Why do gorillas build new nests every night?

The Wildlife Conservation Society announced the discovery of 125,000 western lowland gorillas in the Congo.
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When was the last time you heard promising news about an animal species? Like a baseball franchise stuck in a slump, the number of animals joining the ranks of endangered species has ballooned in recent decades. Granted, the bald eagle and wolf population recoveries in the United States were celebrated comebacks, but tigers, polar bears, rhinos and many more still teeter on the slippery slope toward extinction.

Great apes -- gorillas, chimpanzees, bonobos and orangutans -- all make that disheartening list of endangered animals. Poaching and deforestation in Africa have contributed greatly to the population decline of gorillas in particular. But the Ebola virus sent the species on a nosedive toward disappearance in 2006. The virus, named for the Ebola River in the Democratic Republic of the Congo, killed an estimated 90 percent of gorillas it infected [source: Ravilious]. One study found that the virus claimed the lives of 5,000 gorillas in one geographical area and possibly 25 percent of the world's gorilla population [source: Ravilious].


There are four gorilla subspecies: eastern lowland gorillas, western lowland gorillas, mountain gorillas and cross-river gorillas. Western lowland gorillas primarily inhabit rainforests in Western and Central African countries including the Congo, Gabon and Central African Republic [source: National Geographic]. Twenty years ago, population surveys indicated that this subspecies was critically endangered with fewer than 100,000 remaining [source: Revkin].

But in August 2008, the Wildlife Conservation Society announced what could be a ninth-inning home run for the great apes. Its survey of the western lowland gorilla subspecies found at least 125,000 living in a densely forested area of the Congo, referred to as the "green abyss" [source: Nielsen]. Few people live in this nearly impassable swampy landscape, insulating the gorilla's habitat, and the Ebola virus has yet to reach the area [source: Nielsen].

Although gorillas are the largest of the great ape species, they can be difficult to track down. They change locations every day in search of food, and the dense foliage they live in doesn't make finding them any easier. So how did the surveyors pinpoint a population number for these apes?

Gorilla Nest Building

Counting abandoned gorilla nests is the most common way that wildlife experts estimate the size of gorilla populations.
Cyril Ruoso/JH Editorial/Getty Images

One of the most common ways that wildlife experts estimate the number of gorillas in an area is to count the number of nests, which are often easier to spot than the actual apes [source: Sanz et al]. Gorilla nest building usually occurs in trees or on the ground, and these apes use branches and leaves as their mattresses. Usually, ground nests are made of herbaceous plants, but the materials vary according to season and what's available [source: Tutin et al]. Generally, they construct their beds more often on the ground than in trees [source: Iwata and Ando]. This nest-building behavior isn't unique to gorillas. In fact, all great apes build nests [source: Iwata and Ando]. Just like your bed at home, ape nests provide comfort, warmth and protection.

However, the handmade lodgings are more like sleeping bags than beds since they're intended for temporary use. Great apes practice nomadic living patterns, migrating from place to place in accordance with fruit or vegetation abundance. On average, gorilla troops travel 0.24 to 0.60 miles (400 to 1,000 meters) daily [source: SeaWorld]. As vegetarians, their diet relies on seasonal crops and plants. And as the largest of the great ape species, gorillas can put away a heap of food. The average adult male gorilla will eat up to 40 pounds (18 kilograms) of fruits and veggies each day [source: San Diego Zoo].


Chimpanzees may reuse old nests, especially near trees that are bearing fruit [source: Iwata and Ando]. Gorillas, on the other hand, rarely sleep in the same nest twice and build a fresh one every evening. In a study on the reuse of western lowland gorilla nests, these animals slept in only 4.1 percent of existing beds more than once [source: Iwata and Ando]. Illustrating their preference for fresh bedding, the gorillas returned to more than a third of original bedding sites and made new ones [source: Iwata and Ando].

Abandoned nests hold a wealth of information about their former tenants that researchers use to assess the health of ape groups. First, there's the number of nests, which gives an approximation of the population size. Adults sleep in their own individual nests, but baby gorillas sleep with their mothers until the age of 3, after which they'll build their own nests [source: Estes and Otte]. Second, researchers can extract hair samples in order to examine individuals' health. And finally, we can't leave out the importance of feces. Droppings indicate food abundance and contain olfactory distinctions between the different species for precise identification.

Some primatologists and evolutionary biologists consider nest building to be a form of behavioral adaptation passed along to hominids [source: McGrew et al]. Observation of nesting behavior has highlighted social aspects involved with this form of bedding. Within these nests, gorilla troops sleep in groups, groom each other, play together and sometimes mate [source: Pi et al]. Also, since all great apes share this nest-building behavior, that makes it more probable that the behavior evolved from a common ancestor rather than developing independently in each species [source: McGrew et al].

So the next time you feel a pang of guilt about splurging on your 500-thread count Egyptian cotton sheets and down comforter, you can put your mind at ease. Comfortable bedding is a natural creature comfort.

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