Factors affecting nest height and ground nesting behaviour in Eastern chimpanzees (Pan troglodytes schweinfurthii) of the northern Democratic Republic of the Congo

In order to achieve a better understanding of the factors that might have led our hominin ancestors to transition to a more terrestrial niche, including sleeping on the ground, we have conducted a study on the ground nesting behavior of chimpanzees (Pan troglodytes). Chimpanzees, like all other species of great apes, build nests in which to sleep each night, but little is known about regional differences in their nesting habits. Previously, nesting on the ground was considered typical of gorillas, but rare in most populations of chimpanzees. Using data acquired during our extensive chimpanzee nesting survey conducted between 2004 and 2013 across a > 50 000 km2 region in northern Democratic Republic of the Congo, we report a distinctive ground nesting behaviour of eastern chimpanzees (P. t. schweinfurthii). We have mapped the geographical distribution of ground nesting and compared its frequency at 20 survey areas on both sides of a large river, the Uele. We found that ground nests made up more than 1% of total nests at 15 of the 20 survey regions. For a subset of 16 of these regions, we utilized statistical models to investigate whether forest type and structure, as well as the abundance of carnivores and large herbivores, and the activities of humans impacted the frequency of ground nesting and nest height. We predicted that higher encounter rates of human and dangerous animal signs would be associated with lower rates of ground nesting as well as increased nest height. Overall, 10.4% of the Bili-Uere chimpanzee nests were terrestrial, but the frequency of ground nesting varied extensively between the survey areas (0-29% of nests). The occurrence of ground nests was positively associated with denser forests (p = 0.004), herb patches (p < 0.001), and light gaps (p < 0.001). Light gaps (p < 0.001), herb patches (p = 0.044), and vine tangles (p = 0.016) also had a strong negative effect on nest height. Hunting by humans had a negative effect on the probability of the occurrence of ground nests (p = 0.001) and a positive one on nest height (p = 0.013), with a similar but likely marginal effect of large herbivores on nest height (p = 0.023). In addition, the chimpanzees nested at significantly lower heights with increasing distance from roads and settlements (p < 0.001). Carnivore encounter rates, however, had no significant impact on ground nest frequency or nest height. Our results indicate that ground nesting can no longer be considered a rare and patchily-occurring phenomenon in Pan troglodytes, but is instead a major component of the chimpanzee behavioural repertoire across a considerable fraction of the range of the Eastern subspecies. Our study highlights that neither the large body size of gorillas nor the taming of fire are necessary conditions for hominids to sleep overnight on the ground, even in areas inhabited by multiple species of large carnivore. Human hunting, however, appears to reduce the probability of ground nesting, or eliminate the behavior altogether.