Norman Owen-Smith
email: norman.owen-smith@wits.ac.za

Thomas Morrison
email: thomas.morrison@glasgow.ac.uk

Simon Chamaillé-Jammes Cefe

Robyn Hetem
email: robyn.hetem@wits.ac.za

Emily Bennitt
email: ebennitt@ub.ac.bw

Frank Van Langevelde
email: frank.vanlangevelde@wur.nl

Movement ecology of large herbivores in African savannas: current knowledge and gaps

Keywords: Daily activity, home range, migration, predation, population dispersion

1. Nearly 90% of the world's large herbivore diversity occurs in Africa, yet there is a striking dearth of information on the movement ecology of these organisms compared to herbivores living in higher latitude ecosystems.

2. The environmental context for movements of large herbivores in African savanna ecosystems has several distinguishing features. African large herbivores move in landscapes with high spatiotemporal variability, low predictability, seasonal restrictions in surface water as well as food resources, and exposure to a diverse assemblage of competitors, predators and pathogens. These features influence mobility, diel activity routines, home range fidelity and exposure to predation.

3. We review the knowledge we have gained about the movements of African herbivores from GPS telemetry and identify important gaps in knowledge that exist. Topics addressed include seasonal movement patterns, daily activity schedules, space utilization, water dependency, responses to risks of predation, pathogen transmission, social affiliations and local population density determination.

4. While the growing number of GPS telemetry studies have addressed a wide range of topics in Africa, they remain fragmentary in terms of places and species represented. African landscapes are changing perhaps faster than any other region on Earth, with rapidly expanding human populations, massive infrastructure development projects and changes in climatic regimes. There is a crucial need to establish relationships between herbivore movements and their changing environments especially in Africa where most of the world's large herbivore diversity resides.

5

INTRODUCTION

The advent of Global Positioning System (GPS) technology is enabling new insights into the ecology of animal movements [START_REF] Kays | Terrestrial animal tracking as an eye on life and planet[END_REF]. Studies on large herbivores using this technology have largely been concentrated on ungulates inhabiting northern temperate or arctic environments. Fewer studies have been conducted on large herbivores occupying African savanna environments. Yet, Africa houses 90% of the world's large herbivore diversity, and relatively intact populations of herbivores and carnivores --a global heritage that is increasingly at risk [START_REF] Ripple | Saving the world's terrestrial megafauna[END_REF]. Mounting threats include climate shifts in temperature and precipitation [START_REF] Hetem | Responses of large mammals to climate change[END_REF]) coupled with fences around reserves, roads and expanding human settlements and infrastructure (Ogutu et al. 2011[START_REF] Fyumagwa | Roads as a threat to the Serengeti ecosystem[END_REF].

These developments prevent animals from moving as widely as they did in the past [START_REF] Tucker | Moving in the Anthropocene: Global reductions in terrestrial mammalian movements[END_REF][START_REF] Veldhuis | The Serengeti Squeeze: crossboundary human impacts compromise an iconic protected ecosystem[END_REF]) and may even threaten the persistence of ecosystems [START_REF] Løvschal | Fencing bodes a rapid collapse of the unique Greater Mara ecosystem[END_REF].

African savannas are distinctive in several features. Environmental variation over space and through time is driven largely by fluctuations in rainfall, governing plant growth and surface water availability. Their tree and grass cover is modified locally by fire and herbivory [START_REF] Sankaran | Determinants of woody cover in African savannas[END_REF]. These savanna ecosystems uniquely retain a diverse mix of grazing, browsing and mixed feeding herbivores plus the large carnivores that feed on them.

In this review, we synthesize findings from recent research on the movements of African large herbivores based on GPS telemetry and identify gaps in knowledge needing to be addressed. Topics covered include seasonal movements over varying distances, activity schedules, space occupation at different scales, surface water dependency, thermoregulation, responses to predation risk, mechanisms of pathogen transmission and spatial limitations on abundance. We do not attempt inter-continental comparisons, nor do we cover the large amount of information gathered on the movements of African elephants. The large herbivores we have in mind are ungulates weighing over 10 kg in adult body mass.

WHAT DO WE KNOW AND STILL NOT KNOW?

When do seasonal movements represent migration?

According to classic definitions, migration entails regular seasonal movements of animals between widely separated ranges [START_REF] Avgar | On the adaptive benefits of mammal migration[END_REF]. Especially notable are coordinated mass shifts of populations between distant regions [START_REF] Harris | Global decline in aggregated migrations of large terrestrial mammals[END_REF]. But how far apart must these ranges be to qualify as migration [START_REF] Cagnacci | How many routes to migration? Comparison of methods to assess and characterize migratory movements[END_REF])? Movements that are less regularly expressed in time or space are better interpreted as nomadic [START_REF] Teitelbaum | Beyond migration: Causes and consequences of nomadic animal movements[END_REF]. Migrations may also be partial, with a variable portion of the population remaining behind after the migrants have moved away [START_REF] Chapman | The ecology and evolution of partial migration[END_REF]. Is there any fundamental distinction between mass migrations and more local shifts between seasonal ranges?

What we know

Based on the published literature, as few as eight [START_REF] Teitelbaum | How far to go? Determinants of migration distance in land mammals[END_REF] or as many as 13 species [START_REF] Harris | Global decline in aggregated migrations of large terrestrial mammals[END_REF] of African large herbivore have been regarded as having migratory populations. The archetypal migration by ~1.3 million wildebeest (Connochaetes taurinus) in the Serengeti region of Tanzania covers a linear distance of 210 km, or round-trip circuit spanning 650 km [START_REF] Thirgood | Can parks protect migratory ungulates? The case of the Serengeti wildebeest[END_REF][START_REF] Hopcraft | Why are wildebeest the most abundant herbivore in the Serengeti ecosystem?[END_REF]Fig. 1A). This distance is coupled with a rainfall gradient from 550 mm between the south-eastern plains occupied during the wet season months to over 1000 mm in the northern Serengeti-Mara region inhabited during the dry season. Much smaller populations of wildebeest remain resident year-round within the western corridor, Ngorongoro highlands and Mara regions of the ecosystem.

Nevertheless, GPS tracking has revealed how some of the supposedly resident animals show wide-ranging but opportunistic movements in response to prevailing conditions (Hopcraft unpublished; Fig. 1B).

Elsewhere in Tanzania, wildebeest have been tracked migrating over distances of 40-120 km from a dry season range in the vicinity of the Tarangire River to occupy either the Simanjiro Plains in the east or plains near Lake Natron in the north during the wet season [START_REF] Morrison | Wet season range fidelity in a tropical migratory ungulate[END_REF]. In Kenya, wildebeest migrations from the Maasai Mara Reserve northward to the Loita Plains and from Nairobi National Park southward to the Athi-Kaputiei Plains have effectively collapsed due to human settlements, agriculture, roads and fencing (Ogutu et al. 2013[START_REF] Stabach | Variation in habitat selection by white-bearded wildebeest across different degrees of human disturbance[END_REF].

Within South Africa's Kruger National Park, wildebeest that had migrated 50 km seasonally towards surface water provided by a river to the south-west had this movement blocked by the completion of a fence along the western park boundary [START_REF] Whyte | Blue wildebeest population trends in the Kruger National-Park and the effects of fencing[END_REF]. Although the fence was removed in 1994, the migration has not been re-established.

Nevertheless, some wildebeest fitted with GPS collars moved distances of 4-10 km between seasonally distinct home ranges, while other animals remained year-round within home ranges covering only a few square kilometres [START_REF] Yiu | Restricted habitat use by an African savanna herbivore through the seasonal cycle: key resources concept expanded[END_REF], Owen-Smith et al. 2015[START_REF] Owen-Smith | Space use patterns of a large mammalian herbivore distinguished by activity state: fear versus food?[END_REF]Fig. 1C). Despite the vast difference in scale compared with the Serengeti migrants, the seasonal alternation in habitat conditions was basically similar: concentration in open regions with short grasses while sufficient green forage remained, followed by movement to places where some green grass was retained through the dry season along with perennial surface water.

