Is Being a Baboon Bully Bad for Business?
Interest in animal personalities – that is, stable individual differences in patterns of behavior among non-human animals, as opposed to people with party-going dispositions – has been on the rise in recent years. A new paper in the Proceedings of the National Academy of Sciences by two of my colleagues at the University of Pennsylvania – Robert Seyfarth and Dorothy Cheney – and Joan Silk (ASU) adds to this literature, investigating personalities among baboons.
If you scratch my armpit…
They had an enormous dataset, meticulous records of the behavior of 45 female baboons over the course of seven years, including frequency counts of behaviors such as biting or attacking another animal, touching another animal, grunting to higher or lower ranked animals (which are ways to signal that one’s intentions are benign), and so forth. In all but one of the seven years, they also had data from “weekly fecal samples,” allowing the researchers to assay hormones among the animals.
They took all these data and conducted a principal component analysis to look for regularities in behavior across the animals. They found that the data could be reasonably well characterized by three dimensions. One dimension they dubbed “Nice” – animals high on this dimension did a lot of benevolent grunting and weren’t alone very often. A second was “Loner,” which captured how much time the animal spent, well, alone. The third factor captured the degree to which the animal was aggressive, which Seyfarth et al. labeled “aloof,” but I think is better glossed as “Bullying,” if for no other reason than to motivate the alliteration in my title here. (Bullies also grunted a lot when they approached infant-holding higher ranking females.)
Their year-to-year analyses suggest that animals’ personalities were relatively consistent over time. For the animals for whom they had the most data, 27 of these 33 animals stayed in the same cluster for a majority of years for which they had observations. Further, personality could not be predicted well by its rank or by whether an animal had a lot of kin nearby. These results make the individual differences seem more like personalities as commonly understood, as opposed to strategic responses to these two important elements of baboon social life. It could have been, for instance, that having low rank and few kin turned a baboon into a loner; that’s not what the data suggested.
The next step was to relate personality to other measures. Being a loner, for instance, was correlated with levels of glucocorticoids. (Recall that they had hormone assays from the fecal samples.) Again, this relationship was not due to the fact that loners tended to be of low rank; their GC levels were lower than what one would have predicted based on rank as the sole predictor. This leaves open the possibility that being a loner caused a change in stress hormone levels (or, of course, the reverse).
Seyfarth et al also provide interesting evidence that baboons recognize other baboons personalities. For instance, baboons high on the dimension I’m calling “Bullying” were approached less often, a finding that, again, can’t be accounted for by differences in rank.
They summarize these patterns of results this way:
…borrowing terms from studies of human personality, the three personality dimensions showed: relative stability over time (test-retest reliability) ; discriminant validity because they were not redundant with dominance rank or the availability of kin; and predictive validity because they were correlated with one physiological measure and two measures of dyadic social bonds that were not used to construct the personality dimensions and are known to be associated with reproductive success.
How should these personality dimensions be conceptualized? I had a chance to talk to Robert Seyfarth, and he indicated to me that they’re currently looking at whether close kin are more likely to be located more closely to one another in the three dimensional personality space, which could potentially help to clarify questions of heritability. Indeed, Seyfarth et al signal some sort of conception along these lines, indicating that the personality variables are potentially under selection, writing that “selection would appear to act against females scoring high on Loner, because these individuals were under more stress than others and formed dyadic bonds that yielded low CSI scores and low partner stability” but that “In contrast, selection would appear to favor individuals scoring high on Aloof and Nice. (p. 5). By the same token, they also imply that individuals can choose to “adopt” different strategies, wondering “why any female would adopt the Loner strategy. Loners were not isolated and unfriendly solely because of their subordinate status or lack of kin. Although these demographic factors contributed to their scores on Loner, their behavior exacerbated them.”
As they say, “It remains for future research to determine which of these personality styles is more adaptive, or whether variation in personality styles is maintained by contrasting effects on fitness.” (p. 5). Having systematically described variation in this species, certainly an important next step is explaining this variation. Is being a Loner best thought of as a strategy that a baboon can choose? If variation in being a Loner is heritable – and leads to lower reproductive success, as implied by the correlates Seyfarth et al. found – then what is maintaining this variation?
That last question, by the way, was recently addressed in the context of human personality variation in a new paper by Verweij et al. in Evolution. Spoiler: it’s mutation-selection balance.
Seyfarth, R. M., Silk, J. B., Cheney, D. L. (2012). Variation in personality and fitness in wild female baboons. PNAS.