ms, echo time= 4.4 ms, number of signals averaged= 3, matrix size = 320 X 320). Additionally, 139 NCCC and 13 YNPRC chimpanzees were scanned using a 1.5 Tesla Phillips or GE machine. T1-weighted images were collected in the transverse plane using a gradient echo protocol (pulse repetition= 19.0 ms, echo time= 8.5 ms, number of signals averaged= 8, and a 256 x 256 matrix). After completing MRI procedures, the subjects were temporarily housed in a single enclosure for 6-12 h to allow the effects of the anesthesia to wear off, after which they were returned to their social group. Post-mortem T2-scans were obtained from 64 chimpanzees that had died from natural causes. For the post-mortem scanning, either 4.7 or 7T magnets were used and T2-weighted images were collected in the transverse plane using a gradient echo protocol (pulse repetition = 22.0 s, echo time = 78.0 ms, number of signals averaged = 8-12, and a 256 × 192 matrix reconstructed to 256 × 256).

Sulci Extraction and Measurement

The sequence of post-image processing steps performed on the images are shown in Figures 3a to 3h and have been described in detail elsewhere (Hopkins et al. 2010;[START_REF] Bogart | Different early rearing experiences have long term effects on cortical organziation in captive chimpanzees (Pan troglodytes)[END_REF]Hopkins, Coulon, Meguerditchian, Autrey, Davidek, Mahovetz, S., et al. 2017). The processing used to extract the sulci from the raw T1-weighted image derives from a pipeline initially dedicated to the human brain and freely distributed as a BrainVISA toolbox (http://brainvisa.info) [START_REF] Mangin | Object-based morphometry of the cerebral cortex[END_REF]). To account for the differences in chimpanzee anatomy compared to humans, a number of adjustments were performed before the scans were processed using the pipeline procedure within BrainVISA (BV). Specifically, chimpanzee MRI volumes were skullstripped, cropped, denoised [START_REF] Coupe | An optimized blockwise nonlocal means denoising filter for 3-D magnetic resonance images[END_REF], and reformatted at 0.625 mm isotropic resolution using ANALYZE 11.0 software and subsequently imported into BV. The pipeline process of extracting the sulci from the cortex involved a number of steps [START_REF] Mangin | Object-based morphometry of the cerebral cortex[END_REF]) (see Figure 3a to 3h). To align the template brain, the anterior and posterior commissures were manually specified on the MRI at the point where they intersect with the mid-sagittal slice. The first step was to correct for spatial inhomogeneities in the signal intensity, providing a spatially smooth bias field with a stable distribution of tissue intensities (Figure 3b). Next, the analysis of the signal histogram and mathematical morphology were performed using an automatic analysis of the voxel intensities for the entire brain to obtain a binary mask of the brain (Figure 3c). Adjustments were sometimes needed in the histogram process to determine gray and white matter means for chimpanzee brain scans. The mask was then split into the left and right hemispheres and the cerebellum (Figure 3d). A negative cast of the white matter was computed from the split-brain mask. The outside boundary of this cast results from a 5 mm morphological closing of the masked hemisphere, filling up the folds. The gray/white interface is the inside boundary and is computed using deformations that assure the spherical topology of the cast (Figure 3e). Finally the cast was skeletonised to detect cortical folding, while topological constraints guaranteed the resulting surfaces would have no holes [START_REF] Mangin | Entropy minimization for automatic correction of intensity nonuniformity[END_REF][START_REF] Mangin | Object-based morphometry of the cerebral cortex[END_REF]) (Figure 3f & 3g). The STS and the CS in each hemisphere was selected manually (Figure 3h) by the user, using a 3D visualization interface [START_REF] Bailey | The isocortex of the chimpanzee[END_REF][START_REF] Hopkins | Evolution of the central sulcus morphology in primates[END_REF]. To divide the STS into its anterior and posterior sections, we used a sulcus that was present in the majority of brains (lablled as A3 in [START_REF] Connolly | The fissural pattern of the primate brain[END_REF] and could be used to divide the STS into anterior and posterior regions (see Figure 4). In more than 75% of the hemispheres, the BV pipeline produced a natural break in the STS at the A3 sulcus. In the absence of a natural break, we used the scissors tool in BV to manually bisect the fold along the medial-lateral axis of A3. Using this landmark, the STS was divided into the anterior (STS_Ant) and posterior (STS_Post) regions. For the STS_Post, we included all the folds that were dorsal to the STS_ventral, including those that were disconnected from the main STS fold. The STS_Post excluded any sulci that were part of the lunate, posterior central and intraparietal sulci. The STS_Ant and STS_Post were manually labelled for the left and right hemisphere and we calculated the surface area and mean depth for each region.

Parameterization of the STS_Ant

The selected STS_Ant was meshed using a triangular mesh, and the resulting surface was parameterized in order to create a longitudinal coordinate system [START_REF] Coulon | Quasi-isometric length parameterization of cortical sulci: application to handedness and the central sulcus morphology, 12th IEEE International Synposium on Biomedical imaging[END_REF] (Figure 5a to 5d). The parameterization process was constrained by the anterior and posterior sulcus extremities, automatically detected using the extrema of the first non-zero eigenfunction of the mesh Laplacian (e1 and e2, Fig. 5b). From these points, a smooth and quasi-isometric (i.e. with minimal metric distortions) coordinate field is extrapolated, that localizes all mesh surface points according to their relative position along the sulcus between the two extremities (Fig. 5c). The coordinate field extends along the length of the STS_Ant from the anterior (y = 1) to the posterior (y = 100) ends of the sulcus. Depth was measured at 100 sulcal length positions in an anterior-to-posterior progression along the parameterized sulcal mesh surface. Position 1 was located at the most anterior point of STS_Ant, while position 100 was located at the most posterior point of the STS_Ant where the A3 sulcus was located (see Fig 5c).

At each position, y, along the length, the depth is computed by measuring a geodesic distance (in millimeters) from the brain envelope to the fundus of the sulcus (Fig. 5d).

To simplify the analyses, within the left and right hemispheres, we averaged every 5 depth measures from point 0 to 100 to create 20 STS_Ant regions along the anteriorposterior plane.

Heritability Analyses

Consistent with previous studies, we used the software package SOLAR to estimate heritability in the STS (Almasy and Blangero 1998) (http://solar-eclipsegenetics.org). SOLAR uses a variance component approach to estimate the polygenic component of variance when considering the entire pedigree (see [START_REF] Rogers | Heritability of brain volume, surface area and shape: An MRI study in an extended pedigree of baboons[END_REF][START_REF] Fears | Identifying heritable brain phenotypes in an extended pedigree of vervet monkeys[END_REF][START_REF] Fears | Anatomic brain asymmetry in vervet monkeys[END_REF]Hopkins, Keebaugh, et al. 2014). Total additive genetic variance (h 2 ) is the amount of total phenotypic variance that is attributable to all genetic sources. Total phenotypic variance attributable to genetic and non-genetic variables is constrained to a value of 1; therefore, all non-genetic contributions to the phenotype are equal to 1 -h 2 . We used SOLAR to determine heritability in the average Heritability estimates and genetic correlations at p < .05 were considered significant.

Results

Quantitative Genetic Analyses

Global Results

For descriptive purposes, shown in Table 1 are the average mean surface areas and mean depth for the STS_Ant and STS_Post regions in males and females, and between each rearing group. Heritability results on the combined sample showed the average surface area and mean depth of the STS_Ant and STS_Post regions were significantly heritable, while AQ values for both measures were not (see Table 2). With respect to average surface area and mean depth, scanner magnet was found to be a significant covariate, an expected finding, given the known differences in gray and white matter resolution as a function of field strength. Sex was also found to be a significant covariate, which was also expected, because we did not adjust for brain/body size between males and females (see Table 1). Neither age nor rearing history accounted for a significant proportion of variance in average surface area or mean depth. Also shown in Table 2 are the heritability results within the YNPRC and NCCC cohorts.

