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Identifying ecological gradients at the range edge of a species is an essential step in revealing the underlying mechanisms and constraints that limit the species' geographic range. We aimed to describe the patterns of variation in chimpanzee (Pan troglodytes verus) density and habitat characteristics perpendicular to the northern edge of their range and to investigate potential environmental mechanisms underlying chimpanzee distribution in a savanna–mosaic habitat. We estimated chimpanzee densities at six sites forming a 126-km latitudinal gradient at the biogeographical range edge of the western chimpanzee in the savanna–mosaic habitats of southeastern Senegal. To accompany these data, we used systematically placed vegetation plots to characterize the habitats at each site for habitat heterogeneity, tree density and size, and floral assemblages, among other variables. We found that both biotic and abiotic factors are potential determinants of the chimpanzee range limit in this ecoregion. Specifically, chimpanzee-occupied landscapes at the limit had smaller available floral assemblages less habitat heterogeneity, and contained fewer closed canopy habitats in which chimpanzees could seek refuge from high temperatures than in landscapes farther from the range limit. This pattern was accompanied by a decline in chimpanzee density with increasing proximity to the range limit. Our results provide several indications of the potential limits of food species diversity, thermal refuge, and water availability to the chimpanzee niche and the implications of these limits for chimpanzee biogeography, especially in the face of climate change predictions, as well as for species distributional modeling more generally. [ABSTRACT FROM AUTHOR]

While West African chimpanzees (Pan troglodytes verus) were elevated to Critically Endangered status by the IUCN in 2016 as a result of significant population declines in recent decades, little is known about the population in Senegal. We surveyed the West African chimpanzees outside protected areas in the Kedougou region of southeastern Senegal between November 2014 and July 2015 using recce walks (n = 57, totalling 300 km of pedestrian survey distance) in potential chimpanzee habitats.We recorded direct observations of chimpanzees or indices of chimpanzee activity, such as sleeping nests, footprints and faeces. We accumulated 21 direct contacts with chimpanzees and recorded 3489 chimpanzee nests. We mapped the distribution of chimpanzee sleeping sites and indicators of anthropogenic activity by regional administrative units to facilitate species management and conservation planning in the immediate future. In addition, we identified the habitats and tree species used by chimpanzees to construct their nests in order to explore nesting tree preferences. Chimpanzees used almost 40 tree species in the Kedougou region but 84% of nests were associated with eight tree species, namely Pterocarpus erinaceus, Diospyros mespiliformis, Anogeissus leiocarpus, Hexalobusmonopetalus, Cola cordifolia, Lannea sp., Parkia biglobosa and Piliostigma thonningii. Among these eight nesting tree species, 60%of the corresponding nests were located in three species: P. erinaceus, D. mespiliformis and A. leiocarpus. Chimpanzees nested more often in woodland habitat than in gallery forests, although the latter accounts for only a small percentage of available habitat. This study is the most geographically extensive survey of chimpanzees in Senegal to date, broadening our knowledge of the species' northwestern-most distribution in West Africa. [ABSTRACT FROM AUTHOR]

Objectives: Food scarcity is proposed to be a limitation to chimpanzees at the limits of their range; however, such a constraint has never been investigated in this context. We investigated patterns of δ 13 C and δ 15 N variation along a latitudinal gradient at the northwestern West African chimpanzee (Pan troglodytes verus) range limit with the expectation that isotope ratios of chimpanzees at the range limit will indicate different dietary strategies or higher physiological constraints than chimpanzees further from the edge., Materials and Methods: We measured δ 13 C and δ 15 N values in hair (n = 81) and plant food (n = 342) samples from five chimpanzee communities located along a latitudinal gradient in Southeastern Senegal., Results: We found clear grouping patterns in hair δ 13 C and δ 15 N in the four southern sites compared to the northernmost site. Environmental baseline samples collected from these sites revealed overall higher plant δ 15 N values at the northernmost site, but similar δ 13 C values across sites. By accounting for environmental baseline, Δ 13 C and Δ 15 N values were clustered for all five sites relative to total Pan variation, but indicated a 13 C-enriched diet at the range limit., Discussion: Clustering in Δ 13 C and Δ 15 N values supports that strategic shifting between preferred and fallback foods is a likely ubiquitous but necessary strategy employed by these chimpanzees to cope with their environment, potentially allowing chimpanzees at their limits to avoid periods of starvation. These results also underline the necessity of accounting for local isotopic baseline differences during inter-site comparison., (© 2019 Wiley Periodicals, Inc.)

Adaptations associated with shifting from a predominately forested habitat to a more open environment are considered a crucial step in hominin evolution. Understanding how chimpanzees, one of our closest-living relatives, are exposed to the selection pressures associated with living in a relatively sparse, hot, and dry environment can inform us about the relative importance of potential environmental stressors involved in adaptations to drier environments. We investigated the extent to which chimpanzees living in an extreme savanna habitat experience seasonal variability in either energy balance or thermoregulation (dehydration and heat exposure), as well as whether these potential environmental constraints are taxing to chimpanzee individuals. Specifically, we tested the hypothesis that savanna environments impose seasonally-relevant costs to chimpanzees. To this end, we collected 368 urine samples from one community of chimpanzees at Fongoli, Senegal, and measured c-peptide, creatinine, and cortisol as measures of physiological responses to environmental food, water, and heat constraints, respectively. We then evaluated the influence of climatic and phenological factors on these indicators. Results illustrated significant seasonal variation in all biomarkers, which corresponded to relevant ecological correlates. Furthermore, creatinine but not c-peptide correlated with cortisol levels, suggesting that chimpanzees in this environment endure periods of heat and dehydration stress, but are able to avoid stressful levels of negative energy balance. Using savanna chimpanzees as a referential model, our research lends support to the notion that thermoregulatory challenges were a significant factor in hominin evolution, and suggests these challenges may have overshadowed the challenges of maintaining adequate energetic balance during the expansion of the hominin range from wetter to drier environments., (Copyright © 2018 Elsevier Ltd. All rights reserved.)