The finding was gleaned from 10 years of observing dominant male chimpanzees in Gombe National Park, Tanzania, looking at behaviors they used to compete for alpha male status relative to their size. Analysis showed that larger males relied more on physical attacks to dominate while smaller, gentler males groomed other chimpanzees, both male and female, to gain broad support.
The study focused on three alpha males who reigned between 1989 and 2003. Frodo, one of the largest and most aggressive male chimpanzees ever observed at Gombe, weighed 51.2 kg (112.6 lbs.) at his peak. He relied on his size and aggression to rule. While he allowed other chimpanzees to groom him, he seldom returned the favor. At the other end of the spectrum, Wilkie, who weighed only 37 kg (81.4 lbs.), obsessively groomed both male and female chimpanzees to maintain his top position. And Freud, who weighed 44.8 kg (98.6 lbs.), used a combination of the two strategies. (The average male chimp in Gombe weighs about 39 kg (85.8 lbs.).
The findings are reported in the February issue of the American Journal of Primatology. While it's widely known that grooming plays an important role in chimpanzee social interaction, this study is the first to show that it can be a strategy for achieving dominance.
The actual paper is here.
The authors suggest that the grooming behavior among the smaller males helps them form coalitions to guard against larger males. Larger males, by contrast, may not need coalitions as much so they groom less. Here is the key figure (Figure 2 from the paper):
WL = Wilkie, FD = Freud, and FR = Frodo
We suggest that body size may partially explain this persistence in male grooming habits. Coalition formation may be one of several behaviors a small male uses to attain and maintain dominance, as he might have difficulty intimidating other males with agonistic or aggressive tactics. If males trade grooming and coalitionary support [Watts, 2002], we would therefore expect a small alpha male to exhibit high grooming rates. Indeed, as predicted, the smallest male in our study, WL, had the highest (predominantly mutual) grooming rates, whereas both FD and FR tended to receive grooming. This suggests that grooming others was particularly important for WL because of his low rates of contact aggression. We suggest that because most other males had the physical potential to dominate him, WL avoided contact aggression and instead, used grooming to form alliances and placate rivals. His high rates of dominance displays (and associated pilo-erection that exaggerates size) would serve to intimidate others without physical aggression. In contrast, coalitions should be less important for a large male who can easily intimidate others through directed charging displays and attacks. Indeed, the largest male in the study, FR, had the lowest grooming rates, received more grooming than he gave, displayed less than WL and had the highest rates of contact aggression. (Emphasis mine.)
The authors also report a novel result: that grooming patterns tend to be stable in individuals. This differs from other work in chimps that showed that grooming behavior in individuals is related to how high their rank in the social ladder isat the time. The authors explain this difference by saying that they watched these particular animals over much larger intervals than previous studies.
From the perspective of a neuroscientist, that finding is particularly intriguing. How and when does the set-point for grooming get established? One would assume that if it is proportional to the animal's size, this set-point would be established around puberty. You could visualize such a set-point actually being counter-productive in some cases. Say an animal is aging and slowly becoming less powerful than it was in youth. If it wanted to maintain alpha status, it might want to slowly substitute increased grooming for outright aggression as the power associated with size waned.
But this is not what the author's found, and it would be intriguing from a neurological perspective to figure out why. Maybe animals develops habits with respect to how much grooming they give and receive vs. how much aggression. Maybe chimps learn to solve problems in a certain way and keep solving it that way throughout their lives.
To be fair, the author acknowledge that these findings may not generalize to all chimps. They only looked at the interactions between these three individuals -- not all the males in the group -- and other larger or smaller groups of chimps might have different dynamics.
Still, I think this is a really cool and interesting finding.
Take home: if you are a small chimp, make friends.
M.W. Foster, I.C. Gilby, C.M. Murray, A. Johnson, E.E. Wroblewski, A.E. Pusey (2009). Alpha male chimpanzee grooming patterns: implications for dominance "style" American Journal of Primatology, 71 (2), 136-144 DOI: 10.1002/ajp.20632