Heritability in the surface areas were largely consistent between the two cohorts, but this was not the case for the average depth measures for the STS_Ant and STS_Post measures. --------------------------------------------------------------------------------------------------------------

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Values in parentheses represent standard errors. MR = mother-reared, NR -nurseryreared, WB = wild-born. Surface area and mean deoth are -------------------------------------------------------------------------------------------------------------------se = standard error

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We note here that surface area for the entire brain was significantly heritable (h 2 = .298, se = .148, p = .011). To determine whether the surface area heritability values for the STS were region specific or could be accounted for by overall surface area, we calculated the percentage of STS_Ant and STS_Post by dividing the mean surface areas for each area by the total surface area of the brain and multiplying by 100. We then re-ran the heritability analyses using the percentage STS_Ant and STS_Post measures with sex, age rearing and scanner as covariates. For both the STS_Ant (h 2 = .325, se = .116, p = .0008) and STS_Post (h 2 = .223, se = .129, p = .033) regions, the adjusted surface as measures remained significantly heritable. Thus, the heritability in the STS surface area appears to be independent of genetic influences on total surface area.

Parameterized STS_Ant Depth

We next considered heritability in the STS_Ant depth measures at the 20 points along the anterior-posterior plane. Results for the average STS_Ant depth for the entire sample, as well as for the separate results for the NCCC and YNPRC cohorts, are

shown in Figure 6a. For the combined sample, significant heritability was found for 13 of the 20 regions (regions 6 to 18); thus, the majority of regions along the anteriorposterior axis of the sulcus were significantly heritable with values ranging from .294 to .616. As with the global results and as expected, sex and scanner were significant covariates for 17 and 15 regions, respectively (see Table 3). Rearing was a significant covariate for regions 18 and 19 (see Table 3). The mean depth at the 20 STS regions for males and females and between the three rearing groups are shown in Table 4. As can be seen, when sex was a significant covariate, males had higher values than females. Further, for STS 18 and 19, MR apes had higher values than NR individuals who, in turn, had higher values than WB apes. Between the NCCC and YNPRC chimpanzee cohorts, the results were largely consistent, with significant heritability found in both the NCCC and YNPRC cohorts for STS regions 7 through 18 (see Figure 6b and6c). For the AQ measures for each STS_Ant region, we found no evidence of significant heritability for any of the 20 STS regions, after adjustment for multiple comparisons, in either the combined sample or in each separate cohort. ----------------------------------------------------------------------------------------------- --------------------------------------------------------------------------------------------------------------- -------------------------------------------------------------------------------------------------------------------

-------------------------------------------------------------------------------------------------- STS1 .

Population-Level Asymmetry

Because [START_REF] Leroy | New human-specific brain landmark: The depth asymmetry of superior temporal sulcus[END_REF] emphasized the significance of asymmetries in the STAP as a uniquely human feature, we initially tested for population-level asymmetries in the global measures of the STS_Ant as well as its 20 antero-posterior depth measures in our sample. After adjustment to alpha using Bonferroni's correction procedure (adjusted p = .002), significant rightward asymmetries were found for STS regions 12 t(290) = 3.555, p < .001, 13 t(290) = 4.065, p < .001, 14 t(290) = 4.183, p < .001, 15 t(290) = 3.863, p < .001 and 16 t(290) = 3.310, p < .001 (see Figure 7).

Separate one-sample t-tests within the NCCC and YNPRC cohorts revealed a consistent pattern of results (see Figure 8). For the NCCC sample, significant rightward asymmetries were found for STS 13 t(157) = 2.201, p < .029, 14 t(157) = 2.598, p < .010, 15 t(157) = 2.873, p < .005 and 16 t(157) = 2.740, p < .007. For the YNPRC sample, significant rightward asymmetries were found for STS regions 12 t(128) = 2.702, p < .008, 13 t(128) = 3.21, p < .002, 14 t(128) = 3.068, p < .003, and 15 t(128) = 2.305, p < .023. To test for rearing and sex effects, a MANCOVA was performed with the 20 STS regions serving as the dependent measures. No significant main effects or interactions were found.

KIAA0319 SNPs and STS Morphology

Global RS_P1: To test for the effect of the RS_P1 and RS_63 SNPs on STS_Ant and STS_Post, separate mixed model analyses of co-variance were performed with STS region as the repeated measure and genotype as between-group factors. Sex, rearing and scanner magnet were covariates. Further, because the STS measures were heritable, we also included the genetic relatedness score of each chimpanzee as an additional covariate. The genetic relatedness scores were based on the entire pedigree and reflected how related each chimpanzee subject was to all other individuals in the population. We found a significant main effect for RS_P1 genotype on average mean depth F(2, 242)=4.508, p = .012, as well as a two-way interaction between STS region and genotype F(2, 242)=7.179, p < .001. Post-hoc analysis indicated that, for STS_Ant, average mean depth was significantly greater in chimpanzees with the AA compared to TT, but not the AT genotypes (see Table 5). In contrast, for the STS_Post, average mean depth was significantly higher in chimpanzees with the TT compared to AA, but not the AT genotypes (see Table 5). For the average surface area, as well as the AQ scores for the surface area and mean depth measure, we found no significant main effects or interactions for the RS_P1 genotypes. ------------------------------------------------------------------------------------------------------------

-------------------------------------------------------------------------------------------------------------- RS_P1 AA AT TT --------------------------------------------------------------------------------------------------------------
-------------------------------------------------------------------------------------------------------------- RS_63 AA AC CC -
--------------------------------------------------------------------------------------------------------------
Bolded values indicated significant effect of KIAA0319 genotype. Values in parentheses represent standard errors.

RS_63: No significant main effects or interactions were found for this SNP.

STS_Ant Parameterization

To test for the effect of the RS_P1 and RS_63 SNPs on STS depth, separate mixed model analyses of co-variance were performed with STS region as the repeated measure and genotype as between-group factors. As above, genetic relatedness, sex, rearing and scanner magnet were covariates. For RS_P1, we found a significant main effect for genotype on average mean depth F(2, 242)= 7.553, p = .021 (see Figure 9a).

Post hoc analysis indicated that chimpanzees with the AA allele had deeper folds than TT, but not AT individuals. No significant difference was found between chimpanzees with the AT and TT alleles. For the AQ values, we similarly found a main effect for genotype F(2, 239) = 3.234, p = .041 (see Figure 9b). Chimpanzees with the TT and AT genotypes showed significantly greater rightward asymmetries compared to the AA individuals. No significant difference was found in AQ scores for the chimpanzees with the AT and TT alleles. For RS_63 SNP, there were no significant main effects or interactions for either the average or AQ depth scores on the different alleles.

AVPR1A and STS Morphology

Global

A mixed model analysis of variance was performed on the AVPR1A gene with STS region as the repeated measure while genotype (DupB+/-, DupB-/) and sex served as between-group factors. Genetic relatedness, rearing history and scanner magnet were covariates. Separate analyses were performed for the average and AQ measures because they reflected different dimensions of STS morphology. There was a significant main effect for AVP1A genotype on average surface area F(1, 232) = 4.407, p = .004, but not average mean depth. DupB-/-chimpanzees had larger values than DupB+/-apes (see Table 6). Regarding the AQ data, we found a significant main effect for AVPR1A genotype on surface area F(1, 232) = 9.644, p < .001 and mean depth F(1, 232) = 7.127, p = .008. For both measures, DupB-/-chimpanzees had greater rightward asymmetries than DupB+/-apes (see Figure 10a and10b). ------------------------------------------------------------------------------------------------------------ -----------------------------------------------------------------------------------------------------------Bolded values indicated significant effect of AVPR1A genotype. Italicized values indicate significant interaction between STS region and AVPR1A genotype. Values in parentheses represent standard errors.