The Kalahari area in southern Botswana formerly accommodated well over a hundred thousand wildebeest, until fences blocked their movement towards remaining water sources during a severe drought [START_REF] Spinage | Is the Kalahari cornucopia fact or fiction? A predictive model[END_REF].

However, these seasonal movements were evidently nomadic, rather than regularly migratory [START_REF] Verlinden | Seasonal movement patterns of some ungulates in the Kalahari ecosystem of Botswana between 1990 and 1995[END_REF]. One of the remaining wildebeest herds equipped with a GPS collar travelled a distance exceeding 200 km to seek water after surface water sources had dried up in its wet season range [START_REF] Selebatso | Water provision alters wildebeest adaptive habitat selection and resilience in Central Kalahari[END_REF].

The longest migration documented for any African ungulate is by white-eared kob (Kobus kob leucotis) in South Sudan [START_REF] Fryxell | Seasonal migration by white-eared kob in relation to resoures[END_REF][START_REF] Schapira | Wildlife migration in Ethiopia and South Sudan longer than " the longest in Africa": a response to Naidoo et al[END_REF][START_REF] Morjan | Armed conflict and development in South Sudan threatens some of Africa's longest and largest ungulate migrations[END_REF]. These animals are forced to move from the Boma region, where they congregate in the dry season, by rising floodwaters during the wet season. They shift to uplands in or near Bandigo National Park to the west, until the flood recedes sufficiently. Their seasonal movement spans a distance of up to 400 km, with the round-trip circuit double this. Accompanying the 800,000 kob are 160,000 tiang (Damaliscus lunatus tiang) covering nearly the same distance, plus around 100,000 Mongalla gazelles (Eudorcas albonotata). Migratory populations of kob and tiang or topi have not been documented elsewhere, and movements of Thomson' gazelles (Eudorcas thomsoni) in Serengeti seem to be opportunistic rather than migratory [START_REF] Fryxell | Landscape scale, heterogeneity, and the viability of Serengeti grazers[END_REF].

Zebra (Equus quagga) family herds also migrate seasonally in the Serengeti ecosystem, but with a less coherent pattern than shown by wildebeest. While some collared zebra groups move with the wildebeest as far as the Mara region in Kenya, others relocate over shorter distances (Hopcraft, unpublished data). The longest migration for zebra is by animals moving between the Chobe River floodplain in north-east Namibia, occupied during the dry season, and Nxai Pan National Park in Botswana, where the animals remain for several weeks during the wet season --a linear distance of ~250 km [START_REF] Naidoo | A newly discovered wildlife migration in Namibia and Botswana is the longest in[END_REF]. Other zebra herds within Botswana moved between a dry season range in the Okavango Delta and the Makgadikgadi Pan over a straight-line distance of ~200 km [START_REF] Bartlam-Brooks | Will connecting ecosystems allow long-distance animal migrations to persist? A case study of a zebra migration in Botswana[END_REF][START_REF]Tragelaphus strepsiceros Greater Kudu[END_REF]. These longdistance movements are newly established, or reinstated, because they would have been blocked by fences less than a decade ago. However, the majority of the zebra inhabiting the Okavango Delta region remain resident there year-round.

Within the Makgadikgadi Pan National Park, approximately 15,000 zebra migrate ~100 km from their dry season range on the Boteti River to a wet season range in short grassland to the east [START_REF] Bradley | The effect of environmental variability on the foraging behaviour of plains zebra (Equus quagga) in the Makgadikgadi[END_REF]. Further north, several thousand zebra migrate from wet season ranges around the Mababe Depression to dry season ranges within the Linyanti floodplains [START_REF] Sianga | Vegetation heterogeneity and its effects on large herbivores in the Savuti-Mababe-Linyanti ecosystem, northern Botswana[END_REF]. In Etosha National Park in Namibia, about 70% of the zebra migrate over a distance of 60-80 km to where numerous waterholes are available through the dry season while the remainder of the population remains within the wet season range year-round [START_REF] Zidon | Zebra migration strategies and anthrax in Etosha National Park, Namibia[END_REF]. In contrast, zebras equipped with GPS collars in Kruger National Park remained within fixed but quite large home ranges year-round, apart from excursions to and from surface water during the dry season (Macandza et al. 2012a, Cain et al. 2012, Owen-Smith et al. 2015). Similarly sedentary movement patterns were exhibited by collared zebra in Hwange National Park (Chamaillé-Jammes, unpublished data).

African buffalo (Syncerus caffer) herds also exhibit a diversity of seasonal movement patterns. In northern Botswana and north-eastern Namibia, some herds moved over distances of up to 100 km between seasonally distinct home ranges, thus being labelled migratory (Naidoo et al. 2012b[START_REF] Bennitt | Effects of divergent migratory strategies on access to resources for Cape buffalo[END_REF]. Other buffalo herds in the same region, labelled 'expanders', traversed wider areas during the wet season overlapping with the range they occupied during the dry season. In W National Park in West Africa, collared buffalo herds remained close to rivers in the dry season but moved as far as 35 km away during the wet season (Cornelis et al. 2011). In northern Kruger National Park, collared buffalo shifted their home range occupation by merely 10 km to the vicinity of a seasonal river during the dry season (Macandza et al. 2012a). Where surface water is widely available, buffalo herds stayed in the same home range yearround, not only in Kruger National Park but also in Hwange and Gonarezhou national parks in Zimbabwe (N Owen-Smith, unpublished data, Chamailles-Jammes et al. submitted). Nevertheless, under severe drought conditions buffalo herds in Kruger National Park moved to distant areas that retained adequate forage [START_REF] Staver | Grazer movements exacerbate grass declines during drought in an African savanna[END_REF].

Although labelled as migratory [START_REF] Harris | Global decline in aggregated migrations of large terrestrial mammals[END_REF], hartebeest (Alcelaphus buselaphus) and eland (Tragelaphus oryx) tracked by VHF telemetry in the Kalahari region of Botswana showed no seasonal regularity in their home range occupation (Verlinden 1988), and neither did eland in East Africa [START_REF] Hillman | Home range and movement of the common eland in Kenya[END_REF].

Gaps in knowledge

GPS tracking studies have revealed a diverse range of seasonal movement patterns, blurring the demarcation between 'genuine' migrations and more local shifts in seasonal home ranges. Can seasonal movements between home ranges a few kilometres apart, a distance that can be travelled within a few hours, justifiably be interpreted as migratory? All of the claimed migrants are grazers, constrained in the range extent they can occupy during the dry season by their surface water dependency. The Serengeti wildebeest abandon the south-eastern plains occupied during the wet season before all of this nutritious grass has been consumed, when remaining water becomes too saline [START_REF] Gereta | Wildlife -water quality interaction in the Serengeti National Park[END_REF]. Grazers less dependent on access to surface water, like various gazelles, show nomadic movements in search of green forage rather than being confined near surface water during the dry season [START_REF] Fryxell | Landscape scale, heterogeneity, and the viability of Serengeti grazers[END_REF]. The fabled springbok treks of the past apparently represented opportunistic mass movements between summer and winter rainfall regions in search of food [START_REF] Skinner | Springbok treks[END_REF]).

In the light of more detailed information on movement patterns available from GPS tracking, some qualifications are needed for the labels applied: short-versus long-distance migrants, partial versus mass migrations, and opportunistic versus regular seasonal range shifts. Exactly where these divisions should be placed requires more information on the seasonal movement patterns of individuals, populations and species. The particular gaps that need to be filled include (1) routes, timing and distribution of migratory or less regular movements, (2) trade-offs in benefits and costs between migratory and resident animals, and (3) understanding relationships between movement and resource availability, including water as well as food, plus exposure to predation.

