Baboon Sleeping Site Preferences and Relationships to Primate Grouping Patterns

1. What they did

Where an animal spends the night can matter as much for survival as where they feed. The author asked whether baboons pick their sleeping sites to stay safe from predators, and whether the way good sleeping sites are spread across a landscape could help explain why some primates live in large groups and others in small ones.

He grouped baboon sleeping sites into categories (Fig. 1): caves (over 5 m deep), open cliffs (rock faces within 20° of vertical), wooded cliffs, emerging trees that rise a little above the forest and have strong branches for moving sideways, closed canopy that baboons can cross without coming to the ground, open woodland where they cannot reach the next tree without coming down, and rock overhangs (no deeper than 5 m).

He then gathered descriptions of sleeping sites for many baboon troops across Africa from the published literature, from colleagues, and from his own observations in Botswana (1972 to 1980) and Namibia (Table I). For each troop he recorded which site types were available and which the troop used most. The observations were not originally collected to study sleeping sites, so he presents the findings as hypotheses.

2. What they found

The preference order

• Where caves were present (a small number of sites), they were chosen about as often as cliff faces.
• Where open cliffs were present but caves were not, cliffs were preferred over emerging trees (12 troops to none).
• Emerging trees were preferred over closed canopy (23 to none) and over open woodland (22 to none) when both were available.
• The only exception was F troop in Botswana, which chose scattered palm trees over shorter closed canopy.
• No baboons were reported to use wooded cliffs or rock overhangs for sleeping, and free-ranging baboons have never been reported sleeping on the ground.

Details from the field

• At one mixed site in the Okavango, independent two to three year old juveniles mostly slept in isolated palms, while adults and mothers with infants used the emerging trees and palms that touched each other.
• At Savuti Marsh, one troop slept in dead acacia snags standing in the flooded Savuti Channel, a shallow river about 20 m wide. Some baboons swam out to them and others crossed on fallen logs. Leopards were seen chasing baboons arriving at this roost.
• After night attacks by leopards, troops were seen changing roosts.
• Many sleeping sites were open to wind and weather, such as leafless treetops and bare cliffs.
• When disturbed at night in trees, baboons moved sideways into the next trees without coming down. On open cliffs in Namibia, they moved less when disturbed by spotted hyenas and people, gave alarm calls and released stones, and never abandoned the cliffs at night.
• When Okavango Swamp troops met at sleeping sites (N = 18 in the paper), the losing troop always moved from an emerging Acacia nigrescens roost to a closed canopy roost.
• During an outbreak of scale insects (a favoured food) in mopane forest, one troop slept seven times in the mopane forest, a closed canopy site, when this food took up over 50% of their foraging time. On 42 other days of feeding there, they returned to their two preferred emerging tree roosts. They did not use closed canopy roosts when the insects were not available.

3. What they concluded

• Baboons seem to choose sleeping sites mainly for safety from predators, especially leopards at night. The preferred sites let them escape upward or sideways where predators cannot easily follow.
• Caves may be favoured for a different reason: a few individuals could block the entrance against predators. This remains speculative, because no one has reported how baboons respond when predators enter their caves.
• Rock overhangs, used by Bushmen and other hunter-gatherers, are not used by baboons, probably because they can only be defended actively. When early humans first used such shelters, they may have been able to defend themselves together, perhaps linked to a shift towards group hunting.
• Where good sleeping sites are few and far apart relative to food, groups may gather at them, which may allow large groups to develop. In tropical forests, where suitable sleeping trees are many and evenly spread, small groups become possible.
• Sleeping sites are a constraint that makes some grouping patterns possible, not a cause of social structure.
• Baboons use less suitable sites when they forage far from good ones, so poorer feeding conditions should mean more use of poorer sites. Vulnerability to leopards should be highest at the least preferred sites, but no data were available to test this.

6. Other technologies, skills or innovations

• A simple classification of sleeping site types by their structure (Fig. 1), which can be applied across habitats and species.