Male-Female Aggression in Chimpanzees
Principal Investigator: Nicholas E Newton-Fisher
Project dates: 2003-2007 (+ ongoing)
Funding: HF Guggenheim, Wenner-Gren
Aims
Does the aggression directed by male chimpanzees against con-specific females function as sexual coercion? This study aimed to quantify the extent to which female chimpanzees experience such aggression and to investigate whether it is in fact sexual coercion. It then evaluates the proposal that this behaviour provides a good model for the evolutionary function of aggression and violence in human heterosexual relationships, variously described as domestic violence, dating violence or battery.
Background
Violence and aggression within a (potentially) sexual relationship, including both physical assault and threats, remains a poorly understood and problematic aspect of human society. Such violence may be experienced by as many as two fifths of women (Bradley et al. 2002) and similar behaviour is widespread among mammalian species. In non-human primates, documented costs for females include disruption of estrus cycles (Goodall 1986), abortion (Pereira 1983), severe injury and death (Rajpurohit and Sommer 1991, Smuts and Smuts 1993). More subtle costs may be incurred through chronically elevated levels of stress hormones, associated with negative health and reproductive effects (Dunbar 1996, Packer et al. 1995, Sapolsky 1996, Schapiro et al. 1998).
Chimpanzees are genetically the most similar species to humans with the two more than 99.5% identical in functional nuclear genes (Goodman, McConkey, and Page 2002), and Wrangham & Peterson (1996) have proposed that chimpanzees may provide a good model for understanding the evolution of relationship (domestic) violence in humans. Chimpanzees have been studied in detail, much of their social behaviour is well documented. They live in characteristically fission-fusion unit-groups/communities (Goodall 1973, Nishida 1968), adult female chimpanzees are typically less gregarious than males, are 20-25% lighter (from data in Bean 1999) and consistently hold lower social status (Goodall 1986, Takahata 1990).
Male chimpanzees sometimes show extreme levels of aggression and violence towards females (Muller 2002). Cycling female chimpanzees advertise approaching ovulation with an ano-genital swelling. These swellings make females attractive to males, and males target aggression towards these females. Males also launch apparently unprovoked attacks against non-cycling females and those approaching ovulation. It has been suggested that this aggression may function as sexual coercion whereby the male intimidates the female into biasing her mating effort so that male receives a higher proportion of copulations either immediately, or in the longer term. Females are thought to be vulnerable to such coercion because they are often alone and without allies (Goodall 1986, Smuts and Smuts 1993, Wrangham and Peterson 1996).
Findings
Focal subjects were observed for over 1,600 hours. During observations, a total of 473 aggressive interactions between males and females were recorded. Males were the aggressors in 72% of these interactions, while females were aggressors in 28%. Female aggression towards males was often provoked by male aggression.
There is good evidence from my preliminary analysis that male aggression in chimpanzees does function as sexual coercion, in all three forms. Male chimpanzees use aggression to achieve mating – grabbing a female and holding her down while copulating (forced copulation), and repeatedly targeting a female until she copulates (harassment) after which the aggression stops. There are also indications that aggression against females not showing sexual swellings (intimidation) leads to future compliance. Such aggression is a “consistent and regular” aspect of the lives of female chimpanzees, to the extent that they will experience it repeated in a time-frame of days to weeks. Males therefore gain a direct mating benefit from aggression. Ongoing collaborative genetic work addressing paternity suggests further that consistently pursuing a sexual coercion strategy may have be an effective strategy for male reproductive success. Male aggression imposes costs on females: females show clear evidence of mating preference and engage in furtive copulation, suggesting that male aggression compromises female reproductive strategies. Females also show evidence of minor injury after beatings, and use behavioural strategies to counter male aggression. This aggression does therefore appear to be functioning as sexual coercion, and this is the first systematic demonstration, and quantification of the extent, of this behaviour in chimpanzees.
Contrary to ideas that coercive aggression is something females can do little about and is just a price to be paid for attracting mates (Wrangham 2002), the female chimpanzees in this community showed clear evidence of counter strategies to male aggression. Females will retaliate to male aggression with aggressive behaviour of their own. The level and occurrence of retaliation varies with both the individual female and identity of the aggressive male. Further, females will retaliate cooperatively, forming coalitions to threaten and attack aggressive adult males. This finding is a striking exception to the accepted view of female chimpanzee behaviour, particularly for East African chimpanzees.
Importance
These findings are particularly important for our understanding of chimpanzee society and suggest that a reassessment of female chimpanzee behavioural strategies, and chimpanzee mating system, is required. This will allow better tests of hypotheses relating ape behaviour to the evolutionary roots of human behaviour. With respect to the origins of domestic violence, these findings suggest that human studies, rather than focusing on identifying potential risk factors, should focus on male aggression as a contingent strategy and attempt to identify those contingencies, and examine the range and effectiveness of female counter strategies. This study finds some support for the ape-behaviour component of the demonic males hypothesis (Wrangham and Peterson 1996) but demonstrates a more complex interplay of behavioural strategies than has been assumed previously.