How are activities scheduled daily and seasonally?

Savanna herbivores exhibit an activity routine entrained by the diel cycle of ambient temperature and illumination.

Foraging typically predominates in the early morning and late afternoon, while resting prevails through midday and during much of the night (Owen-Smith 1998). Some feeding is necessary during the night in order to maintain digestive fermentation. High midday temperatures can restrict the time available for foraging during the day [START_REF]How high ambient temperature affects the daily activity and foraging time of a subtropical ungulate, the greater kudu[END_REF], Fuller et al. 2016), as does time taken up by travel to and from remaining water sources in the dry season [START_REF] Cain | The costs of drinking: Comparative water dependency of sable antelope and zebra[END_REF]. Daily time spent foraging can be adjusted to compensate for reduced food availability to some extent, but at a cost in exposure to predation and heat stress. Both the scheduling and proportion of time devoted to foraging versus other activities may change seasonally, perhaps most crucially with regard to nocturnal activity.

What we know

GPS telemetry allows the movements of animals bearing collars to be documented throughout the day and night over complete annual cycles, generating vastly more information on movement patterns than can be obtained through direct observations. The challenge is to infer the activity states generating the movements. Because bouts of foraging and resting generally endure for 2-3 hours, intervals between successive GPS locations need to be hourly or less for activity states at this level of resolution to be recogised. For large herbivores, foraging activity generates a slow drift in location, while directed travel produces greater step displacements. Some fuzziness in activity state assignments is inevitable because of shifts in activity taking place during the intervals between successive GPS locations. Nevertheless, spells of resting can minimally be distinguished from more mobile activities [START_REF] Patin | Identifying stationary phases in multivariate time-series for highlighting behavioural modes and home range settlements[END_REF], and directed travel from foraging or various minor activities generating slow movements [START_REF] Owen-Smith | Applying mixture models to derive activity states of large herbivores from movement rates inferred using GPS telemetry[END_REF].

GPS telemetry has been especially useful for revealing nocturnal activity patterns. In northern Kruger National Park, buffalo exhibited similar levels of activity through the day and night, while zebra and sable antelope (Hippotragus niger)

were less active at night than during the day (Owen-Smith & Goodall 2014; Fig. 2). Collared buffalos in Zimbabwe showed a similar activity pattern to the Kruger National Park animals [START_REF] Valls-Fox | Water and cattle shape habitat selection by wild herbivores at the edge of a protected area[END_REF]). In the Schwelle region of Botswana, gemsbok (Oryx gazella), but not wildebeest, became proportionately more active at night during the hot dry season than during the cooler months [START_REF] Boyers | How free-ranging ungulates with differing water dependencies cope with seasonal variation in temperature and aridity[END_REF]). These ungulates were all least mobile pre-dawn, indicating that this is the time when large herbivores sleep. Travelling becomes most prominent around dawn and dusk. Foraging activity typically shows peaks during the early morning and late afternoon, although some foraging evidently continues during the night [START_REF] Owen-Smith | Coping with savanna seasonality: comparative daily activity patterns of African ungulates as revealed by GPS telemetry[END_REF][START_REF] Owen-Smith | Space use patterns of a large mammalian herbivore distinguished by activity state: fear versus food?[END_REF][START_REF] Selebatso | Adaptive activity patterns of a blue wildebeest population to environmental variability in fragmented, semi-arid Kalahari, Botswana[END_REF]. In Kruger National Park, zebra devoted more time overall to foraging than did sable antelope or buffalo, in accordance with their non-ruminant digestion (Owen-Smith & Goodall 2014). Sable antelope, but not zebra, foraged for longer per day towards the end of the dry season when food became sparse. In Mapungubwe National Park in northern South Africa, high ambient temperatures depressed the diurnal activity of wildebeest, eland and impala (Aepyceros melampus), particularly during summer (Shresta et al. 2014).

Gaps in knowledge

Studies inferring activity patterns from movements provide a baseline for comparisons among species, populations and conditions. More investigations are needed on how different ungulate species respond in their movements and activities to diel and seasonal variation in resources, thermal conditions, surface water availability and the threat of predation. How restricted is the time allocated to foraging activity while animals are migrating, or wandering nomadically in search of food, water or shade? Inferred activity states can be bolstered by accelerometers placed on or in animals to document fine-scale body movements [START_REF] Tatler | High accuracy at low frequency: detailed behavioural classification from accelerometer data[END_REF]. Thermal loads influencing activity can be recorded using miniature black bulb thermometers attached to GPS collars, which combine the effects of radiant heat, ambient temperature and convective cooling [START_REF] Hetem | Validation of a biometric technique, using ambulatory miniature black bulb thermometers to quantify thermoregulatory behaviour in ungulates[END_REF]. Distinctions in the conditions selected for specific activities need to be established.

How do herbivore species use space within home ranges?

Spatial utilization patterns have widely been assessed in terms of within loosely bounded areas called home ranges [START_REF] Spencer | Home ranges and the value of spatial information[END_REF]. Kernel density estimators reveal relative intensities of use within home range limits. However, the places occupied for different activities are likely to vary, and change over the seasonal cycle, as influenced by patterns or resource renewal and depletion. Travelling to and from water may take animals well beyond the bounds of their usual home ranges. Over longer periods, animals adjust their space use in response to habitat changes, predator appearances or human intrusions.

What we know

Home ranges documented by GPS tracking overcome the sampling limitations of data obtained by VHF telemetry, being free of the constraints associated with human operators. They have revealed wide variation in how concentrated or expansive home range utilization can be among grazers with overlapping home ranges. Wildebeest herds in Kruger National Park concentrated their activities within open areas covering only a few square kilometres while sufficient grass remained. Zebra family units moved over a larger set of more diffuse patches, not only in Kruger National Park (Owen-Smith & Martin 2015, Owen-Smith et al. 2015), but also in the Makgadikgadi Pan region of Botswana [START_REF] Brooks | Directed movement and orientation across a large natural landscape by zebras[END_REF] and Hwange National Park in Zimbabwe (Chamaillé-Jammes unpublished). Similarly to wildebeest, sable antelope herds in Kruger National Park confined their foraging to fairly small patches, but favoured quite different habitat with taller grass and denser woody plant cover (Macandza et al. 2012a). The huge herds formed by buffalo move systematically over large home ranges, mostly separate from those of other buffalo herds (Owen-Smith, unpublished).

Although the home ranges occupied by sable antelope herds overlapped spatially with those of buffalo herds, the sable restricted competitive overlap by avoiding areas recently grazed by the buffalo (Macandza et al. 2012b).

Gaps in knowledge

A reassessment of home range concepts is needed in the light of GPS data (Fiberg & Borger 2012). For sedentary herbivores, more attention needs to be given to gaps representing unutilized patches within the home range extent, as well as areas not occupied by any home range. These gaps become evident only after GPS tracking has spanned a sufficient period. They need to be related to habitat features. For example, places no longer supporting sable antelope herds showed no differences in tree cover as revealed by LiDAR imagery from areas within occupied home ranges [START_REF] Asner | Habitat differences do not explain population declines of sable antelope in African savanna[END_REF], suggesting that features of the grass cover must be more influential . Conditions leading animals to wander beyond their usual home ranges need to be identified, because opportunities for wider space use may be critical for survival during extreme conditions. For migratory animals, routes followed and staging areas occupied on route between seasonal home ranges need to be documented and related to prevailing circumstances.

Grazing rotations among sequences of exploited patches contribute to determining the overall home range extent needed by an individual animal or herd. An investigation of such patterns is needed especially for buffalo, because they are the archetypal large herbivores guiding rotational grazing principles for domestic livestock [START_REF] Fynn | Functional resource heterogeneity increase livestock and rangeland productivity[END_REF]. Why do buffalo herds seldom over-graze, while domestic cattle commonly do so?

How water-dependent are herbivores?

