Rank affects a female's likelihood to seek out "friendships"

Photo: Chris Allen

Chimpanzees (Pan troglodytes) are social creatures, living in a fission-fusion society, meaning one large group often breaks into smaller ones to form feeding parties. In the chimp world, males outrank females and there is a strict linear hierarchy to male relationships (Goldberg & Wrangham 1997). Whereas males can remain in their natal group, females migrate around the time when they reach adolescence (years 9-14) (Nishida et al. 2003). Thus, females eventually need to make new friends, whereas males have the benefit of life-long buddies. In most chimpanzee populations, females spend a lot of time by themselves (Williams, Liu, and Pusey, 2002). At Gombe, the famous site where Jane Goodall first went to the wild and studied chimpanzees, females spend the majority of their time alone or with a few other family members (Wrangham and Smuts, 1980; Goodall, 1986).

Gombe National Park in Tanzania, Google Earth

A study due to be published next month in the journal Animal Behaviour reports new insights into the "friendships" of female chimpanzees. (Friendship may very well be too strong of a word. Acquaintance or association is likely more accurate.) Foerster and colleagues used thirty-eight years of data from Gombe National Park and show that female chimpanzees aren't just bonding randomly with each other. Unsurprisingly, mothers, daughters, and sisters form the strongest bonds. However, among unrelated females, those of a low rank were more likely to seek out other females than were mid or high-ranking females. They also sought out other low-ranking females. 

There are multiple reasons low-ranking females might seek out others of the same rank. As Foerster and colleagues suggest, perhaps having an additional low-ranking pal makes competing for food easier or perhaps they seek others to dissuade higher-ranking members from bullying. The reason for their increased likelihood to seek out other female friends remains unknown at the time. It would be interesting to know how long these partnerships last. If they're temporary, why? What circumstances or events cause the end of the partnership?

 Finally, the presence or absence of offspring affects the likelihood a female will seek out other unrelated females. Females with young female offspring associate with other females less than expected, but females with young male offspring seek each other out. This may because it is especially important for males to interact socially with each other and bond, given that those bonds have the potential to last a life time. The same cannot be said for young female chimpanzees because they will eventually leave and find a new troop. Thus, it apparently is worth forming relationships with other young females within the troop.

Food for thought: What are the benefits and disadvantages to spending time with others (if you're a chimpanzee)?
How might this study differ if the study species had been bonobos? Remember, they have a very different social system!

Links of interest:
ScienceDaily article
Chimpanzee social systems
Chimps create traditions video clip
IUCN Redlist page on chimpanzees
Female chimps more likely to use tools
How human are chimps?


References:
Foerster, S, McLellan, K, Schroepfer-Walker, K, Murray, CM, Krupenye, C, Gilby IC, Pusey, AE. Social bonds in the dispersing sex: partner preferences among adult female chimpanzees. Animal Behaviour, 2015; 105: 139 DOI: 10.1016/j.anbehav.2015.04.012
Goldberg TL, Wrangham RW. 1997. Genetic correlates of social behavior in wild chimpanzees: evidence from mitochondrial DNA. Anim Beh 54: 559-70. 
Goodall J. 1986. The chimpanzees of Gombe. Cambridge (MS): Belknap Pr.
Nishida T, Corp N, Hamai M, Hasegawa T, Hiraiwa-Hasegawa M, Hosaka K, Hunt KD, Itoh N, Kawanaka K, Matsumoto-Oda A, et al. 2003. Demography, female life history and reproductive profiles among the chimpanzees of Mahale. Am J Prim 59(3): 99-121. 
Williams, J. M., Liu, H. & Pusey, A. E. 2002. Costs and benefits of grouping in female chimpanzees at Gombe. In: Behavioral Diversity in Pan. (Ed. by Boesch, C., Hohmann, G., and Marchant, L. F.) pp. 192-203. Cambridge: Cambridge University Press.
Wrangham, R. and Smuts, B.B. (1980). Sex differences in the behavioural ecology of chimpanzees in the Gombe National Park, Tanzania. Journal of Reproduction and Fertility. Supplement, 28, 13-31.