STS Parameterization

A mixed model analysis of variance was performed on the AVPR1A gene with STS region as the repeated measure while genotype (DupB+/-, DupB-/) and sex served as between-group factors. Genetic relatedness, rearing history and scanner magnet were covariates. For the average depth measures, a significant main effect for AVPR1A genotype was found F(1, 234)= 7.602, p = .006, as well as a two-way interaction between AVPR1A and STS region F(19, 4446)= 1.747, p = .023. For the AVPR1A genotype by STS region interaction, post-hoc analysis indicated that DupB-/apes had greater mean depth values than DupB+/-individuals for regions 7 through 19 (see Figure 11, left panel). Regarding STS asymmetry, the analysis revealed a significant two-way interaction between AVPR1A genotype and STS region F(19, 4446)= 2.378, p < .001 (Figure 11, right panel). For regions 12, 13, 14 and 18, post-hoc analysis indicated that DupB-/-apes had greater rightward asymmetries than Dup+/-anterior-posterior plane. We also computed AQ values at each of the 20 CS regions following the formula described above for the STS.

Global Measures

In the initial analysis, we tested whether the influence of polymorphisms in the 7). -------------------------------------------------------------------------------------------------------------

--------------------------------------------------------------------------------------------------------------
Bolded values indicated significant effect of AVPR1A genotype. Values in parentheses represent standard errors.

CS Parameterization

For these analyses, the mean depth and AQ values at the 20 CS locations were the repeated measures while sex and genotype were the between group factors.

Rearing, scanner and genetic relatedness were the covariates. Regarding KIAA0319, we found no significant main effects or interactions for RS_P1 nor RS-63 on either average depth or the depth AQ values. By contrast, for the AVPR1A genotypes, we found a significant main effects for genotype on the AQ score F(1, 237) = 4.002, p = .047. DupB-/-apes had significantly greater rightward asymmetry values (Mean AQ = .128, s.e. = .075) compared to DupB+/-(Mean AQ = -.123, s.e. = .100) chimpanzees.

Discussion

The results of this study are straightforward. First, the surface area and depth of the STS, particularly in the central portions, were found to be significantly heritable and the STS in the manner that was utilized in this report. Future studies in other species should seek to clarify the nature of STS asymmetry in the context of the location of STAP region.

We also found that the overall surface area, overall mean depth and the mean depth for the majority of STS depth regions were significantly heritable. Further, heritability results were similar between the NCCC and YNPRC samples, suggesting that the contributions of genetic factors to individual variation in STS morphology are consistent and repeatable. The evidence of consistent and repeatable heritability in the STS is not surprising, in some ways, because the STS is a conserved fold that evolved early in primate brain evolution [START_REF] Zilles | Gyrification in the cerebral cortex of primates[END_REF][START_REF] Gilissen | The neocortical sulci of the capuchin monkey (Cebus): evidence for asymmetry in the sylvian sulcus and comparison with other primates[END_REF][START_REF] Sawada | Ontogenetic pattern of gyrification in fetuses of cynomologous monkeys[END_REF] and, ontogenetically, emerges during embryonic development in human and nonhuman primates [START_REF] White | The development of gyrification in childhood and adolescence[END_REF][START_REF] Habas | Early folding patterns and asymmetries of the normal human brain detected from in utero MRI[END_REF][START_REF] Meng | Discovering cortical sulcal folding patterns in neonates using large-scale dataset[END_REF]). More evolutionary conserved sulci have been found to be more strongly heritable in humans and chimpanzees [START_REF] Gomez-Robles | Relaxed genetic control of cortical organization in human brains compared with chimpanzees[END_REF] and our findings on the STS appear to be consistent with these previous results. It is of further note that the heritability in the overall surface area and depth of the STS in our chimpanzee sample are within the range of values reported in humans in a recent paper by [START_REF] Pizzagalli | The reliability and heritability of cortical folds and their genetic correlations across hemispheres[END_REF] who similarly used BrainVisa to quantify the STS (and many other sulci).

We failed to find any evidence whatsoever that asymmetries in the STS were significantly heritable, a finding consistent with previous results for sulci within the inferior frontal gyrus [START_REF] Hopkins | Behavioral and brain asymmetries in primates: A preliminary evaluation of two evolutionary hypotheses[END_REF] and central sulcus (Hopkins, Coulon, Meguerditchian, Autrey, Davidek, Mahovetz, S., et al. 2017) of chimpanzees. This implies that there are no genes that specifically code for left-right difference in surface area and depth within the STS and other sulci of the chimpanzee cerebral cortex.

Some have hypothesized that one evolutionary shift that occurred in brain organization after the split between chimpanzees and humans from their common ancestor was a genetic basis for the expression of population-level lateralization in structure and function [START_REF] Warren | Handedness and laterality in humans and other animals[END_REF][START_REF]Handedness and brain asymmetry: The right shift theory[END_REF][START_REF] Crow | A theory of the origin of cerebral asymmetry: Epigenetic variation superimposed on a fixed right-shift[END_REF][START_REF] Corballis | Right hand, left brain: genetic and evolutionary basis of cerebral asymmetries for language and manual action[END_REF]. The results reported here are consistent with this view, at least as it pertains to a lack of evidence for a genetic basis for asymmetries in the STS of chimpanzees. However, there are at least two caveats to this conclusion. First, recall that we found that polymorphisms in the RS_P1 SNP of the KIAA0319 gene and the indel deletion of the AVPR1A gene were associated with asymmetries in the overall anterior portion of the STS. Thus, though AQ values were not heritable, at least one SNP in the KIAA0319 and a deletion in the AVPR1A gene were associated with variation in the AQ values which would suggest a genetic basis for STS asymmetry. Second, it is worth noting that the evidence for a genetic basis in determining directional asymmetries in humans is not terribly robust [START_REF] Armour | Genome-wide association study of handedness excludes simple genetic models[END_REF][START_REF] Pletikos | Temporal specification and bilaterality of human neocortical topographic gene expression[END_REF][START_REF] Carrion-Castillo | Genome sequencing for rightward hemispheric language dominance[END_REF][START_REF] De Kovel | The molecular genetics of hand preference revisited[END_REF][START_REF] Schmitz | Building an Asymmetrical Brain: The Molecular Perspective[END_REF][START_REF] Carrion-Castillo | Genetic effects on planum temporale asymmetry and their limited relevance to neurodevelopmental disorders, intelligence or educational attainment[END_REF][START_REF] Sha | The genetic architecture of structural left-right asymmetry of the human brain[END_REF]) and certainly does not support single or dual allele models of asymmetry that have been postulated in previous genetic models of behavioral and brain asymmetry. In short, the role of genetic factors on the determination of population-level structural and functional asymmetries remains unclear but merits further investigation [START_REF] Ocklenburg | The Lateralized Brain: The Neuroscience and Evolution of Hemispheric Asymmetries[END_REF][START_REF] Schmitz | Building an Asymmetrical Brain: The Molecular Perspective[END_REF].

In our view, the overall findings of this study support the view of [START_REF] Redcay | The superior temporal sulcus performs a common function for social and speech perception: implications for the emergence of autism[END_REF] with respect to the diverse roles and function that are subserved by the STS, particularly in the context of the role of the KIAA0319 and AVPR1A genes on behavior.

Specifically, [START_REF] Redcay | The superior temporal sulcus performs a common function for social and speech perception: implications for the emergence of autism[END_REF] has suggested that the STS plays a fundamental role in the processing of visual and auditory sequential information and interpreting the meaning or communicative function of these stimuli. This general neural processing principle would apply to a broad range of stimuli and arguably across species but particularly among other closely-related primate species such as chimpanzees. For instance, chimpanzee communication is multimodal and involves the processing and integration of facial, vocal and gestural signals. Within this framework, the computational need to interpret these sequential, multimodal signals might explain why polymorphisms in purported language genes, such as KIAA0319, would have anatomical consequences on STS morphology in species that lack full-blown language.