Herbivore species differ in their physiological capacity to extract moisture during passage of food residues through the large intestine [START_REF] Woodall | Dimensions of the intestine, diet and faecal water loss in some African antelope[END_REF] and in their ability to concentrate urine [START_REF] Maloiy | Physiology of the dik-dik antelope[END_REF]). This influences their ability to survive without drinking water during the dry season. The frequency with which large grazers need to drink can form a major constraint on their spatial distribution and movements during dry times of the year [START_REF] Gaylard | Surface water availability: Implications for heterogeneity and ecosystem processes[END_REF].

What we know

GPS tracking enables intervals between successive visits to water as well as ranging distances from water to be established. Sable antelope in northern Kruger National Park undertook journeys to pools remaining in a seasonal river every 3-4 days during the dry season, while zebra drank daily or every second day [START_REF] Cain | The costs of drinking: Comparative water dependency of sable antelope and zebra[END_REF]. The one-way distance travelled to water by sable antelope averaged 5-9 km, depending on the circumstances, while individual zebra herds travelled distances averaging from 3 to 8 km. These journeys were undertaken during the latter part of the morning, and animals frequently remained in the vicinity of the water until conditions became cooler in the afternoon before embarking on the return journey back to their foraging areas. Thus the time lost for other activities in order to satisfy water requirements was greater than the travel time alone. Time available for foraging or resting was reduced by ~ 8 hours for sable antelope and ~5 hours for zebra on the days when journeys to water occurred. In Hwange National Park, zebras similarly visited waterholes almost daily (Chamaillé-Jammes, unpublished data). In the Makgadikgadi Pan region of Botswana, wildebeest drank only every 2-4 days and grazed 10-15 km from a river [START_REF] Curtin | Remarkable muscles, remarkable locomotion in desert-dwelling wildebeest[END_REF], and zebra went as long as 3-4 days between visits to water [START_REF] Brooks | Directed movement and orientation across a large natural landscape by zebras[END_REF]. The basis for this unusual tolerance remains unclear.

Buffalo herds in Hwange National Park remained close to water through the dry season, and sometimes drank twice per day both in the early morning and at dusk [START_REF] Valls-Fox | Water and cattle shape habitat selection by wild herbivores at the edge of a protected area[END_REF]. At the other extreme, gemsbok in the Kalahari region fitted with GPS collars rarely visited places where water was available, except during an extremely dry year [START_REF] Boyers | How free-ranging ungulates with differing water dependencies cope with seasonal variation in temperature and aridity[END_REF].

Gaps in knowledge

Much more information is needed to define the specific drinking requirements of various large herbivores under different conditions. Some browsers may go for prolonged periods without drinking, provided they can access sufficient green leaves. But under extremely dry conditions, even gemsbok and many browsers appear at waterholes. What are the thresholds in green forage availability controlling surface water dependence, and how do these differ among ungulate species? Further information is needed on how water quality influences the use of particular water sources.

The relative use of particular water sources also needs to be considered, comparing drinking at artificially maintained pools or troughs with visits to large dams and rivers. Wildlife managers frequently install additional water points to draw wildlife into unoccupied areas, but these seem to benefit the more common, water-dependent grazers (like zebra and buffalo) at the expense of less common species (like roan (Hippotragus equinus) and sable antelope: [START_REF] Harrington | Establishing the causes of the roan antelope decline in the Kruger National Park, South Africa[END_REF], Owen-Smith & Mills 2006). More information is needed on how far animals move between foraging sites and places where they drink in order to provide guidance on the spacing of water sources (Owen-Smith 1996).

How pervasive is the landscape of fear?

It has been proposed that the 'landscape of fear' generated by the presence of predators strongly affects movements and habitat occupation by large herbivores [START_REF] Laundre | The landscape of fear: Ecological implications of being afraid[END_REF]. Herbivores may reduce the risk of predation proactively, by occupying more secure habitats, being vigilant and by restricting their movements at times when carnivores are most active (Owen-Smith 2019). Space use patterns may be governed by habitat features that enhance the ability of animals to detect and escape from predators. Trade-offs between food acquisition and risks of predation are likely to exist between day and night, depending on whether the major predators hunt diurnally or nocturnally.

Diminishing food resources during the dry season may force animals to forage in more risky habitats [START_REF] Sinclair | Population consequences of predation-sensitive foraging: The Serengeti wildebeest[END_REF]. Prey responses may differ depending on whether the predators hunts by stalking or ambush versus prolonged pursuits [START_REF] Thaker | Minimizing predation risk in a landscape of multiple predators: effects on the spatial distribution of African ungulates[END_REF]. Exposure to predation based on the frequency of encounters needs to be supported by assessments of the vulnerability of the prey to being killed contingent on the habitat conditions and time of day [START_REF] Hoffmann | How diverse large herbivores coexist with multiple large carnivores in African savanna ecosystems: demographic, temporal and spatial influences on prey vulnerability[END_REF].

What we know

In Serengeti, collared wildebeest showed little response to risks associated with cover for stalking lions, perhaps gaining security from their aggregation in enormous herds [START_REF] Hopcraft | Competition, predation, and migration: individual choice patterns of Serengeti migrants captured by hierarchical models[END_REF]. Zebra moving in smaller herd units on the short-grass plains avoided situations where predation risk was high, but showed less response to spatial variation in risk while in woodlands. In central Kenya, zebra herds tended to occupy more densely wooded areas at night, despite the presence of lions [START_REF] Fischhoff | Habitat use and movements of plains zebra in response to predation danger from lions[END_REF]). In contrast, in Hwange National Park zebras remained in open localities during the night [START_REF] Courbin | Reactive responses of zebras to lion encounters shape their predator-prey space game at large scale[END_REF][START_REF] Davidson | Environmental determinants of habitat and kill site selection in a large carnivore: scale matters[END_REF], and moved away from the waterholes where lions hunted [START_REF] Courbin | Zebra diel migrations reduce encounter risk with lions at night[END_REF]. In Kruger National Park, wildebeest confined their movements within the central regions of their diurnal foraging ranges during night, away from the edges where greater tree and grass cover could hide stalking lions (Owen-Smith & Traill 2017). In the Central Kalahari region of Botswana, collared wildebeest herds similarly remained in pan habitats with clear visibility during the night [START_REF] Selebatso | Adaptive activity patterns of a blue wildebeest population to environmental variability in fragmented, semi-arid Kalahari, Botswana[END_REF]. Impalas shifted into more open areas at night from the more densely wooded areas that they occupy during the day [START_REF] Riotte-Lambert | Periodicity analysis of movement recursions[END_REF].

Despite being highly apparent in the short grass lawns that they inhabited, wildebeest herds in west-central Kruger National Park did not encounter lions (Panthera leo) more frequently than did the more elusive zebra herds [START_REF] Martin | Habitat selectivity influences the reactive responses of African ungulates to encounters with lions[END_REF]. Encounters with lions closer than 1 km occurred at mean intervals of 9 days for zebra and 7-8 days for wildebeest. Following encounters, wildebeest herds returned to the open patches where they typically resided, while zebra herds moved away from the vicinity of the contacts. In Hwange National Park, contacts between collared zebra herds and lions occurred at mean intervals of 35 days [START_REF] Courbin | Reactive responses of zebras to lion encounters shape their predator-prey space game at large scale[END_REF]. Most of the encounters took place <1 km from water, and zebras tended to flee into more open areas several kilometres away following encounters [START_REF] Courbin | Reactive responses of zebras to lion encounters shape their predator-prey space game at large scale[END_REF].

