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.
Nesting behaviour
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.
 |
| Credit: Miquel Llorente, Mona Foundation. |
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
Reference:
Fascinating! Thank you for this
ReplyDelete