The potential specificity of the role of KIAA0319 and AVPR1A on STS morphology is further supported by the findings (or lack thereof) of the effect of the polymorphisms for each these genes on variation in the CS. Notably, neither of SNPs for the KIAA0319 gene had a significant effect on CS surface area, mean depth or asymmetries in these measures. A significant main effect for AVPR1A was found for the overall CS depth asymmetry values with DupB-/-individuals showing greater rightward asymmetries. This pattern in directional asymmetry is similar to the findings reported for the STS_ant parameterization results; however, the influence of the AVPR1A polymorphisms were specific to the posterior regions and not the entire fold as was the case for the CS. Therefore, interpreting the collective effect of AVPR1A on asymmetries in sulci surface area and depth is not straight forward. Conservatively, when comparing the overall influence of these genes on sulci surface area and depth, our findings suggest a more region-specific impact of KIAA0319 on STS morphology and asymmetry compared to AVPR1A.

In summary, we found that chimpanzees show a small but significant rightward asymmetry in the STAP region within the STS. Additionally, the surface area and depth of the STS was found to be significantly heritable and these results were repeatable in two genetically isolated populations of chimpanzees. Asymmetries in the surface area and depth of the STS were not found to be significantly heritable. Single nucleotide polymorphisms in two genes, KIAA0319 and AVPR1A, were also found to be associated with individual variation in the depth of the STS. Collectively, the data provide new insights into factors that influence individual variation and evolution of STS morphology in hominids.

Figure 1 :

 1 Figure 1: 3D lateral views of the superior temporal sulcus (STS) in different primate

Figure 2 :

 2 Figure 2: Four different example of variation in the morphology of the STS. (a) typical

Figure 3 :

 3 Figure 3: Pipeline process in BrainVisa for sulci extraction.

Figure 4 :

 4 Figure 4: Left panel: 3D rendering of chimpanzee brains with the STS extracted and the landmarks identified to distinguish between the anterior and posterior sections. Right panel: 3D rendering of chimpanzee brain showing the STS_Ant section that the parameterization analyses were performed for each subject.

Figure 5 :

 5 Figure 5: Parameterization steps of the STS_Ant of the chimpanzee brain (see text for description).

  surface area and average depth of the overall STS_Ant and STS_Post regions by adding the left and right hemisphere values and dividing by two. We also computed an asymmetry quotient (AQ) for the surface area and mean depth measures for both the STS_Ant and STS_Post region following the formulas [AQ = (R -L) / ((R + L ) *.5)] where R and L represent the right and left hemisphere values. Positive AQ values indicated rightward and negative values indicated leftward biases. We also used SOLAR to estimate heritability in the STS_Ant depth values based on the parameterization data by calculating the average for the right and left hemisphere and AQ values for each of the 20 regions. For all heritability analyses, scanner strength (1.5T, 3T, and 7T), sex, age, and rearing history served as covariates in the analyses.

  -Values in parentheses represent standard errors. MR = mother-reared, NR = nurseryreared, WB = wild-born.

Figure 6 :

 6 Figure 6: Heritability (+/-s.e.) for each of the 20 regions within the parameterized STS in the chimpanzees for the combined sample (a), NCCC (b) and YNPRC (c) samples.

Figure 7 :

 7 Figure 7: Upper panel: 3D rendering of chimpanzee brain with the STS_Ant region identified and that portion on which significant rightward asymmetries are evident. Lower panel: Mean AQ (+/-s.e.) scores for each of the 20 STS regions for the combined sample.

Figure 8 :

 8 Figure 8: Mean AQ (+/-s.e.) scores for each of the 20 STS regions for the NCCC (upper panel) and YNPRC (lower panel) samples.

Figure 9 :

 9 Figure 9: (a) Mean depth (+/-s.e.) and (b) Mean AQ (+/-s.e.) for chimpanzees with different polymorphism in RS_P1 in the KIAA0319 gene.

Figure 10 :

 10 Figure 10: Mean surface area AQ (+/-s.e.) (a) and Mean depth AQ (+/-s.e.) (b) for the STS_Ant and STS_Post measures in chimpanzees with different alleles for the AVPR1A gene.

  KIAA0319 and AVPR1A genes influenced the average surface area (CS_ASA), average mean depth (CS_AMD), overall surface area AQ (CSSA_AQ) and overall mean depth AQ (CSMD_AQ) values of the CS. To test for the effect of the RS_P1 and RS_63 SNPs on these 4 CS measures, multiple analyses of covariance (MANCOVA) was performed with CS_ASA, CS_AMD, CSSA_AQ and CSMD_AQ as dependent measures while sex and genotype were between-group factors. Rearing, scanner magnet and relatedness coefficients were covariates. No significant main effect or interactions were found for either of the KIAA0319 SNP polymorphisms. With respect to AVPR1A, the MANCOVA revealed a significant main effect for AVPR1A F(4, 228) = 2.575, p = .03. Subsequent univariate F-test revealed a significant effect for the CSMD_AQ values. DupB-/-individuals had greater rightward asymmetry values compared to DupB+/-apes (see Table

Table 1 :

 1 Average Mean Surface Area and Mean Depth (+/-s.e.) for the STS_Ant and STS_Post as a Function of Sex and Rearing History

	Sex			Rearing
	Female	Male	MR	NR	WB
	-

  -

	Surface Area (mm 2 )
	STS_Ant	1339.28	1483.65	1462.47	1419.75	1397.19
		(19.54)	(22.84)	(22.09)	(27.37)	(42.63)
	STS_Post 584.46	666.36	641.57	628.90	605.74
		(13.35)	(15.60)	(15.09)	(18.69)	(29.11)
	Mean Depth (mm)
	STS_Ant	11.57	12.13	12.02	11.78	11.76
		(.094)	(.110)	(.106)	(.132)	(.205)
	STS_Post 7.98	7.94	8.37	8.08	7.43
		(.106)	(.124)	(.120)	(.148)	(.231)

Table 2 :

 2 Heritability in STS_Ant and STS_Post Surface Area and Mean Depth for the

	Combined, YNPRC and NCCC Samples
	h 2	se	p	Covariates Variance
	-

Table 3 :

 3 Heritability in STS_Ant Parameterized Regions and Variance Accounted for By

	Covariates

Table 4 :

 4 -Mean (+/-s.e.) STS Depth at the 20 STS locations as a Function of Sex and

	Rearing Group
	Sex			Rearing History
	Male	Female	MR	NR	WB

Table 5 :

 5 Descriptive Data for STS_Ant and STS_Post Average and Asymmetry Data for Chimpanzees with Different KIAA0319 Genotypes Genotype

  -

	Mean STS_Ant	1454.34 (25.38)	1408.65 (21.05)	1382.29 (30.23)
	Mean STS_Post	623.26 (17.52)	605.38 (14.53)	602.86 (20.87)
	AQ STS_Ant	-0.006 (.016)	+0.018 (.013)	+0.027 (.019)
	AQ STS_Post	+0.001 (.040)	-0.025 (.033)	-0.033 (.047)
	Mean Depth
	Mean STS_Ant	12.01 (.122)	11.69 (.101)	11.70 (.145)
	Mean STS_Post	8.12 (.142)	8.03 (.118)	8.07 (.169)
	AQ STS_Ant	+0.024 (.010)	-0.002 (.009)	-0.012 (.012)
	AQ STS_Post	-0.035 (.025)	-0.020 (.021)	-0.029 (.030)
	Surface Area

Table 6 :

 6 Descriptive Data for STS_Ant and STS_Post Average and Asymmetry Data for

	Chimpanzees with Different AVPR1A Genotypes
		Genotype
	AVPR1A	DupB -/-	DupB +/-

Table 7

 7

	: Mean CS_ASA, CS_AMD, CSSA_AQ and CSMD_AQ Values (+/-s.e.) for
	Chimpanzee with Different KIAA0319 and AVPR1A Genotypes
	CS_ASA	CS_AMD	CSSA_AQ CSDM_AQ

Acknowledgement This work was supported, in part, by NIH grants AG-067419, NS-073134, NS-42867, and NS-092988. Chimpanzee maintenance at the National Center for Chimpanzee Care was previously funded by NIH/NCRR U42-OD-011197. For portions of this work, W.D.H. was also supported by (1) the IMERA/ILCB residency program (ANR-16-CONV-0002) of Aix-Marseille Universite and its Excellence Initiative (A*MIDEX) and (2) the Blaise-Pascal Chair of Region Ile de France and Université Paris-Saclay. A.M has received funding from the European Research Council under the European Union's Horizon 2020 research and innovation program grant agreement No 716931 -GESTIMAGE -ERC-2016-STG. All aspects of this research conformed to existing US and NIH federal policies on the ethical use of chimpanzees in research. Reprint requests may be sent to: William D Hopkins, Department of Comparative Medicine, Keeling Center for Comparative Medicine and Research, Bastrop, Texas 78602 Email: wdhopkins@mdanderson.org

apes. By contrast, DupB-/-apes had greater leftward asymmetries than Dup+/-apes for region 3.