Gaps in knowledge

GPS telemetry opens new opportunities for documenting the topology of landscapes of fear based on the movement of predators, as well as how potential prey respond to the spatial threat. However, few studies in Africa have as yet deployed GPS collars on both predators and prey in the same region. The problem to be surmounted is dealing with multiple predators and prey species in the same area. Lions are the main threat for ungulates weighing over 100 kg, but lions prey on a diverse suite of ungulates [START_REF] Owen-Smith | Predator-prey size relationships in an African large-mammal food-web[END_REF]. Comparisons among species are also critical for understanding the non-consumptive effects of predation [START_REF] Chamaillé-Jammes | Predator Effects on Herbivore Dynamics and Behavior. What Mechanisms Lead to Trophic Cascades in Savannas?[END_REF][START_REF] Say-Sallaz | Non-consumptive effects of predation in large terrestrial mammals: Mapping our knowledge and revealing the tip of the iceberg[END_REF].

Establishing the true frequency of encounters between predators and prey requires locations recorded more frequently than hourly, because much ground can be covered by both predator and prey during the dynamics of the interaction. Accelerometers could help document the specific movements of both interactors during a hunt, successful or otherwise [START_REF] Wilson | Biomechanics of predator-prey arms race in lion, zebra, cheetah and impala[END_REF][START_REF] Hetem | Body temperature, activity patterns, and hunting in free-living cheetah: biologging reveals new insights[END_REF]. Kills can be inferred from subsequent lack of movement by the predator provided that the carcass takes several hours to consume [START_REF] Davidson | Environmental determinants of habitat and kill site selection in a large carnivore: scale matters[END_REF]. This would not be meaningful for spotted hyenas (Crocuta crocuta) or African wild dogs (Lycaon pictus), which typically devour carcasses within a few minutes.

How do mobile herbivores transmit pathogens?

The potential for wild ungulates to infect domestic livestock with pathogens depends on direct or indirect contacts affecting pathogen transmission. Buffalo are centrally involved as carriers for bovine tuberculosis as well as foot-and-mouth disease, both transmittable to cattle [START_REF] Bengis | Wildlife diseases and veterinary controls[END_REF]. The fence along the western border of Kruger National Park was constructed mainly to restrict contacts between buffalo and cattle. In Botswana, veterinary cordon fences cross the country to separate wildlife areas from those given over to cattle ranching, largely to enable beef exports to Europe. Restrictions on wildlife movements can also increase the likelihood of infections becoming endemic within populations.

What we know

GPS telemetry has revealed that female buffalo dispersing between herds could spread pathogens over distances up to 100 km [START_REF] Caron | African buffalo movement and zoonotic disease risk across transfrontier conservation areas, southern Africa[END_REF][START_REF] Spaan | Why did the buffalo cross the park? Resource shortages but not infections drive dispersal in female African buffalo[END_REF]. In Zimbabwe, contacts between buffalo and cattle take place mainly at waterholes on the periphery of protected areas during the dry season, where foot-and-mouth disease can potentially be transmitted to the livestock (Miguel et al. 2013). The mechanism for infection seems to be via the persistence of the virus in the soil rather than direct contacts between the wild and domestic ungulates [START_REF] Miguel | Driers of foot-and-mouth disease in cattle at wild/domestic interface: insights from farmers, buffalo and lions[END_REF]. In Etosha National Park in Zimbabwe, migratory zebra escaped infections with anthrax because the bacillus did not persist in the soil where these hosts were present only seasonally in the dry months [START_REF] Zidon | Zebra migration strategies and anthrax in Etosha National Park, Namibia[END_REF]. Zebra that remained in the wet season range year-round were subject to continuing mortality from the disease.

Gaps in knowledge

Opportunities for investigating how wild herbivore movements affect pathogen transmission and contagion using GPS tags remain largely unexplored. Fences are being expanded throughout Africa without much understanding of how they will affect disease ecology. Fencing has wide implications for the establishment of Transfrontier Conservation Areas, which anticipate communities of people with cattle and other domestic livestock living alongside protected areas with 20 abundant wildlife. Fences help by blocking incursions of livestock into protected areas, but at the cost of restricting wider movements by the wild herbivores. This is a blank area requiring further studies.

How socially persistent are herds?

Herd formation is a proactive response to the risk of predation, increasing vigilance and diluting the risk [START_REF] Jarman | The social organization of antelope in relation to their ecology[END_REF], as well as facilitating social interactions. Many African ungulates form quite small but cohesive herds that move as a unit within a shared home range; for example greater kudu (Tragelaphus strepsiceros; Owen-Smith 2013), sable antelope [START_REF] Estes | The Behavior Guide to African Mammals[END_REF][START_REF] Hensman | Home range occupation and habitat use of sable antelope in the Okavango Delta region of northern Botswana[END_REF]) and plains zebra [START_REF] Klingel | Soziale organisation und Verhalten freilebender Steppenzebras[END_REF], and even sedentary wildebeest [START_REF] Owen-Smith | Space use patterns of a large mammalian herbivore distinguished by activity state: fear versus food?[END_REF] . Herd members may, however, split and re-join over time. Female impala inhabit a common home range without necessarily moving together (Murrray 1982). Most male ungulates are associated only temporarily with breeding herds of females and young [START_REF] Choe | Age, size, dominance and reproduction among male kudus: mating enhancement by attrition of rivals[END_REF]. Herd members share a common pool of memory guiding their movements [START_REF] Riotte-Lambert | How memory-based movement leads to non-territorial spatial segregation[END_REF], supported by cultural transmission across generations [START_REF] Jesmer | Is ungulate migration culturally transmitted? Evidence of social learning from translocated animals[END_REF]).

What we know

GPS telemetry has revealed how ungulate herds can be open to transfers of animals between herd ranges as well as showing splitting and re-joining among herd members, most notably for buffalo [START_REF] Winnie | Habitat quality and heterogeneity influence distribution and behaviour in African buffalo[END_REF], Caron et al. 2016, Bennitt et al. 2018, Wielgus et al. submitted). Although the home ranges occupied by buffalo herds in Kruger National Park are largely discrete, individual females may disperse between the distinct herds [START_REF] Spaan | Why did the buffalo cross the park? Resource shortages but not infections drive dispersal in female African buffalo[END_REF]. Dispersal movements tend to occur during times of resource stress such as the transition from the late dry season into the early wet season.

Gaps in knowledge

Much more research is needed to establish how cohesive the herds formed by different ungulate species are, factors influencing their spacing and conditions promoting dispersal movements between herds. Multiple members of the same herds need to be tagged with GPS devices, or have proximity monitors placed, in order to establish relative frequencies of association among herd members [START_REF] Torney | From single steps to mass migration: the problem of scale in the movement ecology of the Serengeti wildebeest[END_REF].

Findings that sedentary ungulate herds comprising females and young may occupy largely discrete home ranges without overt defence need to be explored further, and underlying mechanisms (such as memory-driven foraging:

Riotte-Lambert et al. 2015) investigated. Studies are also needed to show how independently males move relative to herds of females plus young. Additional devices such as animal-borne microphones [START_REF] Wijers | Listening to lions: Animal-borne acoustic sensors improve bio-logger calibration and behaviour classification performance[END_REF], cameras [START_REF] Ossi | Biologging and Remote-Sensing of Behavior[END_REF] or proximity tags [START_REF] Ossi | Understanding and geo-referencing animal contacts: proximity sensor networks integrated with GPS-based telemetry[END_REF]) could help interpret social contexts. Long-term observations spanning multiple years are required to establish how persistent herd home ranges are.

What limits population abundance?

Animal movements and associations contributed towards determining locally effective population densities from the extent of the area actually occupied and the number of animals sharing this range [START_REF] Borger | Stuck in motion? Reconnecting questions and tools in movement ecology[END_REF][START_REF] Riotte-Lambert | Spatial memory shapes density dependence and population dynamics[END_REF]. Populations can grow in density both through increases in herd size and through expansions or contractions in the ranges covered or occupied by these herds [START_REF] Chirima | Recent distributions of larger ungulates in the Kruger National Park from ecological aerial survey data[END_REF]. The kinds of habitats they favour play a major role in governing local concentrations and gaps in use. When habitats deteriorate, animals must move over wider areas to secure their needs, perhaps leaving larger unutilized gaps within or between occupied ranges. Thus animal movements represent small-scale processes governing changes in population densities and distribution patterns, apart from the demography of births and deaths.

