Why do chimpanzees build nests?

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

www.mona-uk.org

Reference:

Stewart F.A. & Pruetz J.D. (2013). Do Chimpanzee Nests Serve an Anti-Predatory Function?, American journal of primatology, PMID: 23471670

Giant bubbles protect fish from scoliosis

Sea squirts, fish and mammals don't look much alike, but glimpse at their embryos and you probably couldn't tell them apart. Among other similarities, all sport a tube-like structure stretching from head to tail - the notochord - that serves as a backbone, before being replaced by the spine. New research now shows that mysterious bubble-like structures in the notochord are critical to make a straight spine.

In the centre of the notochord there are unusual cells packed with huge fluid-filled vacuoles (a sort of bubble) that press against a fibrous elastic wrapping. This makes the notochord stiff but also flexible, much like the sausage-shaped balloons used to make balloon animals. A tube like this can come handy when you don’t have a bony spine, for instance if you are a wiggly sea squirt larvae swimming away from a hungry predator.

Zebrafish embryo with the notochord marked by a green fluorescent protein.
(Credit: Kathryn Ellis/ Duke University Medical Center) 

Even though the notochord vacuoles were first described decades ago, they have remained a bit of a mystery. Michel Bagnat and colleagues at Duke University Medical Center are interested in understanding how these strange structures form, and what they are for. “The dogma was that the notochord is an evolutionary relic that falls apart and is replaced by the spine […] so there was not much interest,” Bagnat says. In vertebrates such as humans, the notochord eventually turns into the jelly that fills up the space between the spine vertebrae. Because it seemingly ‘disappears’, many scientists had brushed aside the possibility that the notochord might have other important functions besides its well-known roles in early embryogenesis (in making muscle and nerve cells for instance).

Bagnat’s team took advantage of the zebrafish (Danio rerio) photogenic qualities to study the notochord vacuoles. Zebrafish is a small aquarium fish widely used in research because it is transparent and grows fast, so it is possible to image cells and tissues developing live throughout embryogenesis. By making movies of zebrafish embryos, Bagnat and his colleagues Kathryn Ellis and Jennifer Bagwell showed in a new study how the notochord vacuoles start off as small bubbles, then fuse together and finally inflate until they fill up the cell. The researchers then used fluorescent markers to disclose the identity of the vacuoles. They discovered they are related to lysosomes - the cell’s rubbish bins where unwanted molecules and foreign invaders like viruses are destroyed - but have a completely different function. So what do the notochord vacuoles do?

Notochord vacuoles marked with a lysosome-specific protein (green).
(Credit: Kathryn Ellis/ Duke University Medical Center) 

Earlier studies with dissected frog notochords suggested that, as the vacuoles swell up, they somehow force the embryo to elongate, but these experiments had caveats because the right tools simply didn’t exist at the time. Bagnat’s team was now in a good place to test this in living fish. Disrupting the vacuoles (without affecting the rest of the notochord) using genetic tricks produced dwarf embryos, confirming the old study in frogs. However, something unexpected happened to these embryos. “We tried to raise those very short larvae and we saw that they developed kinks in the spine,” Bagnat says. This surprising finding showed that, contrary to what was previously believed, the notochord has a function later in development in making a straight spine. These fish without notochord vacuoles had the equivalent to human scoliosis, a spine deformity of unknown cause that affects over 2% of the adult population. But how do the vacuoles ensure the spine develops straight? “We think it is the internal pressure of the vacuoles that allows the axis of the embryo to remain straight,” Bagnat explains. He suggests that the notochord vacuoles act as an inflatable scaffold to build the spine.

Zebrafish embryos without vacuoles are short and develop kinked spines.
(Credit: Kathryn Ellis/ Duke University Medical Center) 

Richard Adams, a developmental biologist from the University of Cambridge (UK) who works on zebrafish tissue morphogenesis thinks studies like this could help us understand what causes human scoliosis “This study raises the possibility of a new underlying cause for this condition. […] The zebrafish provides a powerful model organism with which to rapidly test the roles of many such molecular pathways.”

In the most severe cases, scoliosis can be painful and debilitating, and patients might need corrective surgery. Scoliosis research is difficult in part because the spine is not readily accessible in mammalian animal models, such as mouse. But because zebrafish are transparent, researchers can follow spine formation in living animals, and see what goes wrong when they get a kink in the spine. And this is exactly what Bagnat’s team plans to do in the future “We think that we should be able to image scoliosis as it happens in the fish.”

Reference:

Ellis K., Bagwell J. & Bagnat M. (2013). Notochord vacuoles are lysosome-related organelles that function in axis and spine morphogenesis, The Journal of Cell Biology, 200 (5) 667-679. DOI: 10.1083/jcb.201212095

I wrote a short article about this study for ScienceNow that includes a movie of the vacuoles forming. You can read it here.