The counterstrategies to sexual conflict are as diverse as the manifestations of conflict itself. They may be morphological, such as the thick skin of female blue sharks (Prionace glauca) vulnerable to bites from "courting" males (Pratt 1979), or the large body size of some female lemurs, which is argued to limit sexual coercion during the mating season (Foerg 1982; Taylor and Sussman 1985; Richard 1992; Morland 1993; Brockman 1999).
Female sexual behavior - particularly promiscuity - can limit sexual conflict, both in the form of precopulatory coercion and postcopulatory infanticide. The convenience polyandry hypothesis holds that conceding copulations allows females to avoid the costs of resistance to coercive males (Thornhill and Alcock 1983; Mesnick and le Boeuf 1991; Blyth and Gilburn 2006). This explanation is less often invoked as an anticoercion counterstrategy in primates than in other animals, but one example is Eberle and Kappeler's (2004a, p 97) argument that the multimale mating of female mouse lemurs reflects " 'making the best of a bad job' in the face of male harassment." The counteractive value of convenience polyandry is improved when it is supplemented with postcopulatory mechanisms of cryptic female choice (e.g., spermicides) (Holman and Snook 2006), but this remains unstudied in nonhuman primates. In the postcopulatory domain, both theoretical models and empirical evidence suggest that female promiscuity offers significant potential to limit infanticide by confusing paternity (Hrdy 1979; Ebensperger 1998; van Schaik and Janson 2000; Wolff and MacDonald 2004; Pradhan and van Schaik 2008).
Association with males is a hypothesized female counterstrategy to sexual conflict, again in both the form of sexual coercion and of male infanticide. Sustained proximity to a large, dominant male reduces estrous female exposure to male harassment and intimidation in Japanese macaques (Matsubara and Sprague 2004) and chimpanzees (Wrangham 1986), as well as in many other taxa (insects: Thornhill and Alcock 1983; fish: Pilastro et al. 2003; Dadda et al. 2005; birds: Gowaty and Buschhaus 1998; bighorn sheep: Reale et al. 1996; elephant seals: Mesnick and le Boeuf 1991). This function has also been suggested for the temporary consortships of female orangutans at risk of forced copulation (Mitani 1985; Fox 2002; Setia and van Schaik 2007). Thus, protection from sexual coercion is an alternative functional hypothesis for consortships, independent (though not mutually exclusive) of mate guarding, and female choice hypotheses (Manson
1997). The relevance of this hypothesis for understanding variation in consortships has not been explored thoroughly.
Reducing the costs of precopulatory sexual harassment may similarly underlie sexual swellings. Previous analyzes have suggested that sexual swellings might benefit females because they incite male-male competition, which then facilitates insemination by high-quality males (Clutton-Brock and Harvey 1976) or copulation with many males (Hrdy and Whitten 1987). Alternatively, sexual swellings might serve to reduce the costs of harassment or intimidation by ensuring mate guarding by a dominant male who keeps other males away. The adaptive value of this counterstrategy, however, must be measured against the (coercion) costs of advertising estrous, the benefits of multimale mating, and the benefits of the alternative counterstrategy of reducing coercion via concealment of receptivity.
Male-female association is also a proposed counterstrategy to postcopulatory conflict in the form of infanticide (Wrangham 1979; van Schaik and Dunbar 1990; van Schaik and Kappeler 1997). Empirical evidence supports this hypothesis in numerous taxa, including insects, birds, and rodents, and a few primate species (reviewed by Palombit 2000). Mountain gorilla (Gorilla beringei) groups have long been viewed as associations of females with a male protector, but whether he deters infanticide or predation is debated. A recent mathematical simulation supports the antiinfanticide hypothesis (Harcourt and Greenberg 2001), but Harcourt and Stewart (2007) argue that rejection of the antipredation hypothesis is premature. Recently, this argument was extended to orangutans with Setia and van Schaik's (2007) suggestion that lactating females use male long calls to stay loosely associated with adult male protectors.
Van Schaik and Dunbar's (1990) hypothesis that social monogamy is an antiinfanticide strategy remains one of the most interesting versions of this hypothesis. Evidence that infanticide has selected for social monogamy is strong in some nonprimate taxa such as burying beetles (Nicrophorus spp.) and tropical house wrens (Troglodytes aedon), but interpretations of the gibbon data have generated divergent conclusions (Palombit 1999, 2000; Sommer and Reichard 2000; Fuentes 2002; van Schaik and Kappeler 2003). Recent tests of the hypothesis in prosimians, such as fork-marked lemurs (Phaner furcifer), avahis (Avahi occidentalis), and spectral tarsiers (Tarsius spectrum), have not consistently supported the hypothesis (Schulke and Kappeler 2001; Thalmann 2001; Gursky 2002). However, this intriguing hypothesis awaits further direct testing in the taxa it primarily addresses: the gibbons.