What we know

Spatial utilization patterns can help explain the huge contrast in regional abundance between wildebeest in Serengeti and wildebeest inhabiting Kruger National Park. The population density attained by migratory wildebeest in Serengeti (>50 individuals per km 2 ) is more than 100 times the regional density exhibited by wildebeest in Kruger National Park (<0.5 individuals per km 2 ). Nevertheless, herds of 15-35 wildebeest in Kruger National Park remain within areas of 1.5-3 km 2 for 6-10 months of the year [START_REF] Yiu | Restricted habitat use by an African savanna herbivore through the seasonal cycle: key resources concept expanded[END_REF], so that their locally effective density is almost 10 individuals per km 2 ., much closer to the pressure on food resources within Serengeti. This suggests that the extent of sufficiently open habitat conferring security from predation, rather than solely contrasts in the nutritional value of food resources, contributes to governing the contrast in population abundance [START_REF] Owen-Smith | Space use patterns of a large mammalian herbivore distinguished by activity state: fear versus food?[END_REF][START_REF]Ramifying effects of the risk of predation on African multi-predator, multi-prey large-mammal assemblages and the conservation implications[END_REF].

Gaps in knowledge

Situations where ungulate herds occupy largely discrete home ranges open opportunities for investigating further how space utilization affects local population densities, and hence pressures on resources, in the context of predation and competition. The mechanisms enabling migratory populations to vastly exceed the abundance levels attained by resident populations also need to be established. Does this contrast arise from the effect of better nutrition in the wet season range on reproductive success? Might the reduced grazing pressure in the expanded dry season range support survival through this critical period of the year? Or is it a result of the alleviation of predation pressure as a consequence of population shifts beyond the ranges of settled predators?

OVERVIEW

Much is being learnt about the movement patterns of certain large herbivores inhabiting African savannas from GPS telemetry, covering many aspects of ecology, as has been documented above. But studies have been restricted both in geographic coverage and in duration, and limited mainly to just three species. They inadequately represent the wide variation in movement patterns shown by different ungulate species, and by populations of the same species. Space occupation patterns range from settlement within small home ranges covering a few square kilometres to migratory shifts over thousands of square kilometres. Wide contrasts exist among species and local populations, and even within subpopulations. Space occupation patterns are contingent on environmental conditions and largely determine the regional and local density levels attained. There are suggestive indications that niche separation among these species is enabled largely by the spatial distinctions in their movements, related to security from predation as well as their food and water requirements.

How well these species cope with the strictures imposed by climate change and expanding human settlements with associated fencing and roads depends largely on how their movements are constrained. The crucial circumstances arise during extreme droughts or conditions where rains are unusually late, prompting animals to wander beyond their usual home ranges. If wider movements are blocked by fences or settlements, populations can crash dramatically.

Baseline documentation of movement patterns by large mammalian herbivories is urgently needed before the amplifications of global change restrict their movements even further. It is generally anticipated that species will show range shifts along temperature gradients in response to climate change [START_REF] Parmesan | A globally coherent fingerprint of climate change impacts across natural systems[END_REF][START_REF] Walther | Ecological responses to recent climate change[END_REF].

However, for savanna herbivores food production is controlled largely by rainfall, and forage quality depends on soil fertility. Exposure to predation is governed largely by the woody vegetation cover. Thus changes in habitat suitability independent of bioclimatic regimes need to be taken into account in projecting likely movement responses. A particularly insidious threat to savanna grazers lies in the bush thickening fuelled by elevated carbon dioxide levels in the atmosphere [START_REF] Buitenwerf | Increase tree densities in South African savannas: >50 years of data suggests CO 2 as the driver[END_REF].

Large herbivores inhabiting African savanna environments pose an especially great challenge to research because of the diverse range of species represented and the influences of unpredictable rainfall on prevailing environmental conditions. Funding restricts the deployment of GPS collars to represent this heterogeneity, and sharing data is sorely needed to overcome this limitation in individual studies. A start has been made through the formation of the "AfriMove" network (http//afrimove.org), drawing from the Eurodeer/Euromammals experience [START_REF] Cagnacci | Partial migration in roe deer: migratory and residency tactics are end points of a behavioural gradient determined by ecological factors[END_REF].

Once problems of restricted data availability have been overcome, the challenges of how best to analyse and interpret the data remain to be resolved. Seasonal movements tend to be less regular and more opportunistic when controlled by rainfall than in the temperature-driven seasons typical of higher northern latitudes. Daily activity routines get constrained within diel cycles of temperature and illumination. Places where food is secured during early morning and late afternoon foraging sessions may differ from the localities where animals are most secure from nocturnally active predators or effectively shaded from midday temperature extremes. Access to surface water may drive animals beyond their favoured habitats during dry season conditions. For grazers, habitat suitability depends both on the composition and structure of the grass layer providing food and on features of the woody vegetation cover affecting securing from predation. Prevailing fire regimes modify both.

Without more informed knowledge of the movement patterns of large herbivores in response to rapidly changing environments, we are likely to see a progressive loss of this legacy, as protected areas lose their effectiveness under shifts in climate, atmospheric CO 2 , and human activities plus infrastructure [START_REF] Veldhuis | The Serengeti Squeeze: crossboundary human impacts compromise an iconic protected ecosystem[END_REF]. A far-sighted perspective is needed, conserving wildlife populations not only as a cultural heritage, but also as an essential resource for sustainable food production in increasingly hot and arid savannas. Africa's large mammal diversity constitutes both

FIGURESFigure 1

 1 FIGURES

an unrivalled World Heritage and a valuable future resource, needing adequate resources marshalled internationally to ensure its persistence. Space use patterns documented in great detail by GPS telemetry are central to this endeavour.

ACKNOWLEDGEMENTS

This review was stimulated by a workshop held at the Wits Rural Facility in March 2018 to share data and collaborate in writing publications based on GPS data gathered for African large mammals. We acknowledge helpful suggestions for the manuscript provided by Melinda Boyers, Francesca Cagnacci, Victoria Goodall, Thomas Mueller, Francesca Parrini, Moses Selebatso and Maartin Strauss, and from the external reviewers.

Figure 2 Translating hourly movement rates into activity states for wildebeest, zebra and buffalo herds in Kruger National Park fitted with GPS collars by fitting independent mixture models. For zebra and buffalo, activity has been apportioned among the three basic activities. For wildebeest, mixed movement has been retained as a distinctive state.


Alexandre Bachelet

Marion Chabot

Abduleziz Ablat

Kazuo Takimiya

Lionel Hirsch

Mamatimin Abbas
email: mamatimin.abbas@ims-bordeaux.fr

Low Voltage Operating Organic Light Emitting Transistors with efficient charge blocking layer

Keywords: organic light emitting transistor, low voltage operation, hole blocking layer

published or not. The documents may come

Low voltage operating organic light emitting transistors with efficient charge blocking layer