Effect of KIAA0319 & AVPR1A on Central Sulcus (CS) Surface Area and Depth

In this final analysis, we tested for the influence of the KIAA0319 and AVPR1A SNPs on the surface are and depth of the CS. The details for extraction, labeling and parameterization of the CS were identical to those used for the STS and have been described in detail elsewhere (REFS). The only major difference was in the parameterization direction which was performed along the superior-inferior plane for the CS as opposed to the anterior-posterior plane for the STS. As with the STS, depth of the CS was measured at 100 sulcal length positions in an superior-to-inferior progression along the parameterized sulcal mesh surface. Position 1 was located at the most superior point of CS while position 100 was located at the most inferior point of the CS. As with the STS, to simplify the analyses, within the left and right hemispheres, we averaged every 5 depth measures from point 0 to 100 to create 20 CS regions along the these findings were consistent between the NCCC and YNPRC cohorts of chimpanzees. Second, and consistent with the previous report by [START_REF] Leroy | New human-specific brain landmark: The depth asymmetry of superior temporal sulcus[END_REF], chimpanzees show a small, but nonetheless significant, population-level rightward asymmetry in the STAP region within the STS. Lastly, one SNP within the KIAA0319 gene was found to have a significant impact on the depth and asymmetry in the STAP regions within the STS. Similarly, an indel deletion with the arginine vasopressin receptor 1A gene, AVPR1A, was found to be associated with both the mean depth and asymmetry within the STS.

Regarding the results on lateralization, the significant rightward asymmetries in the STAP region are of interest for two reasons. First, as we have reported for measures of the planum temporale in chimpanzees [START_REF] Spocter | Reproducibility of leftward planum temporale asymmetries in two genetically isolated populations of chimpanzees (Pan troglodytes)[END_REF], the findings were largely consistent between the NCCC and YNPRC cohorts of captive chimpanzees suggesting that this bias is repeatable in at least two genetically isolated populations of apes. Second, the STS was quantified slightly differently than the method used by [START_REF] Leroy | New human-specific brain landmark: The depth asymmetry of superior temporal sulcus[END_REF]. Therefore, it also appears that the small but significant rightward asymmetries in the STAP region are evident independent of the methods used to quantify it. More generally, the overall significant rightward asymmetries in the STAP region clearly challenge the claim that these asymmetries are uniquely human, though admittedly, the magnitude of the bias is decidedly smaller in chimpanzees compared to humans. We also note that significant rightward asymmetries have been reported in the overall surface area of the STS in macaques [START_REF] Bogart | Cortical sulci asymmetries in chimpanzees and macaques: A new look at an old idea[END_REF][START_REF] Suwada | Cerebral sulcal asymmetry in macaque monkeys[END_REF], though it is unclear if these effects are attributable to lateralization within the STAP region because these previous studies did not delineate


W D Meguerditchian

Michele M Mulholland
email: mmmulholland@mdanderson.org

A Meguerditchian

& Hopkins

Michale E Keeling

Age-and sex-related differences in baboon (Papio anubis) gray matter covariation

Keywords: aging, nonhuman primate, baboon, source-based morphometry, gray matter covariation

Introduction

Both humans and nonhuman primates show changes in cognition, brain morphology, and behavior with age, both as a part of normal aging and as a result of age-related disease. Normal human aging is characterized by declines in cognition, global brain volume, gray matter volume, and functional activation associated with increasing age [START_REF] Abe | Aging in the CNS: Comparison of gray/white matter volume and diffusion tensor data[END_REF][START_REF] Bigler | Temporal lobe morphology in normal aging and traumatic brain injury[END_REF][START_REF] Bigler | Hippocampal volume in normal aging and traumatic brain injury[END_REF][START_REF] Bishop | Neural mechanisms of ageing and cognitive decline[END_REF][START_REF] Brickman | Structural MRI covariance patterns associated with normal aging and neuropsychological functioning[END_REF][START_REF] Chen | Brain aging in humans, chimpanzees (Pan troglodytes), and rhesus macaques (Macaca mulatta): magnetic resonance imaging studies of macro-and microstructural changes[END_REF][START_REF] Ge | Age-related total gray matter and white matter changes in normal adult brain. Part I: Volumetric MR imaging analysis[END_REF][START_REF] Good | A voxelbased morphometric study of aging in 465 normal adult brains[END_REF][START_REF] Hawkins | Diffusion Tensor Imaging Correlates of Cognitive-Motor Decline in Normal Aging and Increased Alzheimer's Disease Risk[END_REF]. Most studies on nonhuman aging have utilized non-primate model species such as yeast, nematodes, fruit flies, and mice due mostly to availability, cost, short lifespan and well-documented genetic and/or physiological characteristics. While these models have provided important data and have advanced our understanding of aging, they do not closely model the human aging process. Utilizing nonhuman primate models (particularly those more closely related phylogenetically) is critically important given their genetic similarities to humans, longer lifespans, larger and more complex brains and more sophisticated cognitive abilities compared to other model species. Among nonhuman primates, baboons are particularly well-suited as a model for human aging. First, unlike other monkey species, there is evidence that baboons undergo well-coordinated growth, growth spurts, and an order of growth cessation like humans [START_REF] Leigh | Growth and development of baboons, The baboon in biomedical research[END_REF]. Second, the baboon genome is very similar to humans with over 90% homology of coding segments and 85% homology of non-coding regions. For this reason, the baboon has served as a model to study the genetics and epigenetics of several human diseases [START_REF] Cox | Baboons as a Model to Study Genetics and Epigenetics of Human Disease[END_REF][START_REF] Cox | A second-generation genetic linkage map of the baboon (Papio hamadryas) genome[END_REF][START_REF] Robinson | Analysis of 100 high-coverage genomes from a pedigreed captive baboon colony[END_REF][START_REF] Rogers | The Development and Status of the Baboon Genetic Linkage Map[END_REF][START_REF] Rogers | A Genetic Linkage Map of the Baboon (Papio hamadryas) Genome Based on Human Microsatellite Polymorphisms[END_REF]. In addition, both human and baboon lifespan is moderately heritable (0.23-0.26 and 0.23, respectively) and differences in frailty likely causes variation in individual mortality risk within both species [START_REF] Bronikowski | The aging baboon: Comparative demography in a non-human primate[END_REF][START_REF] Martin | Lifespan in captive baboons is heritable[END_REF]. Third, baboons have been shown to develop similar age-related diseases and conditions as humans, including osteoporosis and arthritis, menopause, endometriosis, obesity, diabetes, cardiovascular disease, and Alzheimer's-like pathology [START_REF] Martin | Lifespan in captive baboons is heritable[END_REF]Schultz et al., 2000a;Schultz et al., 2000b;[START_REF] Schultz | Tau pathology in neurons and glial cells of aged baboons[END_REF][START_REF] Yeung | A Cross-Sectional Study of Ageing and Cardiovascular Function over the Baboon Lifespan[END_REF]. For example, postmortem studies have shown that baboons develop tau pathology in both neurons and glia and Aβ plaques as they age (Schultz et al., 2000a;[START_REF] Schultz | The brain of the aging baboon: A nonhuman primate model for neuronal and glial tau pathology, Aging in nonhuman primates[END_REF]Schultz et al., 2000b;[START_REF] Schultz | Tau pathology in neurons and glial cells of aged baboons[END_REF].

