Tuesday, June 30, 2015

Olive baboons can be democratic when it comes to troop movement

Olive baboons (Papio anubis) live in a world where social rank matters. They live in troops that can have over one hundred individuals, with females remaining in their natal group and males dispersing. Males are dominant over females but females have a strict linear dominance hierarchy, meaning who your mother is matters. A lot. Related females groom each other and have each others backs in agonistic meetings.

Photo credit: Nevit Dilmen
A new study out in Science by Strandburg-Peshkin and colleagues shows that, much to everyone's surprise, olive baboons are democratic when it comes to troop movement. Yep, these primates, which can be known for their aggression (just look at those canines), don't bully when it comes to choosing where to go.

What we'd expect in a primate with such a strict social system is that the group would go where dominant individuals want to go. Those lower on the social totem pole aren't going to act against a more dominant individual's decision nor are they going to attempt to make these decisions themselves. However, using GPS technology, researchers found that the troop tends to go where multiple initiators want to go and individuals who move in a directed manner are more likely to be followed.

When two groups within a troop are moving in different directions, the larger group is more likely to win, and this likelihood grows as the difference in size between the two groups of initiators grows. The authors failed to clarify if, of those decisions with multiple initiators, the number of dominant individuals within groups of initiators made a difference. I'd like to know if five low-ranking individuals could top two high-ranking individuals, when determining which direction to go. Does social rank really have no effect? I'm not sure I'm entirely convinced that it doesn't, but I also don't study baboons.

Now, this study was conducted by studying 33 out of 46 members of one baboon troop over nine days, so it'd be interesting to see if results differ in other troops, in troops of differing sizes, or during different times of the year (for example, when food resources are low). It'd also be interesting to see if this finding holds true for other species of baboons. For example, previous research has shown chacma baboons (Papio ursinus) do pay attention to social relations when deciding which patches of food to target (Marshall et al., 2012).

Food for thought:
Strandburg-Peshkin and colleagues didn't find any differences based on sex when it came to an individual initiating troop movement. Why is this surprising?
Why might this system (in which the troop is democratic in deciding where to go) be adaptive to baboons? When might it not be?

Links of potential interest:
Olive baboon behavior and social organization
Chacma baboon study

Literature cited:
Ariana Strandburg-Peshkin, Damien R. Farine, Iain D. Couzin, and Margaret C. Crofoot. Shared decision-making drives collective movement in wild baboons. Science, 19 June 2015 DOI: 10.1126/science.aaa5099

Harry H. Marshall, Alecia J. Carter, Tim Coulson, J. Marcus Rowcliffe, Guy Cowlishaw. Exploring Foraging Decisions in a Social Primate Using Discrete-Choice Models. The American Naturalist, 2012; 180 (4): 481 DOI: 10.1086/667587

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