Electronic devices based on organic semiconductors (OSC) have been well studied over the last decades as they enable specific applications those of which their inorganic counterparts cannot fulfil. [START_REF] Forrest | The path to ubiquitous and low-cost organic electronic appliances on plastic[END_REF][START_REF] Khan | A New Frontier of Printed Electronics: Flexible Hybrid Electronics[END_REF] These devices can be easily processed with potential low cost. They can be large area, light weight and flexible. [3][START_REF] Kaltenbrunner | Ultrathin and lightweight organic solar cells with high flexibility[END_REF][START_REF] Yin | Highly Flexible Fabric-Based Organic Light-Emitting Devices for Conformal Wearable Displays[END_REF] An interesting and aspiring device in this field is organic light emitting transistor (OLET), a multifunctional optoelectronic device. It combines the electrical switching function of a field effect transistor (FET) with the light emission property of a light emitting diode (LED). [START_REF] Muccini | A bright future for organic fi eld-effect transistors[END_REF][START_REF] Chaudhry | Organic Light-Emitting Transistors: Advances and Perspectives[END_REF] The potential applications of such a device are numerous, from active matrix display to even electrically pumped lasing. [START_REF] Mccarthy | Low-voltage, low-power, organic light-emitting transistors for active matrix displays[END_REF][START_REF] Kuehne | Organic Lasers : Recent Developments on Materials , Device Geometries , and Fabrication Techniques[END_REF] Since the first report on OLET, there has been enormous progress in this field. [START_REF] Hepp | Light-emitting field-effect transistor based on a tetracene thin film[END_REF][START_REF] Gwinner | Highly efficient single-layer polymer ambipolar light-emitting field-effect transistors[END_REF][START_REF] Sobus | High Performance p-and n-Type Light-Emitting Field-Effect Transistors Employing Thermally Activated Delayed Fluorescence[END_REF][START_REF] Liu | Organic Light-Emitting Field-Effect Transistors: Device Geometries and Fabrication Techniques[END_REF][START_REF] Yuan | Design of High-Performance Organic Light-Emitting Transistors[END_REF] To further enhance the performance of these devices, incorporation of interfacial layers such as charge injection and blocking layers is essential, as they play rather important role in device performances of organic electronic devices being almost indispensable in some cases, such as in organic LEDs. [START_REF] Fahlman | Interfaces in organic electronics[END_REF] However, one limitation thus far has been the rather high operating voltages of OLETs in many reported studies (even above 100 V in some cases). [START_REF] Ullah | UV-Deep Blue-Visible Light-Emitting Organic Field Effect Transistors with High Charge Carrier Mobilities[END_REF][START_REF] Hsu | Ordered polymer nanofibers enhance output brightness in bilayer light-emitting fieldeffect transistors[END_REF][START_REF] Maruyama | Ambipolar light-emitting organic single-crystal transistors with a grating resonator[END_REF] Such an operating condition limits the potential areas of their application (displays require below 10 V operations) and restrains the efficiency and stability of these devices under strong electrical stress, but also obscures the role of charge injection/blocking layers. That is why developing low voltage operating OLETs can be regarded as a major focus in research towards technological applications. Two aspects have to be considered to realize low voltage operating OLETs. One is to apply high mobility transport layers, such as metal oxides, small molecules or polymers. [START_REF] Kyndiah | A Multifunctional Interlayer for Solution Processed High Performance Indium Oxide Transistors[END_REF][19][START_REF] Chen | Dithiopheneindenofluorene (TIF) Semiconducting Polymers with Very High Mobility in Field-Effect Transistors[END_REF] Another is that the capacitance of the dielectric layer should be augmented, either by minimizing the thickness or using high-k dielectrics. [START_REF] Houin | Device Engineering for High Performance, Low Voltage Operating Organic Field Effect Transistor on Plastic Substrate[END_REF][START_REF] Ortiz | High-k Organic , Inorganic , and Hybrid Dielectrics for Low-Voltage Organic Field-Effect Transistors[END_REF] Up to now, only one work reported OLET operating below 10 V. [START_REF] Chaudhry | Low-Voltage Solution-Processed Hybrid Light-Emitting Transistors[END_REF] Low voltage operating devices provide ideal platform to explore the potential of charge injection/blocking layers in further enhancing the electrical and optical performances of OLETs. However, incorporation of them are lacking among those very few OLET devices operating at relatively low voltages. [START_REF] Chaudhry | Low-Voltage Solution-Processed Hybrid Light-Emitting Transistors[END_REF][START_REF] Soldano | Highly Efficient Red Organic Light-Emitting Transistors (OLETs) on High-k Dielectric[END_REF] In this work, we first realized low voltage operating OLETs using high hole mobility 2,9-didecylnaphtho[2,3b:2',3'-f]thieno [3,2-b]thiophene (C10-DNTT) as transport layer and anodized Al2O3 as dielectric layer. 26,[START_REF] Abbas | Water soluble poly(1-vinyl-1,2,4-triazole) as novel dielectric layer for organic field effect transistors[END_REF] Operating voltage was reduced to as low as 5 V, which allowed us to unambiguously investigate the effect of hole blocking layers in our OLETs. Two molecules: 2,2',2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) were used with the aim to decouple the efficacy of hole blocking from electron injection and transport. We obtained significantly improved optical performance with BCP which proved to be efficient hole blocking/electron injecting layer.

The 15 mm × 15 mm glass substrates were cleaned with Hellmanex soap in ultrapure water for 15 minutes in an ultrasonic bath, then were rinsed in pure water for 5 minutes two times. They were then subjected to 10 minutes of UV-O3 treatment. Aluminum (100 nm) as gate was evaporated in an e-beam deposition system at the base pressure of 1×10 -6 mbar. The aluminum was partially anodized in a citric acid aqueous solution (0.1 mM) to get 35 nm thick Al2O3. A thin polystyrene (3 mg mL -1 in chlorobenzene) layer was spin coated (2000 rpm, 60s) on top to passivate the surface. The transport layer, C10-DNTT (20nm), was evaporated in a thermal evaporator at the base pressure of 1×10 -6 mbar with a controlled rate of 0.1 to 0.2 Å s -1 . MoO3 (10nm)/Ag (40nm) source electrode was evaporated through a shadow mask at a deposition rate of 0.1 nm s -1 . The emissive layer, Alq3 (50 nm), was evaporated thermally at a rate of 0.5 Å s -1 . Finally, the semitransparent drain electrode (LiF (1nm)/Al (15nm)) was evaporated through a shadow mask. Thus defined channel length was 300 µm and channel width was 2 mm. For BCP (10 nm) and TPBi (10 nm) devices, they were evaporated thermally at a rate of 0.1 nm s -1 before the deposition of drain electrode through the same shadow mask. The characterizations were carried out under inert atmosphere (nitrogen glovebox). A 4200 Keithley Semi-conductor analyzer along with a calibrated silicon photodetector on top of the device were used to measure the electrical and optical signals. A Hamamatsu amplifier was used to magnify the photocurrent. The brightness of Alq3 based OLED was first measured via a Konica Minolta Luminance meter LS-100. A correction factor was hence determined to derive the brightness of OLETs. Whole (LiF/Al) electrode area is considered as light emitting area to take into account the change in the area of the light emission. The photographs were taken by using a Nikon D-80 camera with a 50 mm lens.

First, we evaluated the OFET performance of C10-DNTT molecule. Al gate deposited on glass substrate was partially anodized to create Al2O3, which served as dielectric layer. [START_REF] Abbas | Water soluble poly(1-vinyl-1,2,4-triazole) as novel dielectric layer for organic field effect transistors[END_REF][START_REF] Dang | Aluminum oxide film as gate dielectric for organic FETs: Anodization and characterization[END_REF] A thin layer of polystyrene was spin coated to passivate the surface. [START_REF] Houin | Device Engineering for High Performance, Low Voltage Operating Organic Field Effect Transistor on Plastic Substrate[END_REF] Subsequently, C10-DNTT was deposited, followed by the deposition of MoO3/Ag source/drain electrodes. [START_REF] Ablat | Role of Oxide / Metal Bilayer Electrodes in Solution Processed Organic Field Effect Transistors[END_REF] We obtained an average saturation hole mobility of 4.3 cm 2 V -1 s -1 , with low threshold voltage and high current on-off ratio (see supporting information). Such an excellent performance makes the choice of C10-DNTT relevant as the transport layer of our OLETs. The OLET device structure is presented in Figure 1a). Here, we used asymmetric contacts for separate hole and electron injection. After the deposition of MoO3/Ag contact for hole injection, Alq3 emitter was thermally evaporated. The second asymmetric contact for electron injection is composed of hole blocking molecule (or not) and LiF/Al semi-transparent electrode. The choice of BCP and TPBi is mainly due to their deep lying highest occupied molecular orbital (HOMO) levels (6.2-6.4 eV), which enable them to block holes coming through the emissive layer in a rather similar fashion. For that, they both commonly act as hole blocking layers in OLEDs. [START_REF] Jou | Carrier modulation layer-enhanced organic light-emitting diodes[END_REF] However, relatively large difference in lowest unoccupied molecular orbital (LUMO) levels (2.7 eV for TPBi and 3.0 eV for BCP) and in electron transport can manifest in their ability to efficiently provide electrons, thus influencing the rate of recombination that creates the excitons by the accumulation of both charges in the emissive layer. [START_REF] Jou | Highly efficient color-temperature tunable organic light-emitting diodes[END_REF] We carried out experiments with different thicknesses and chose the best performing device (10 nm) in terms of current density and luminance for BCP device.