Furthermore, compared to rhesus macaques (a common nonhuman primate model species), baboons have larger brains making them ideal for imaging studies [START_REF] Black | Neuroimaging in baboons, The baboon in biomedical research[END_REF], and the baboon prefrontal cortex is more developed and more similar to the human prefrontal cortex than the rhesus macaque [START_REF] Brent | The study of captive baboon behavior[END_REF][START_REF] Fridman | Species of the genus Papio (Cercopithecidae, primates) as subjects of biomedical research: II. Quantitative characteristics of contemporary use of baboon species in medical and biological investigations[END_REF]. Due to these characteristics, studies of baboon aging can help bridge the translational gap between other model species and human aging studies.

Like humans, there is evidence of age-related cognitive decline in baboons. In a cross-sectional study of 19 baboons (aged 1-14), [START_REF] Bonté | Age effects on transfer index performance and executive control in baboons (Papio papio)[END_REF] found that executive control (as measured by a transfer index task) declined with age. In another cross-sectional study of 6 baboons, older adults (20-23 years old) performed significantly worse on cognitive tasks (specifically, learning a novel task, precision, and simple discrimination) compared to younger adults (13-16 years old) [START_REF] Lizarraga | Age-related cognitive decline in baboons: modeling the prodromal phase of Alzheimer's disease and related dementias[END_REF]. These findings suggest that there are likely neurobiological changes occurring as baboons age. Although baboon neuroanatomy and function is well-studied (e.g., [START_REF] Amiez | Sulcal organization in the medial frontal cortex provides insights into primate brain evolution[END_REF][START_REF] Atkinson | Cortical folding of the primate brain: An interdisciplinary examination of the genetic architecture, modularity, and evolvability of a significant neurological trait in pedigreed baboons (Genus Papio)[END_REF][START_REF] Black | Neuroimaging in baboons, The baboon in biomedical research[END_REF][START_REF] Blaizot | The Parahippocampal Gyrus in the Baboon: Anatomical, Cytoarchitectonic and Magnetic Resonance Imaging (MRI) Studies[END_REF][START_REF] Cain | An anatomical asymmetry in the baboon brain[END_REF][START_REF] Greer | MR atlas of the baboon brain for functional neuroimaging[END_REF][START_REF] Love | The average baboon brain: MRI templates and tissue probability maps from 89 individuals[END_REF][START_REF] Marie | Left Brain Asymmetry of the Planum Temporale in a Nonhominid Primate: Redefining the Origin of Brain Specialization for Language[END_REF][START_REF] Mcbride | A comparative volumetric analysis of the prefrontal cortex in human and baboon MRI[END_REF][START_REF] Meguerditchian | Baboons (Papio anubis) living in larger social groups have bigger brains[END_REF][START_REF] Rogers | Heritability of brain volume, surface area and shape: An MRI study in an extended pedigree of baboons[END_REF][START_REF] Westerhausen | Corpus callosum morphology across the lifespan in baboons (Papio anubis): A cross-sectional study of relative mid-sagittal surface area and thickness[END_REF], and postmortem studies reveal the development of tau pathology as they age (Schultz et al., 2000a;[START_REF] Schultz | The brain of the aging baboon: A nonhuman primate model for neuronal and glial tau pathology, Aging in nonhuman primates[END_REF]Schultz et al., 2000b;[START_REF] Schultz | Tau pathology in neurons and glial cells of aged baboons[END_REF], to date there have been few in vivo neuroimaging studies focused specifically on the aging baboon brain. Franke et al. [START_REF] Franke | Premature Brain Aging in Baboons Resulting from Moderate Fetal Undernutrition[END_REF] found that prenatal undernutrition (70% of normal daily food intake) had long-term effects on the baboon brain, prematurely aging the young adult female baboon brain by 2.7 years on average. In addition, the authors examined typical brain aging across the baboon lifespan and reported a significant decrease in overall gray matter volume and increase in overall white matter volume with age.

More recently, [START_REF] Westerhausen | Corpus callosum morphology across the lifespan in baboons (Papio anubis): A cross-sectional study of relative mid-sagittal surface area and thickness[END_REF] examined corpus callosum morphology across the lifespan and found that the area of the corpus collosum increases slowly with age, particularly within the anterior section.

Here we examine the relationship between age and whole-brain gray matter covariation using source-based morphometry (SBM). SBM is a multivariate approach which incorporates an independent components analysis to create networks of brain regions that covary among gray or white matter voxels across the whole brain [START_REF] Gupta | Source-based morphometry: a decade of covarying structural brain patterns[END_REF]. Here we examine the contribution of both age and sex to gray matter covariation within the baboon brain. A number of studies in human subjects have reported age-related changes in gray matter covariation networks [START_REF] Dupre | Structural covariance networks across the life span, from 6 to 94 years of age[END_REF][START_REF] Evans | Networks of anatomical covariance[END_REF][START_REF] Hafkemeijer | Associations between age and gray matter volume in anatomical brain networks in middle-aged to older adults[END_REF][START_REF] Koini | Grey-matter network disintegration as predictor of cognitive and motor function with aging[END_REF][START_REF] Liu | Structural brain network changes across the adult lifespan[END_REF][START_REF] Montembeault | The impact of aging on gray matter structural covariance networks[END_REF]Xu et al., 2009b), but there are only a few studies of this type in nonhuman primates. Source-based morphometry-like methods were used to show that older rhesus macaques had reduced gray matter covariation in components including dorsal and ventral prefrontal cortex, superior temporal sulcus, and sylvian fissure [START_REF] Alexander | Age-related regional network of magnetic resonance imaging gray matter in the rhesus macaque[END_REF]. Previous studies in chimpanzees have also reported age-related changes in gray matter covariation, with decreased gray matter covariation reported for the superior frontal, supplementary motor, and anterior temporal cortex as chimpanzees age (Hopkins, W. D. et al., 2019). In addition, they reported increased gray matter covariation in the prefrontal and premotor cortices and part of the cerebellum as chimpanzees age (Hopkins, W. D. et al., 2019). A more recent study of agerelated changes in chimpanzee gray matter (using voxel-based morphometry) found significant reductions in the prefrontal cortex, middle temporal cortex, superior frontal cortex, insula, superior temporal cortex, and entorhinal cortex with increased age [START_REF] Mulholland | Age-and cognition-related differences in the gray matter volume of the chimpanzee brain (Pan troglodytes): A voxel-based morphometry and conjunction analysis[END_REF]. In the current study, we use source-based morphometry to examine the relationship between age and gray matter covariation in a baboon sample. We hypothesized that significant age effects would be found in one or more of the SBM components, specifically those comprised of regions known to be influenced by age in humans and other non-human primates, such as the middle temporal lobe, prefrontal cortex, and the cerebellum.

In addition, we examined sex differences in gray matter covariation. Studies of the baboon brain have indicated that sex may account for variation in overall brain volume and sulcal length [START_REF] Atkinson | Cortical folding of the primate brain: An interdisciplinary examination of the genetic architecture, modularity, and evolvability of a significant neurological trait in pedigreed baboons (Genus Papio)[END_REF][START_REF] Rogers | Heritability of brain volume, surface area and shape: An MRI study in an extended pedigree of baboons[END_REF], differences in the area of the corpus collosum [START_REF] Phillips | Tracking development of the corpus callosum in fetal and early postnatal baboons using magnetic resonance imaging[END_REF], and whole brain gray and white matter volumes [START_REF] Franke | Premature Brain Aging in Baboons Resulting from Moderate Fetal Undernutrition[END_REF]. [START_REF] Franke | Premature Brain Aging in Baboons Resulting from Moderate Fetal Undernutrition[END_REF] found that males had significantly higher absolute gray matter volume than females but did not examine regional differences or gray matter covariation. [START_REF] Atkinson | Cortical folding of the primate brain: An interdisciplinary examination of the genetic architecture, modularity, and evolvability of a significant neurological trait in pedigreed baboons (Genus Papio)[END_REF] reported that sex accounted for a significant proportion of variance in the length of the central sulcus, lateral fissure, lunate, and superior temporal sulcus. In chimpanzees, an SBM analysis revealed significant sex differences in gray matter covariation, including the cerebellum, primary visual, anterior temporal, and frontopolar cortices (Hopkins, W. D. et al., 2019). Similar to chimpanzees, here we hypothesized that there would be significant sex differences in one or more of the baboon SBM components.

