Sleep, Sleeping Sites, and Sleep-Related Activities: Awakening to Their Significance

1. What they did

Primates spend about half of their lives at sleeping sites, yet most field researchers have focused on what monkeys and apes do by day. The author reviewed what was known about sleep and sleeping sites in diurnal monkeys and apes, both wild and captive, building on his own earlier review (Anderson, 1984).

He searched the literature from 1940 to 1995 and kept only studies in which sleep was the main topic. He first summarized the key earlier studies (before 1984), then the more recent ones, under three themes:

• Choosing sleeping sites: safety from predators, and comfort and hygiene.
• Social life at sleeping sites: who sleeps with whom, huddling, and interactions while settling down.
• Sleep itself and night-time activity: physiology, video studies and the effects of rearing.

He ended with how this knowledge could improve sleeping arrangements for captive primates.

2. What they found

Interest is growing

Studies with sleep as their main topic were sporadic in the 1950s and 1960s but became more frequent from the late 1970s, with the most (six) in 1995.

Safety from predators

• Baboons prefer, in decreasing order, cliff faces or caves, emergent trees, closed canopy, then open woodland. This order matches decreasing protection from predators, especially leopards.
• Guinea baboons sleep out on terminal branches, away from the trunk, and refused to leave their tree even when researchers shone lamps and made a disturbance below. Adult males were usually the first down at dawn, which may lower the risk from a predator waiting near the site.
• Leopards have attacked and killed chacma baboons at their sleeping sites, and a leopard was once found with two dead baboons in a sleeping tree. Sites may be abandoned, at least for a while, after a kill.
• Tamarins became more watchful and quieter just before retiring, and in an experiment captive tamarins chose nest boxes that were high, hidden from view and covered overhead.
• Capuchins sleeping on Jessenia palm leaves are hard for cats to reach, and a cat leaping onto a leaf would make noise and vibration.
• On Siberut Island, Kloss’s gibbons used far more liana-free trees than Mentawai langurs. Hunters climbed trees by the lianas, and langurs were killed in disproportionate numbers. The author calls this probably the best example of competition for safe sleeping sites affecting mortality.
• Chimpanzees in open woodland and savanna in Senegal built higher, more open and more clumped nests than those in dense forest, where predation risk was thought to be lower. Orangutan mothers with young and adolescents nested higher and away from fruiting trees.

Comfort and hygiene

• Primates may choose cliffs sheltered from cold wind or that hold the day’s heat, and great apes may nest out of the wind, in the sun, or away from rain and surface water.
• Moving between sites, sleeping over running water, and great apes building a new nest each night may reduce contact with feces and parasites.
• Malaria infection in New World monkeys increased with sleeping group size. Tamarins sleeping in holes or dense tangles may be less exposed to mosquitoes.
• Lowland gorillas in Gabon may choose nest sites that reduce disturbance by elephants.

Social life at sleeping sites

• Night is a time of closer grouping for many, but not all, primates. In macaques, huddles were most often pairs: mothers and infants, same-sex animals, or male and female consorts. Young animals keep returning to sleep with their mothers until they become more independent.
• Huddling may conserve warmth. Guinea baboon huddles were slightly bigger on windy mornings, perhaps for stability on swaying branches.
• Dominant female spider monkeys used the regular sites more often than subordinate females.
• Juvenile Barbary macaques called more as they settled down, linked to being rebuffed by their mothers. A juvenile baboon threw a tantrum when rejected from a huddle.
• Older male olive baboons sometimes gave up a female to a younger male at the sleeping rocks, perhaps to avoid dangerous fighting in bad light, and resumed the consortship in the morning.
• Bonobos groomed and played in nests. Ape nest groups have been compared with human hunter-gatherer camps, and chimpanzees returning to the same nest sites season after season may leave debris similar to that at early archaeological sites.

Sleep and night-time activity

• Guinea baboons slept mostly lightly, possibly to avoid falling from the tree. Infant pigtail macaques of dominant mothers fell asleep faster, and peer-reared young macaques slept more poorly than mother-reared ones.
• Video at very low light showed considerable night activity in captive spot-nosed monkeys, even in a 2 week old infant; rhesus macaques slept only sitting up. Video has been underused.

3. What they concluded

• Sleep and sleeping habits are important in primate behaviour, and studying them shows how individuals and species deal with avoiding predators, staying healthy and managing social relationships.
• Recent studies support the view that primates choose sleeping sites that protect them from predators and behave around those sites so as not to alert or be attacked by predators. Proving that safety is the main factor for any one group will be difficult.
• Sleeping habits can feed into wider accounts of ecological adaptation and of the evolution of human behaviour.
• Much more information is needed on almost every aspect of sleeping habits in the wild. Video recording is a non-invasive tool that has been underused.
• People caring for captive primates should give sleeping sites and sleeping partners more attention; this could improve their welfare.

6. Other technologies, skills or innovations

• Electrophysiological recordings and biotelemetry of sleep in captive primates.
• Video recording at very low light levels to observe night-time behaviour and identify individuals.
• Experiments that vary the height, concealment and cover of nest boxes to test sleeping site preferences.

7. Implications for conservation (from the paper)

• Losing mature trees with cavities big enough for tamarins but not their predators may threaten the tamarins’ survival.
• Competition for safe sleeping sites can influence population numbers in primate communities, as with gibbons and langurs hunted by people on Siberut Island.
• Sleeping facilities have been neglected in the welfare of captive primates. By varying height, location, materials, concealment and protection from the weather, keepers could find sleeping arrangements that suit each group.