Immigration costs for female chimpanzees and male protection as an immigrant counterstrategy to intrasexual aggression

In chimpanzees, Pan troglodytes, females transfer from their natal group shortly after sexual maturity to permanently join another group. A conflict of interest exists between female and male residents over the immigration of new females: additional females increase female feeding competition but provide new mating opportunities for males. Resident females express their interests by directing high rates of aggression towards immigrants, and males could protect their interests by intervening in these conflicts. We studied the Kanyawara chimpanzee community in Kibale National Park, Uganda over 10.5 years to assess whether (1) resident female aggression is costly to immigrants, (2) males are effective in protecting immigrants from aggression and (3) immigrants seek out male protective services. Results showed costs of resident aggression on two levels. Immigrants held low dominance ranks and experienced higher physiological stress than natal residents, as indexed by urinary cortisol. Males were found to be effective protectors for immigrants. They intervened aggressively to curtail female conflicts, more often when conflicts involved immigrants and occurred during periods of heightened female competition. When intervening, males nearly always supported immigrants over residents. As a result, females, especially immigrants, experienced less intrasexual aggression in the presence of males than when males were absent. Immigrants took advantage of male protective services. They associated with males more often than resident females and, unlike residents, did not greatly decrease their level of association outside of oestrus. We suggest male protection is an important strategy used by immigrants to integrate themselves into their new group.