Materials and Methods

Subjects

We utilized MRI data collected from 89 captive olive baboons (56 females, 31 males) from the Station de Primatologie CNRS in Rousset, France (Agreement number for conducting experiments on vertebrate animals: D13-087-7). The baboons ranged in age from 2 to 26 years of age (M = 11.46, SD = 5.89). All subjects were socially housed with free access to indoor/outdoor enclosures furnished with wooden platforms and vertical climbing structures. Care staff fed the baboons a diet of commercial primate chow, seed, and fresh produce four times per day. All baboons had ad libitum access to water.

All procedures complied with the current French laws and the European directive 86/609/CEE and were approved by the Provence Alpes Côte d'Azur ethics committee.

Image Acquisition and Post-Image Processing

Magnetic resonance image (MRI) scans were acquired using a 3.0 Tesla scanner (MEDSPEC 30/80 ADVANCE; Bruker) with a Rapid-Biomed surface antenna located at the Marseille Functional MRI Center. Detailed information about both image acquisition and post-image processing can be found in [START_REF] Love | The average baboon brain: MRI templates and tissue probability maps from 89 individuals[END_REF]. Briefly, each baboon was sedated with an intramuscular injection of ketamine (10 mg/kg) prior to transportation to the imaging facility. Upon arrival, subjects were further sedated with intramuscular injections of tiletamine and zolazepam (Zoletil™, 7 mg/kg) and acepromazine (Calmivet™, 0.2-0.5 mg/kg). During the imaging procedures, subjects were anesthesized with a drip irrigation of tiletamine, zolazepam (Zoletil™, 4 mg/kg/h) and NaCl (0.9% of 4 ml/kg/h). Veterinary staff monitored cardiovascular and respiratory functions with a SpO2 device and respiratory belt. Animals were placed into the scanner in a prone position with their head supported by cushions and Velcro strips. High resolution structural T1-weighted images were acquired (TR: 9.4 ms; TE: 4.3 ms; flip angle: 30°; inversion time: 800 ms); due to size field of view and isotropic voxel size differed for female and young male baboons (fov: 108 × 108 × 108 mm; isotropic voxel size: 0.6 mm3) and adult males (fov: 126 × 126 × 126 mm; isotropic voxel size: 0.7 mm3). Upon completion of the scan, subjects were monitored and fully recovered from anesthesia prior to being reunited with their social groups.

Resulting images were first denoised using an Adaptive Nonlocal Means filter (ANLM; [START_REF] Manjón | Adaptive non-local means denoising of MR images with spatially varying noise levels[END_REF]) before brain extraction using both Multi Atlas Skull Stripping software (http://www.cbica.upenn.edu/sbia/software/MASS/index.html; [START_REF] Doshi | Multi-atlas skull-stripping[END_REF][START_REF] Ou | DRAMMS: Deformable registration via attribute matching and mutual-saliency weighting[END_REF] and ITK-SNAP (version 3.2, www.itksnap.org, [START_REF] Yushkevich | User-guided 3D active contour segmentation of anatomical structures: significantly improved efficiency and reliability[END_REF]. Next, the skull stripped images were bias corrected using an N4 algorithm [START_REF] Tustison | N4ITK: improved N3 bias correction[END_REF]. Finally, we ran these images through the FSL-VBM pipeline using the FMRIB Software library tools (FSL; Oxford; http://fsl.fmrib.ox.ac.uk/fsl/fslwiki/FSLVBM). This process included (1) segmentation of each scan into gray and white matter, (2) linear registration of each scan to a standard baboon template [START_REF] Love | The average baboon brain: MRI templates and tissue probability maps from 89 individuals[END_REF], (3) creation of a study-specific gray matter template [START_REF] Andersson | Non-linear registration aka Spatial normalisation FMRIB Technial Report TR07JA2[END_REF][START_REF] Douaud | Anatomically related grey and white matter abnormalities in adolescent-onset schizophrenia[END_REF][START_REF] Smith | Advances in functional and structural MR image analysis and implementation as FSL[END_REF], (4) non-linear registration of each subject's gray matter image to the study-specific template, (5) modulation of the gray matter volume by use of a Jacobian warp to correct for local expansion or contraction of gray matter within each voxel, and (6) smoothing with an isotropic Gaussian kernel with a sigma of 2 mm. The resulting smooth, modulated gray matter volumes were used in subsequent analyses.

Statistical Analyses

Source-based morphometry. The data were analyzed using source-based morphometry (SBM v 1.0b) performed in the Group ICA of fMRI Toolbox (GIFT; http://icatb.sourceforge.net) (Xu et al., 2009a) in MATLAB R2015b. We imported the smoothed, modulated gray matter volumes for each subject and allowed the software to estimate the number of components based on an independent component analysis using a neural network algorithm. After estimation, the program calculates the parameters and creates a 4D volume containing each of the independent spatial components. At the individual level, SBM produces a weighted score that reflects each subject's relative contribution to the creation of each component, which were then used in subsequent statistical analyses. To examine the brain regions contributing to each component, we registered the component maps (scaled to standard deviation units and z scores) to the standard baboon template brain [START_REF] Love | The average baboon brain: MRI templates and tissue probability maps from 89 individuals[END_REF] and set the z-score threshold at |z| ≥ 3.00 as has been used in previous studies in humans and chimpanzees (see Hopkins, W. D. et al., 2019;Hopkins et al., 2020b;Xu et al., 2009a). All brain regions reaching this threshold were considered significant, and the volume was then measured using the region-of-interest tool in ANALYZE 14.

Age and sex effects. We ran a multivariate analysis of variance (MANOVA) with age group and sex (female, male) as the independent variables and weighted scores for each SBM component as dependent variables. Subsequent univariate ANOVAs were then used to examine differences between the age groups on their contribution to each individual SBM component.

Results

The SBM analysis identified 10 independent gray matter covariation components and corresponding individual weighted scores. The largest regions within each component included 1. cerebellum (bilateral), 2. caudate (bilateral) and putamen (bilateral), 3. brainstem (bilateral) and superior parietal cortex (bilateral), 4. isthmus of the cingulate (bilateral), 5. occipital cortex (bilateral), 6. brainstem (bilateral), 7. thalamus (bilateral), 8. middle frontal and superior frontal cortex (bilateral), 9. amygdala and hippocampus (bilateral), and 10. brainstem and cerebellum (bilateral). Full anatomical descriptions of each component and their respective gray matter volumes can be found in Table 1. 3D renderings of each component can be found in the supplemental materials. Baboons were grouped by age: juvenile (2-6 years old), young adult (7-12 years old), middle age

Discussion

Using source-based morphometry, we identified 10 components of gray matter covariation in the baboon brain and examined the relationship with age. First, contribution to components 3 and 8 decline as baboons age. Juvenile baboons contributed more to gray matter covariation components (compared to all other age classes) which include the middle frontal cortex, superior frontal cortex, brainstem, superior parietal cortex, thalamus and pallidum, precentral cortex, insula, and precuneus. In addition, we found adult baboons (of all age classes) contribute more to components 5 and 7 (occipital cortex, thalamus, putamen, and insula). The current findings expand on those reported by [START_REF] Franke | Premature Brain Aging in Baboons Resulting from Moderate Fetal Undernutrition[END_REF] who found reductions in overall gray matter volume as baboons age. Here we show that gray matter covariation within specific regions of the baboon brain change with increased age. As the baboons in the current study had no known illnesses or injuries, the changes in gray matter covariation here can presumably be attributed to normal aging, similar to neurological changes reported in healthy aging humans and chimpanzees (Hopkins, W. D. et al., 2019;[START_REF] Lemaitre | Normal age-related brain morphometric changes: nonuniformity across cortical thickness, surface area and gray matter volume?[END_REF][START_REF] Smuts | Age-related gray matter volume changes in the brain during non-elderly adulthood[END_REF].

