Male coercion and the costs of promiscuous
mating for female chimpanzees
Martin N. Muller1,*, Sonya M. Kahlenberg2, Melissa Emery Thompson2
and Richard W. Wrangham2
1Department of Anthropology, Boston University, Boston, MA 02215, USA
2Department of Anthropology, Harvard University, Cambridge, MA 02138, USA
For reasons that are not yet clear, male aggression against females occurs frequently among primates with
constrainfemalematechoice.Weuse 10yearsof behavioural andendocrinedatafromacommunityof wild
male aggression against females. Specifically, we show that male aggression is targeted towards the most
be viewed as a counter-strategy to female attempts at paternity confusion, and a cost of multi-male mating.
Keywords: chimpanzee; sexual coercion; intersexual aggression; promiscuous mating; sexual selection;
Females in many mammalian species mate promiscuously,
actively soliciting copulations from multiple partners
(Dixson 1998; Wolff & Macdonald 2004). Primates
represent a particularly interesting group in this regard
because, in many Old World species, females display clear
anatomical and physiological adaptations that promote
multi-male mating (i.e. sexual swellings; Hrdy 1981; Nunn
1999; Zinner et al. 2004). In these species, male aggression
against females is a common occurrence (van Schaik et al.
2004). However, this behaviour has not yet been fully
explained. Hypotheses include male aggression towards
females being an incidental outcome of male–male compe-
tition, or the result of intersexual dominance or feeding
competition. The predominant hypothesis, however, is that
likely to mate with others (Smuts & Smuts 1993). Here, we
test the sexual coercion hypothesis.
Clutton-Brock & Parker (1995) identified three types
of sexual coercion: forced copulation, harassment and
intimidation. These strategies are differentiated primarily
by the temporal proximity of their effects. Forced
copulation involves violent restraint, resulting in immedi-
ate mating. Harassment involves repeated attempts to
copulate that impose costs on females, inducing eventual
female submission. Intimidation involves physical punish-
ment of female refusals to mate, increasing the likelihood
of submission in the future. All of these strategies are
expected to involve non-preferred males, as they presume
Male mate guarding comprises a fourth form of
coercion that involves directing aggression at females to
prevent them from mating with other males (Smuts &
Smuts 1993; van Schaik et al. 2004). Mate guarding may
involve preferred males, as it functions to constrain female
promiscuity, rather than to overcome female reluctance.
This form of coercion is expected to be prominent in
species with high rates of multi-male mating, and can be
viewed as a counter-strategy to female attempts at
paternity confusion (van Schaik et al. 2004).
For male aggression to be interpreted as any form of
sexual coercion, three specific conditions must be satisfied
(Smuts & Smuts 1993). First, male aggression against
females should intensify in reproductive contexts. Speci-
fically, the most fecund females (i.e. those with the highest
probability of conception) should receive the highest rates
of male aggression. Second, male aggression against
females should correlate with increased mating activity.
Specifically, individual males should show higher copu-
lation rates with the females that they are relatively more
aggressive towards. Third, there must be a cost to male
aggression, such that females would be better off not
experiencing it. Although receiving aggression may appear
inherently undesirable, it is possible that it might benefit
females in some way, for example, by allowing them to test
the quality of potential mates (Smuts & Smuts 1993;
Szykman et al. 2003).
Individual predictions of the sexual coercion hypothesis
find support in a range of mammals (Smuts & Smuts
1993; Clutton-Brock & Parker 1995). For example, male
aggression against females frequently correlates with
female fecundity, such that cycling females or females in
oestrus receive more aggression than non-cycling or
anoestrous females (ungulates: Clutton-Brock et al.
1992; primates: Smuts & Smuts 1993; Soltis et al. 1997;
bottlenose dolphins (Tursiops sp.): Connor et al. 1996;
Scott et al. 2005). Furthermore, in some species females
that mate with multiple males experience lower reproduc-
tive rates and higher morbidity and/or mortality (sheep
(Ovis aries): Reale et al. 1996; feral horses (Equus caballus):
Linklater et al. 1999), costs probably associated with male
Proc. R. Soc. B (2007) 274, 1009–1014
Published online 30 January 2007
*Author for correspondence (firstname.lastname@example.org).
Received 31 October 2006
Accepted 21 December 2006
This journal is q 2007 The Royal Society
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