We kept the thickness of TPBi the same for the sake of comparison, since TPBi devices get worse and worse than the reference device with the increase of the thickness.

The electrical and optical characteristics of the three OLET devices (control, with TPBi and with BCP) are shown in Figure 2a) at a drain-source voltage (Vds) of -5 V. All three devices show quite similar transfer curves, with comparable current turn on voltage (Von(I)), threshold voltage (Vth) and maximum drain current (Imax). In contrast, we observed large differences in the optical performances. Maximum luminance of the device with BCP is almost ten times higher than that of the control device, indicating an efficient hole blocking by the BCP layer. However, despite having deeper HOMO level, TPBi device yielded even worse luminance than the control device. If we look at the energy diagram of various layers of our devices (as shown in Figure 2c)), shallow LUMO level of TPBi (2.7 eV) seems to introduce additional energetic barrier for electron injection from LiF/Al contact to Alq3. Apparently, electron injection has stronger effect than hole blocking. Another factor to be considered is the electron mobility of these two HBLs. Earlier reports have shown that BCP has much higher electron mobility than TPBi. [START_REF] Kulkarni | Electron Transport Materials for Organic Light-Emitting Diodes Abhishek[END_REF][START_REF] Rhee | Effect of electron mobility of the electron transport layer on fluorescent organic lightemitting diodes[END_REF] Having almost one order of magnitude higher electron mobility, BCP transports electrons much faster than TPBi, which can be another contributing factor for the better performance of BCP device.

Device performances measured at drain-source voltage of -15 V are presented in Figure 2b). Remarkably, Von(I) and light turn on voltage (Von(L)) almost overlap for all the devices, which is a strong contrast to the solution processed hybrid LETs. [START_REF] Ullah | Hybrid Light-Emitting Transistors Based on Low-Temperature Solution-Processed Metal Oxides and a Charge-Injecting Interlayer[END_REF][START_REF] Ablat | Low optical turn-on voltage in solution processed hybrid light emitting transistor[END_REF] The hole transport layer (C10-DNTT) is the same for all the control, TPBi and BCP devices. In principle they should have similar hole mobilities. Nevertheless, more efficient electron injection can lead to higher recombination rate in the emissive layer, thus further contributing to the current flow. Thus, we obtained a slightly higher mobility for BCP devices when operated at 15 V. Still, the mobility values are comparable to the OFET without the emissive layer. Maximum gate leakage current is around 2 nA, and several orders magnitude lower than the corresponding drain current (see supporting information). Overall optical performances follow the trend observed earlier for low voltage operation, albeit being less significant. Peculiar feature in the control device where a dip appears around gate voltage of 7.5 V is worthy of discussion. We should note that when the gate voltage increases, the carrier concentration of the holes continues to rise, conversely that of the electrons continues to diminish, as it is controlled by the potential difference between the gate electrode and drain electrode (LiF/Al). The equilibrium point is at half gate voltage, where the switch from electron domination to hole domination occurs. At high gate voltages, electron injection is predominantly through tunnelling, driven by the fixed electric field between source and drain electrode over the channel. When electron injection/transport is poor, as in the case of TPBi devices, the increased amount of hole carriers through efficient blockage are compensated, hindering further accumulation of electron/hole pairs (excitons) in the emissive layer.

When electron injection gets better, as in the case of control device, exciton accumulation improves, leading to still enhanced luminance after the switching point for hole domination. The best case is BCP, where continuous enhancement of exciton accumulation is achieved thanks to both efficiently blocked holes and injected/transported electrons. Such luminance characteristics at high gate voltage region again confirm the importance of electron injection/transport abilities of these HBLs. Brightness higher than 700 cd m -2 is obtained for BCP device operating at -15 V. Output characteristics of the devices are provided in the supporting information. Current efficiencies of the devices are also given in the supporting information. Electrical and optical performance parameters are summarized in Table I. Visual observation of the emitted lights from the three devices operating at -15 V are presented in Figure 3 for the control device, the device with TPBi and the device with BCP (a), b) and c)). These photograph images show that the lights are emitted primarily from LiF/Al electrode edge, which is due the high concentration of the excitons near the channel. We actually noticed that light emission started from the edge, slowly diffusing the area under the electrode with the increase of the gate voltage, revealing exciton distribution profile. It is also evident that BCP device is brighter than control and TPBi devices, in agreement with the luminance measurement data. However, a closer look exposes the differences in the colour of the emitted lights of the three devices. To verify that, we recorded electro-luminance (EL) spectra and compared them with photoluminance (PL) spectrum of Alq3 film, as shown in Figure 3d). The main EL peak at 520 nm of all the three devices corresponds to that of the PL peak of Alq3 film. Interestingly, we observed additional red shifted EL peaks. The position of this optical resonance seems to be related to the distance between Al gate electrode and Al top electrode. With 10 nm more thickness due to the HTLs, the peak shifted from 610 nm in the control device to 660 nm for the devices with BCP and TPBi. Such a phenomenon was not observed in Si/SiO2 high operating voltage based devices. The origin of this optical effect is the subject of our further study.

In summary, we presented Alq3 emissive layer based OLETs operating at as low as 5 V, which were realised by adopting C10-DNTT high mobility transport layer and anodized Al2O3 high capacitance dielectric layer. These devices allowed us to investigate the role of hole blocking molecules in these OLETs. Two molecules, BCP and TPBi, were compared, in order to distinguish the separate mechanisms of hole blocking and electron injection/transport. Baring more suitable energy level for electron injection and higher electron mobility, BCP resulted in much improved optical performance than the control and TPBi devices. On the contrary, TPBi displayed even worse performance than control device. Thus, we identified that in low voltage operating OLETs, merely efficient hole blocking is not sufficient to enhance device performance without decent electron injection/transport. Low operating voltage OLETs with efficient charge blocking layer usher these important optoelectronic devices closer to many potential practical applications, on which our study provides one of the roadmaps.

Fig. 1 .

 1 Fig. 1. a) Device structure of the OLET (PS: polystyrene, HBL: hole blocking layers TPBi or BCP). Chemical structures of b): C10-DNTT, c): BCP and d): TPBi. MoO3/Ag contact is grounded (source) and fixed Vds is applied to the LiF/Al contact (drain) during the characterizations.

Fig. 2 .

 2 Fig. 2. a) Transfer characteristics and luminance of control, TPBi and BCP OLETs operating at Vds=-5V. b) Transfer characteristics and luminance of control, TPBi and BCP OLETs operating at V ds =-15V. c) Energy diagram of various layers with respect to the vacuum level.

  Table I. Summary and comparison of electrical and optical properties of control, TPBi and BCP OLETs atVds of -5 V. The values in the brackets are for the operation at Vds of -15 V.

Acknowledgment. A. Bachelet acknowledges financial support of Université Bordeaux Doctoral Grant and A.

Ablat from the French national PAUSE program.