Further, it is possible that these regional changes are related to age-related changes in cognition. In chimpanzees, aged individuals show moderate loss in cognitive and motor functions compared to younger conspecifics (Hopkins et al., 2020a;Hopkins, W.D. et al., 2019;[START_REF] Hopkins | Genetic basis for motor skill and hand preference for tool use in chimpanzees (Pan troglodytes)[END_REF][START_REF] Lacreuse | Age-related decline in cognitive flexibility in female chimpanzees[END_REF][START_REF] Lacreuse | Cognitive and motor aging in female chimpanzees[END_REF], and age-related changes in cognition are related to changes in gray matter volume particularly within the frontal and parietal cortices, and precentral cortex [START_REF] Mulholland | Age-and cognition-related differences in the gray matter volume of the chimpanzee brain (Pan troglodytes): A voxel-based morphometry and conjunction analysis[END_REF]. In rhesus macaques, age-related declines in cognitive performance (i.e., delayed response, recognition memory, executive function) are related to decreases in overall volume, synaptic density, and receptor binding density of the dorsolateral prefrontal cortex [START_REF] Hara | Neuronal and morphological bases of cognitive decline in aged rhesus monkeys[END_REF]. Like humans, chimpanzees, and rhesus macaques, baboon cognition also appears to decline with age; performance on transfer index, novel task learning, precision, and simple discrimination tasks are worse in older adult compared to younger adult baboons [START_REF] Bonté | Age effects on transfer index performance and executive control in baboons (Papio papio)[END_REF][START_REF] Lizarraga | Age-related cognitive decline in baboons: modeling the prodromal phase of Alzheimer's disease and related dementias[END_REF]; however, additional studies are needed to directly test the relationship between these previously reported cognitive declines and the age-related neuroanatomical differences reported in the current study.

In addition, we found significant sex differences in gray matter covariation. Females contributed more to components which included the brainstem (bilateral), superior parietal cortex (bilateral), thalamus and pallidum (right), precuneus (left), middle and superior frontal cortices (bilateral), amygdala and hippocampus (bilateral), superior temporal cortex (bilateral), and rostral middle frontal (left). Whereas males contributed more to components including caudate and putamen (bilateral), cerebellum (bilateral), and lingual cortex (bilateral). These findings are consistent with other studies which show sex differences in baboon overall brain volume and sulcal length, corpus collosum area, and whole brain gray and white matter volumes [START_REF] Atkinson | Cortical folding of the primate brain: An interdisciplinary examination of the genetic architecture, modularity, and evolvability of a significant neurological trait in pedigreed baboons (Genus Papio)[END_REF][START_REF] Franke | Premature Brain Aging in Baboons Resulting from Moderate Fetal Undernutrition[END_REF][START_REF] Phillips | Tracking development of the corpus callosum in fetal and early postnatal baboons using magnetic resonance imaging[END_REF][START_REF] Rogers | Heritability of brain volume, surface area and shape: An MRI study in an extended pedigree of baboons[END_REF]. Baboons are sexually dimorphic in both anatomy and behavior.

Adult male baboons are larger than adult females, and also display different growth trajectories (with females reaching their adult size at earlier ages than males) and life expectancies [START_REF] Alberts | The malefemale health-survival paradox: a comparative perspective on sex differences in aging and mortality, Sociality, hierarchy, health: comparative biodemography: a collection of papers[END_REF][START_REF] Leigh | Growth and development of baboons, The baboon in biomedical research[END_REF][START_REF] Leigh | Sexual dimorphism in the baboon facial skeleton[END_REF][START_REF] Roseman | Genetic and environmental contributions to variation in baboon cranial morphology[END_REF]. Sex differences in baboon behavior are also apparent from infancy to adulthood (e.g. communication, social relationships, aggression, play, etc.) [START_REF] Bentley-Condit | Sex Differences in Captive Olive Baboon Behavior During the First Fourteen Days of Life[END_REF][START_REF] Brent | The study of captive baboon behavior[END_REF][START_REF] Chalmers | Developmental Relationships among Social, Manipulatory, Postural and Locomotor Behaviours in Olive Baboons, Papio anubis[END_REF][START_REF] Coelho | Sexual dimorphism in the activity of olive baboons (Papio cynocephalus anubis) housed in monosexual groups[END_REF][START_REF] Pereira | Effects of age and sex on intra-group spacing behaviour in juvenile savannah baboons, Papio cynocephalus cynocephalus[END_REF][START_REF] Smuts | Age-related gray matter volume changes in the brain during non-elderly adulthood[END_REF][START_REF] Young | Gender and environment as determinants of behavior in infant common baboons (Papio cynocephalus)[END_REF][START_REF] Young | The development of grooming, sociosexual behavior, play and aggression in captive baboons in their first two years[END_REF]. As for cognition, there is limited evidence of sex differences to date. [START_REF] Lacreuse | Sex differences in inhibitory control in socially-housed baboons (Papio papio)[END_REF] found that males were slower but more accurate on a task of inhibitory control compared to females, perhaps due to testosterone mediated sensitivity to rewards. It is possible that the neuroanatomical sex differences reported in the current study could be related to these sex differences in behavior or cognition; however, more research is needed to examine these relationships.

Though the current study has many strengths, it is not without limitations. The sample was skewed toward younger and female baboons, with half the number of geriatric baboons (n=12) compared to other age classes (juvenile n=25, young adult n=30, middle-aged adult n=22), and almost twice as many females (n=58) as males (n=31). Although the proportions of males to females reflect proportions within many captive baboon colonies (baboons are typically housed in single-or two-male multi-female groups, i.e. harems), future studies should attempt to include data from more males and ensure that geriatric baboons are represented at numbers similar to other age classes. Despite these limitations, the current study shows that gray matter covariation differs between the two sexes and across age, with geriatric baboons contributing less to covariation components including the frontal and parietal cortices. Future research should examine the relationship between cognitive performance and both age-related and sex-related differences in gray matter covariation. We are very grateful to the Station de Primatologie CNRS, particularly the animal care staff and technicians, Jean-Noël Benoit, Jean-Christophe Marin, Valérie Moulin, Fidji and Richard Francioly, Laurence Boes, Célia Sarradin, Brigitte Rimbaud, Sebastien Guiol, Georges Di Grandi but also the vet Alice Bertello for their critical involvement in this project, Ivan Balansard, Sandrine Melot-Dusseau, Laura Desmis, Frederic Lombardo and Colette Pourpe for additional assistance.

Disclosure Statement

The authors have no conflicts of interest to report.

Juveniles

Young Adults Middle Aged Adults Elderly

(

  Figure 2.

  16-CONV-0002 (ILCB) and from the Excellence Initiative of Aix-Marseille University (A*MIDEX). MRI acquisitions were done at the Center IRM-INT (UMR 7289, AMU-CNRS), platform member of France Life Imaging network (grant ANR-11-INBS-0006).

FiguresFigure

  Figures

  Figure 1. The average weighted SBM score on Components 3, 5, 7, and 8 for each age class. Elderly baboons contributed more to Components 5 and 7 but contributed less to components 3 and 8 compared to juvenile baboons (p < 0.05).

Figure 2 .-

 2 Figure 2. The average weighted SBM score on each SBM component for males and females. Males contributed more to component 2, whereas females contributed more to components 3, 8, and 9. * p < 0.05, ** p < 0.01

Acknowledgements

AM has received funding from the European Research Council under the European Union's Horizon 2020 research and innovation program grant agreement No 716931 -GESTIMAGE -ERC-2016-STG, from the French "Agence Nationale de le Recherche" ANR-12-PDOC-0014-01 (LangPrimate), ANR-

Tables

 Table 1.Regions and corresponding volumes (≥ 20 mm 3 ) of the 10 baboon SBM components.