Among other things, friends are people you count on to come to your aid when you need help. If you were at a bar and a stranger started acting aggressively towards you, for example, you would expect your friends to rush over to help you rather than just stand there, mojito in hand. Contrary to our feelings of human exceptionalism, however, ours is not the only species of primate to create and maintain friendships.
For years primatologists have been puzzling over "friendship" in baboons. Across baboon species lactating females keep up close social relationships with unrelated adult males. The females are not reproductively available, and by devoting much of their attention to these females the males significantly reduce their opportunities to mate with other females, so why are these males so concerned with mothers and infants? What is the function of this behavior?
Several hypotheses have been forwarded. Perhaps friendship is a defense against infanticide, a way to reduce harassment of mothers and their infants by other group members, or a way for mothers to get their infants to bond with particular males so that they will continue to reap social benefits (such as food sharing and support during fights) as they mature. Of these, however, friendship as an anti-infanticide mechanism appears to be best-supported, especially since infanticide is a major cause of mortality among infant Chacma baboons. Baboon social groups are centered around female families that stick together, but males often move from one group to another. As a result immigrant males occasionally supplant the group's dominant male, and when this happens among Chacma baboons the new alpha picks off the group's infants one-by-one (hence the group's females come back into estrus sooner). In such situations a friendship between a male and female baboon can make the difference between life and death for her offspring.
That male friends provide such protection was confirmed through playback experiments in the field. When the distress calls of female Chacma baboons with infants were played in the vicinity of their friends the males reacted strongly to the sound by trying to find where that female was and determine if she was in danger. The control subjects, males of similar social rank but were not friends, glanced up when the sound was played but quickly went back to whatever they were doing. These results reaffirmed that at least part of the role of male friendship was to offer protection to mothers with infants.
Yet the threat of infanticide varies among baboon species even as friendships remain a common phenomenon. In olive baboons, for example, infanticide by an immigrant male is a rare event rather than the norm, and primatologists A. Lemasson, Ryne Palombit, R. Jubin took this as an opportunity to see if friendships in this species involved protection from harassment or some other social benefit. If male friends provide protection for mothers and infants then they would react strongly to distress calls, just like the Chacma baboons, but if the function of friendship is to provide infants with social benefits later in life it would be predicted that males would not be as concerned upon hearing the female's screams. To test this idea the scientists arranged a second playback experiment published in Behavioral Ecology and Sociobiology.
The experiment took place in Kenya's Laikipia District from February through July of 2004, and the group of approximately 100 baboons to be studied had been observed by the same group of scientists since 1999. This meant that not only were the baboons habituated to the observers moving on foot, but also that the scientists knew how to identify each individual animal and had records of their life history. This was important as the study relied upon identifying individuals that came into close contact with each other (defined as within 2 meters) and how those animals interacted (the number of approaches and withdrawals, and who initiated or broke off contact). Within those subsets the researchers also recorded important social interactions such as grooming, submission displays, and the interactions of males with infants, so altogether these observations could be used to quantify the details of social relationships between females and their male friends.
This data collecting allowed the researchers to identify which males could be classified as friends and singled out for playback experiments (in addition to control males of equivalent rank but that were not friends of females). The setup was relatively simple. One researcher, equipped with an iPod connected to a hidden speaker, would play the distress call of a female while another would record the reaction on video. Both were visible to the baboon, and the scientists were careful to make sure that the male was isolated from other individuals by 50 meters, could not see the female who had made the distress call, and that they did not habituate their subjects to the experiment.
What the results showed was the male friends reacted more strongly to the screams of the females than the control males. While the control males would glance up for a second or two and sometimes move away from the site of the concealed speaker friend males typically looked for the female for between eight and twenty seconds, sometimes moving towards the speaker. This is common baboon behavior individuals use to find out if there is trouble brewing among the group before deciding to intercede or stay out of it. Likewise, during normal observations male and female friends spent a much greater amount of time grooming each other than non-friend pairs, and incidences of agonistic behavior were also reduced among the friend pairs. As the researchers summarized:
Relative to non-friend dyads, friends are characterized by conspicuous spatial proximity initiated by both partners, high rates of grooming, and low rates of agonism. Lactating females were much more tolerant of male friends handling their infants than other males. We similarly found that female olive baboons commonly maintain multiple male friends at the same time.
