By now, I know of few people who haven’t seen the movie, Mean Girls. But in case you haven’t, here’s what you should know about it: the story is essentially an explanation of social cliques and aggressive teenage girl behavior. As a study recently published in Behavioral Ecology suggests, this agonistic behavior between females in cliques is not exclusive to human primates, but is found in our non-human primate kin as well.
Over 18 months and 1027 interactions, Huchard and Cowlishaw (2011) discovered a correlation between sexually receptive female baboons and female-female aggression in groups. In female baboons, sexual receptiveness is typically a period in which individuals are estrous, or “in heat.” A way in which a female can display this information is through sexual swellings, or a swelling of the perineal skin, which indicates ovulation. Additionally, females with wider sexual swellings are perceived as “sexy,” as they attained sexual maturity earlier and generally have more offspring that survive (Domb and Pagel 2001).
With that in mind, enter female-female competition. Female-female competition is thought to occur more often under circumstances where resources for success in reproductive factors might be limited: for example, yielded access to food resources inhibits successful gestation or production of milk or helpful mates that provide more access to resources through social rank.
In the study performed by Huchard and Cowlishaw, sexual receptiveness was perceived to be the driver of aggressive behaviors as sexually receptive females received the most aggression, while lactating mothers received the least. It is thought this might be a tactic to delay conception; thus, females who have already conceived or have offspring would be more likely to receive access to resources and thereof prevents competition. Females who eat less (or would have limited access to food resources) also tend to have less reproductive success (Altmann and Alberts 2003). In addition, it is also possible by inflicting the cost of aggression onto sexually receptive females, the stress may make it more difficult to conceive or support a pregnancy (Beehner et al. 2006). Therefore, by being aggressive to these sexually receptive females, pregnant females or females who have offspring are conserving their resources and limiting the competition.
While no reports of any baboons getting thrown in front of buses have been reported yet, if it does happen—be sure to check the sexual swelling for the baboon version of Regina George.
Altmann, J. & Alberts, S.C. (2003). Intraspecific variability in fertility and offpsring survival in a non-human primate: behavioral control of ecological and social sources. In: Wachter KW, Bulatao RA, editors. Offspring: human fertility behavior in a biodemographic perspective. Washington (DC): National Academy Press; p. 140-169.
Beehner, J.C., Nguyen, N., Wango, E.O., Alberts, S.C., & Altmann, J. (2006). The endocrinology of pregnancy and fetal loss in wild baboons. Hormones and Behav, 49: 688-699.
Domb, L.G. & Pagel, M. (2001). Sexual swellings advertise female quality in wild baboons. Nature, 410: 204-206.
Huchard, E., & Cowlishaw, G. (2011). Female-female aggression around mating: an extra cost of sociality in a multimale primate society Behavioral Ecology DOI: 10.1093/beheco/arr083