One population in which long-term data continue to suggest an antiinfanticide function of male-female bonding is the chacma baboon (Papio hamadryas griseipes) of the Okavango Delta, Botswana (see also Weingrill 2000). Like yellow baboons (P. h. cynocephalus) and olive baboons (P. h. anubis) of east Africa, these baboons live in relatively large, multimale, multifemale groups, with female philopatry and dominance relationships in both sexes. In contrast to its east African cousins, however, the chacma baboon exhibits comparatively high rates of infanticide (Palombit 2003). Infanticide is the primary source of mortality for infants, and accounts for at least 38% of infant mortality, though this rate may be as high as 75%
in some years (Cheney et al. 2004). The patterning of infanticide in this population is more consistent with the sexual selection hypothesis than with alternative hypotheses (Palombit et al. 2000). Infanticide is generally committed by males that have recently immigrated into a group and attained alpha status. The relatively short tenure of alpha males (approximately 7 months, on average) combined with their apparently greater monopolization of matings (Bulger 1993) creates conditions that enhance the potential benefits of infanticide. In other words, a new alpha male is confronted with a short period of relatively exclusive sexual access to females. Conversely, since loss of an infant significantly accelerates resumption of fertile cycling in females, lactating mothers are confronted with a significant threat of infanticide.
Unsurprisingly, lactating females exhibit conspicuous and aroused aversion to newly immigrated alpha males, including continual retrieval of infants, screaming, and tail-up displays (Busse 1984). They almost always establish a "friendship" with an unrelated, adult male shortly after parturition (Busse 1981; Palombit et al. 1997). Friendships can be unambiguously differentiated from a female's relationships with other males in the group on the basis of spatial association, grooming, infant handling, and vocal interaction (reviewed by Palombit 2009). Ad libitum evidence suggests that friendship status increases a male's defense of infants during potentially (or actual) infanticidal attacks. Although several males may rush to the scene of such attacks, it is primarily the male friend of the infant's mother who provides direct, apparently costly forms of defense, such as fighting or threatening the alpha male, or carrying the infant. Experimental playback experiments further showed that male friends were more likely to respond to their female friends' screams than to the screams of other females, and females' screams were more likely to provoke responses form their male friends than from other males (Palombit et al. 1997). These experiments also revealed that the solicitude of male friends was tied closely to the presence of infants: playback of female screams shortly after infants died elicited similarly weak responses from all males, regardless of their friendship status. Alternative benefits of friendships to females, such as protection from harassment from higher-ranking females, lack empirical support (Palombit 2009).
Since these original observations, a series of hormonal studies in this population have further supported the antiinfanticide function of heterosexual friendships. Following the immigration of a new male, glucocorticoid levels rise in females generally, but remain high over subsequent weeks only among anestrous females, not among cycling females (Beehner et al. 2005; Wittig et al. 2008). This is a striking difference because cycling females are the primary targets of the protracted, aggressive chasing that seems to facilitate a new male's rise to alpha status (Kitchen et al. 2009). Thus, hormonal patterns suggest that it is females at risk of infanticide (not simply of aggression) from the new male who experience greater stress upon his arrival in the group. This is further substantiated by additional increases in glucocorticoids among lactating females when a new alpha actually commits an infanticide (Engh et al. 2006) or among the (few) lactating females who lack male friends at the time of male immigration (Beehner et al. 2005).
A final indication of the potential importance of friendships is that females compete with one another for them (Palombit et al. 2001). This is reflected partly by the positive correlation between the dominance ranks of male and female friends, and partly by observations of high-ranking females displacing subordinate rivals from friendship with a particular male. Competition among females for males is relatively rare in mammals (Berglund et al. 1993; Andersson 1994), and in this case, it suggests that males provide a service with important fitness consequences for females. It is not immediately obvious why male protection is not shareable among multiple lactating females, but since friendship status appears so crucial, females may compete for social access to males in order to develop this relationship.
Sexual conflict hypotheses for male-female bonding are potentially relevant to understanding human pair-bonding, although space precludes a thorough treatment of this question here. Early models argued that a durable pairbond between the sexes was part of an adaptive suite of traits including reproductive monogamy and a division of labor between females and provisioning males (Murdock 1949; Washburn and Lancaster 1968; Lovejoy 1981). An alternative hypothesis emphasizes the importance of male protection of females from sexual conflict in the form of sexual coercion and/or infanticide (Betzig 1992; Smuts 1992; Mesnick 1997; Hrdy 1999; Hawkes 2004). A recent cross-cultural analysis rejected the male protection hypotheses partly because pairbond stability (overall divorce rates in a society) was uncorrelated with general male aggressiveness (overall rates of male homicides and assaults) (Quinlan and Quinlan 2007). However, this conclusion is limited in the same way that the lack of a correlation between overall male aggressiveness and mating success in chimpanzees may overlook the fact that sexual coercion significantly increases a male's mating success with the particular females he targets (see above). Thus, the hypothesis must be tested with human data addressing specifically how risk of sexual coercion or infanticide to individual women varies with the nature of their pair bonds. Since male partners are themselves sometimes a source of sexual coercion to women (Rodseth and Novak 2009), these analyses must differentiate between the costs of pair-bonding with men and the protective benefits of pair bonds from other men. The variety of current evidence suggests the possibility that the different selective pressures proposed may each promote pair-bonding under different conditions (Quinlan 2008). This proposition merits greater scrutiny.
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