The reactions of the friend males underscored this conclusion, and since infanticide by immigrant males was rare among this group the researchers concluded that friendships among these olive baboons may serve to reduce the harassment of mothers and their infants. It did not matter that the infants in this population were less likely to be killed by members of the group; the male friends still reacted strongly to the distress of the females they closely associated with. This suggests that friendships function to ensure the protection of mothers and their infants. As the authors state:
The playbacks suggested that male olive baboon friends are more strongly predisposed to supportive responses to female distress in a manner highly reminiscent (both qualitatively and quantitatively) of male chacma baboons in "infanticidal contexts." Thus, friendship status in olive baboons seems to similarly enhance male interest in defending lactating females (and infants) even from non-lethal forms of harassment that are ostensibly less costly to female fitness than infanticide.
What is still unknown, however, is why the males seem to spend so much time and attention in upkeeping these friendships. At present there are three, non-mutually exclusive explanations. The most obvious is that the males are the fathers of the infants, so whatever protection they afford is ensuring the survival of their own offspring. (Although this would not explain why high-ranking females have multiple male friends at any one time.) Similarly, protecting a female and her young during a non-mating period may be rewarded by mating opportunities with that female in the future. Alpha males might try to keep other males from mating, but female choice (and possibly friendships) allow other males to mate surreptitiously. Then again, these friendships may have little to do with mating at all. The presence of an infant might act as a kind of social buffer that reduces harassment of females while immigrant males who strike up friendships with resident females may more easily assimilate into the group.
At present which (if any) of these hypotheses is correct is not known, but the "paternal investment" hypothesis is presently being tested through genetic studies. If the male friends turn out to be the fathers the behavior makes sense, but if not the friendship of males and females remains puzzling. The hypothesis that male friends are protecting mothers and infants from harassment and even death is well-supported, but just why this might be so is still a mystery.
Lemasson, A., Palombit, R., & Jubin, R. (2007). Friendships between males and lactating females in a free-ranging group of olive baboons (Papio hamadryas anubis): evidence from playback experiments Behavioral Ecology and Sociobiology, 62 (6), 1027-1035 DOI: 10.1007/s00265-007-0530-z
Very interesting. I wonder if Hamadryas baboons form these friendships as well.
I think something that is too often overlooked in a search for an evolutionary explanation is that the friendships might fulfill some emotional need to both adults as well. Certainly human friendships between the sexes aren't reliant on protecting against infanticide or paternal investment, and often have no direct evolutionary function when it comes to ensuring the female's reproductive success, but are still valuable in terms of emotional well being. Baboons are highly social, so it may be that these friendships are more than just based on fitness, but also on individual emotional needs.
Great post! I love the literature on baboon friendships, particularly the endocrinology stuff. Some of it is actually what inspired my dissertation research (looking at female friendships and stress).
Raymond; Click the ref. links for the papers. They are freely available and cite some of the other species frienships have been observed in (and just follow the citations from there).
Melanie; I was thinking about the same thing, although that seems harder to detect/quantify. The social signs of frienships seem well-established, but figuring out why the baboons foster them is harder to get at.
In terms of why females may have multiple male friends it may be possible that the male is gambling that the infant is his. If the cost of providing such support by a male were small enough, and there were a reasonable chance the infant was related, it might be worth it to pay the costs of friendship despite the risk that the infant is actually unrelated. He need not always be the father. This is the first that I've read of baboon friendship though, so maybe the people talking about the fathership possibility have already considered this.
In regards to Melanie's point though, the question in my mind becomes: why would the baboons (or we) evolve to value friendship so highly? What's the benefit to having friendships impact our emotional states so strongly, instead of just thinking of them as "easy come, easy go?"
Very nice. It made me look at an old book I have on conflict and blood revenge in Montenegro by Christopher Boehm.
I am not familiar with his other work but he has written a lot with regard primatology and conflict resolution
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