Guest post by Lorraine Docherty
Each day, every wild chimpanzee over the age of weaning builds at least one nest. Why do chimpanzees take time out of their very busy lives to build a nest, sometimes two, every day for most of their lives? New research led by Fiona Steward from the University of Cambridge (UK) shows that shelter construction may have evolved to enable large apes to sleep comfortably while minimising predation risk.
|Female chimp with her young resting on a nest. |
(Credit: Ronan Donovan/ kibalechimpanzees.wordpress.com)
Although chimpanzees have few predators in the wild, and direct evidence of predation on apes is rare, Stewart and colleague Jill Pruetz from the University of Iowa suspected that these low rates of predation could still have a significant impact on chimpanzee populations and behaviours, such as nest building.
In primates, nest building is only displayed in lemurs, lorises, bushbabies, tarsiers and the great apes. Great apes make nests by day or by night, primarily for resting. Nest building is a routine behaviour learned by the young from their mother, and in the case of orangutans and chimpanzees, social influences are essential for the development of successful nest building skills. Chimpanzees and bonobos make their nests by lacing together branches from one or more trees. These nests consist of a mattress, supported on a strong foundation, and lined above with soft leaves and twigs. Nest-counts and faecal analysis at each nest site can be used to estimate great ape population counts and composition.
In a new study published in the American Journal of Primatology, Stewart and Pruetz provide a unique insight into the reasons behind why chimpanzees choose a particular nest site. They compared the nesting behaviour of two communities of wild savannah-living chimpanzees that differ in the presence of predators. Chimpanzees in Issa in western Tanzania live in a predator rich site, but chimpanzees in Fongoli in south-eastern Senegal live in a region relatively free of predators. Potential predators that were directly or indirectly identified in Issa were spotted hyenas, African wild dog, lion and leopard.
Nest site selection
Stewart and Pruetz found that chimpanzees in Issa nest higher and more peripherally within trees than chimpanzees in Fongoli, which supports the hypothesis that nests may function as a refuge to protect against predator attack. David Samson, a primatologist from Indiana University specialised in the evolutionary origin of sleeping platform construction and great ape sleep architectures and function says “Stewart and Pruetz’s observation that predator rich environments are predictive of high, peripherally located sleeping platforms is a very valuable contribution to the field.” The team also provides evidence suggesting that escape route and group size are less important counter-predation strategies than where the chimpanzees choose to sleep within the tree.
Nesting on the ground is not uncommon in wild chimpanzee populations. Stewart and Pruetz show that the chimpanzees of Fongoli more frequently nested on the ground than the chimpanzees living in Issa, which supports recent research by another group. They propose that sleeping on the ground may be more efficient and comfortable and therefore used more frequently by chimpanzees living in habitats with poor predation risk. Samson adds “This interpretation is important to human evolutionary studies because the tree-to-ground transition may have been an important moment in our species’ development towards higher quality sleep and cognition.”
This study shows predation is an important factor for why chimpanzees build nests, but there are, however, other possibilities. For instance, nests could provide insulation against overnight hypothermia, or protect against disease vectors such as mosquitoes, which are less common high up in the trees. Another possibility could be that chimpanzees might need a nest to prevent them from falling out of the tree, not because they have poor balance skills (apes have excellent balance), but because like humans, apes need REM (Repetitive Eye Movement) sleep, which is thought to be important for memory consolidation but causes overall muscle relaxation.
Stewart says “The level of risk of predation likely influences where chimpanzees build their nests, whether terrestrially or in trees but also how peripheral they sleep within trees. However, […] environmental conditions may [also] influence the way in which the chimpanzees build their nests, for example thicker, warmer nests, for thermoregulatory benefits in cold conditions, or more stable support structures during windy conditions”.
Dr. Lorraine Docherty has over ten years experience in the rescue and rehabilitation of chimpanzees. She set-up the charity MONA-UK dedicated to rescue of primates suffering in captivity, and also supports the work of Mona Foundation sanctuary in Spain. Lorraine has a particular interest in chimpanzee welfare in captivity and also works as a consultant advising zoos on implementing species appropriate environmental enrichment programs, and with organisations highlighting primate suffering in captivity. http://chimprescue.wordpress.com