Lemurs can be deceptive. Most animals are—it comes in handy when you're competing for food or mates, or trying to hide from a predator. However, lemurs are particularly interesting because they can possibly help us better understand the development of our own sociality.
The social intelligence hypothesis maintains that primate brains evolved in response to a need to manage the complexities of social life, particularly relationships—alliances and friendships, after all, are instrumental within day-to-day networks. Evolutionary anthropologist Robin Dunbar, the originator of the social brain hypothesis, believes that anthropoid primates may have adapted the pair bond relationship in larger group settings to give rise to non-sexual relationships. In non-human primates, including mammals and birds, larger brains are significantly present in species that are inclined toward pair bonding. Understanding how these types of connections shaped sociality is not an easy task. For example, is sociality tied solely to social intelligence or is it impacted by other types of intelligence? And how do you trace this through the fossil record where evidence can sometimes be scant?
The social intelligence hypothesis proposes that the larger the group the greater the social demands because larger groups have more complex social networks. In turn, this suggests that primates who live in larger groups will be smarter. The social brain is therefore also a larger brain (relative to body size), but this isn't always the case: chimpanzees, for example, have brains that are *only* twice as large while elephants (and humans) tip the scales for mammals.
In a Current Anthropology paper published last December, Gowett, Gamble and Dunbar note that to fully complete the sociality puzzle we need to cover about 15 million years of evolutionary history beginning with the divergence of the great apes. One method of doing this has involves finding primate analogies. Duke University evolutionary anthropologist Evan MacLean recently published a paper in PLoS ONE examining the social brain hypothesis in six lemur species using two tasks: a social cognition task adapted from human infant tests requiring subjects to understand and take advantage of a competitor's visual awareness, and a non-social task where subjects were required to leverage learnings about how to retrieve an object in different environmental contexts.
Why lemurs? Most of the primate studies done previously has focused on species that live in similarly sized groups and whose brain sizes are generally correlated to group size. However, lemurs live in groups of varying sizes, and those groups don't seem connected to brain size. So there are two advantages offered by lemurs in this case: the first is that they offer the chance to observe the influences of group size within a species in relation to sociality and the second is that it offers a chance to observe these influences independent of ties to brain size.
In the first task, food was placed in front of two humans. The catch was that one of the humans was visually blocked from the food in some way (e.g., facing away or blindfolded) while the other was not. Lemurs with a greater social understanding would be more likely to take the food from the human whose view was hampered. In the second task, the lemurs were taught to remove food from an opaque cylinder by reaching around the sides to where the food was accessible. The tube was then replaced with a transparent tube, which required the lemurs to utilize the skills they had learned previously in the absence of clear visual cues. (For more on this particular study, head on over to The Thoughtful Animal by my colleague Jason Goldman. I've only hit the high points here.)
Lemurs from larger social groups performed better at the social cognition task: they more frequently took food from from the human who was visually impaired. However, when it came down to the non-social task, there was not a strong link to group size. Taken together, these findings suggest that social behavior may not necessarily be tied to group size but to the specific environmental and social factors that each group experienced. These findings also demonstrate that brain size is not necessarily a measure for intelligence so we should be cautious about linking the two without fully understanding the contributions of environmental and social factors.
Maclean's study reminds us that primate analogies will only take us so far. Gowett and colleagues argue for reexamining the archaeological record for clues to more general primate-wide "rules" that suggest an organization to primate behavior, ecology, and demography. This requires reading artifacts in a larger time-space grouping. For example, finding the same toolkits in Asia dating to 100,000 years-ago at Neanderthal and Anatomically Modern Human sites may reflect an adaption to particular environmental factors rather than similar cognitive abilities. In thinking this way, we can expand our allowance of social intelligence beyond the Neolithic and perhaps find deeper meaning in primate analogies.
Gowlett, John, Gamble, Clive, and Robin Dunbar. (2012). Human Evolution and the Archaeology of the Social Brain. Current Anthropology, 53 (6) DOI: 10.1086/667994
MacLean E.L., Sandel A.A., Bray J., Oldenkamp R.E., Reddy R.B. & Hare B.A. (2013). Group Size Predicts Social but Not Nonsocial Cognition in Lemurs. PLoS ONE, 8 (6) DOI: 10.1371/journal.pone.0066359