Animals live in a world of limited resources. When resources that affect survival and reproduction–food, water, mates, nest sites, burrows, and so on–are scarce, conflicts of interest arise. Natural selection favors traits that enhance success in competition, and competitive pressures are responsible for some of the most spectacular adaptations that we see in nature: the peacock’s tail, the many variations in the beaks of Darwin’s finches, the ants’ complex colonies, and the baboon’s intimidating canines. Even our large and powerful brains may be the legacy of millions of years of maneuvering for advantage in competitive situations. A growing body of evidence suggests that social bonds may also help animals compete effectively and cope with the consequences of conflict. Some species, including many primates, form coalitions in contests with rivals. A variety of social tactics, including grooming, reproductive concessions, and tolerance at feeding sites, may be used to strengthen relationships with valued allies. The presence of reliable allies may protect animals from harassment and reduce stress.
This general argument implies that sociality will have fitness consequences for the individual. However, there were few efforts to actually test this idea, partly because it seemed unlikely that we would be able to link the short-term benefits that individuals derive from social interactions, such as grooming or greetings, to long-term differences in their fitness. However, as my friend, Lynn Fairbanks, pointed out to me over lunch one day, we ought to look for connections between social bonds and fitness outcomes.
Savanna baboons seemed like ideal candidates for this kind of analysis. Primatologists have been studying baboons for decades, so we already know a lot about their social organization, mating systems, and behavior. Baboons form large multi-male, multi-female groups from which males disperse at puberty. Females remain in their natal group throughout their lives and form stable dominance hierarchies, in which related females occupy adjacent ranks. Coalitionary support plays an important role in rank acquisition, and might also play a role in maintaining dominance rank. Females spend a good part of their day grooming, a behavior that is thought to cement social bonds.
I proposed an analysis of the structure and function of social bonds among female baboons to my friends, Jeanne Altmann and Susan Alberts, directors of the Amboseli Baboon Research Project (ABRP), which has been monitoring yellow baboons, Papio cynocephalus, in the Amboseli basin of Kenya since the early 1970s. I had worked in Amboseli in the early 1980s and knew that the ABRP conducted systematic observations of female social behavior and collected information about various aspects of female reproductive behavior. I suggested that we put these two data sets together. Jeanne and Susan agreed to collaborate on this project, and soon hundreds of thousands of lines of behavioral data representing more than 100 females were sitting on my hard disk.
The first step was to quantify social relationships. Social relationships are abstractions that represent the history of interactions among individuals. We needed to operationalize this concept. Susan had devised an index of “social integration” to characterize the social disposition of Amboseli males, and we modified this procedure for our analyses of females. We used information about the rates of grooming and association to create a composite sociality index for each female. The next step was to evaluate female reproductive success. Infant survival is an important source of variation in lifetime fitness among females in the Amboseli population, so we calculated the proportion of each female’s infants that survived their first year of life.
We found that females that were more socially integrated into their groups had higher survivorship among their infants than females that were less socially integrated. Our results were exciting because they linked sociality with fitness consequences for the first time, and supported the hypothesis that social bonds may be an adaptive means to cope with competitive pressures.
Our findings prompted us to examine females’ social bonds more carefully. We found that females were most likely to form strong ties to close kin and peers who were likely to be their paternal half-sisters. Females also preferentially supported their relatives in agonistic contests. When we examined the distribution of grooming within dyads, we discovered that females with the strongest social bonds also had the most well-balanced grooming relationships. The long-term nature of the Amboseli project also let us explore the stability of females’ relationships across time. Mothers and daughters were quite likely to maintain very close ties for as long as they lived together in the group. In contrast, relationships with unrelated partners were typically ephemeral, with strong relationships existing in one year, but not lasting until the next. Thus, we found that females in Amboseli form strong, supportive, well-balanced, and enduring relationships with selected partners.
Some questions arose as these findings were published and publicized. Some colleagues suggested that the causal arrow might run in the other direction. Baboon females are fascinated by other females’ infants, and cluster around mothers of newborns so that they can touch, sniff, and groom these infants. Perhaps our results reflect elevated levels of sociality for mothers of surviving infants rather than benefits derived from sociality. I also wondered whether this might be a case of beginner’s luck. Would the results hold up in other baboon populations?
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| Anubis baboons grooming. |
I was lucky to be invited to dig into another rich body of data by my friends Dorothy Cheney and Robert Seyfarth, who directed a long-term study of chacma baboons, Papio ursinus, in the Moremi Reserve of Botswana. Robert and Dorothy had invited me to join them in the field as they began their project in the early 1990s. A number of postdoctoral fellows who worked with them in subsequent years (Jacinta Beehner, Thore Bergman, Cathy Crockford, Anne Engh, Liza Moscovice, and Roman Wittig) used the same data collection protocol that we had used, and kindly agreed to let me explore their data.
Dorothy pointed out that we could eliminate the “natal attraction” problem by simply excluding data from females when they had young infants in our analyses. We found striking similarities in the structure of social bonds among females in Amboseli and Moremi. Even more satisfying, however, were analyses that showed that the strength of females’ social bonds was positively associated with the survival of their infants. Thus, for these two baboon populations, which live in very different ecological settings thousands of miles apart from each other, females form close social bonds and sociality seems to enhance female fitness.
We have begun to learn that baboons may not be exceptional in this way. Sociality is associated with higher reproductive success in female horses, house mice, and bottle-nosed dolphins and in male Assamese macaques. There is also abundant evidence that greater social integration is associated with reduced mortality and better physical and mental health in humans. These parallels suggest that the capacity to build strong bonds has been favored by natural selection for millions of years.
So, I offer this advice: make a date to go out for coffee with a friend. It will do you both good.
Joan B. Silk is a Professor in the School of Human Evolution and Social Change and the Institute for Human Origins at Arizona State University. She was elected to the American Ƶ of Arts and Sciences in 2015.
© 2016 by Joan B. Silk