Sexual Harassment and Intimidation in Non Human Primates

Three conditions promote sexual harassment that occurs when multiple males attempt to mate simultaneously with a single female (Riale et al. 1996; Head and Brooks 2006; Smith and Sargent 2006): (1) a male-biased operational sex ratio;

(2) asynchrony in female estrous; and (3) weak dominance among males (i.e., reduced or incomplete male ability to control sexual access to females).

All three conditions prevail in nocturnal mouse lemurs (Microcebus murinus) studied at Kirindy, western Madagascar: reproducing males tend to outnumber estrous females; females breed on only one night each year, but are receptive on individually different nights of the mating season; male-male competition sometimes involves contests, but scramble competition via extensive roaming behavior is more common (Eberle and Kappeler 2004a, b). On her night of receptivity, a female is typically approached by 2-15 males and mates with almost all of them up to 11 times. Notably, the usual social dominance of females wanes during the mating season, prompting Eberle and Kappeler (2004a: 97) to interpret the high rates of mating with multiple males as "harassment" stemming from a temporary female inability to reject suitors. Postcopulatory mate guarding does occur occasionally, raising the possibility of sexual intimidation. But this mate-guarding is based less on aggression directed at the female than on chasing rival males away. Attacks on females occurred in only 4 of the 55 cases of mate guarding and were also largely ineffectual in light of the fact that three of the four females succeeded in deserting the male. These patterns of sexual coercion are generally more consistent with multi-male harassment than with sexual intimidation, as predicted by the demographic, social, and reproductive conditions.

The gregarious (diurnal) strepsirrhines are of comparative interest for distinguishing between harassment and intimidation because intimidation relies particularly on learned cooperation in explicitly gregarious contexts (Clutton-Brock and Parker 1995). Unfortunately, few relevant new data have become available since Smuts and Smuts (1993) to address this question. Brockman's (1999) description of sexual aggression by male sifakas (Propithecus verreauxi) suggests harassment rather than intimidation. Multiple males attempt simultaneously to mate with most estrous females during the mating season. Intersexual sexual aggression increases significantly at this time, but the vast majority of it is female aggression to males (not vice versa). Harassment typically takes the form of disrupting an ongoing copulation, and can be perpetrated by either males or females. Although interfering females direct aggression at either copulating partner, males virtually always focus exclusively on the rival male instead of the female. These patterns are collectively inconsistent with the definition of sexual intimidation. Indeed, the data support Smuts and Smuts's (1993) hypothesis that female dominance in some lemurs effectively deters coercion in the form of sexual intimidation. Even so, indirect costs via sexual harassment apparently persist for female sifakas. The nature and magnitude of these costs for female fitness remain unclear, however. Limitation of female choice seems likely, but this possibility needs to be clarified quantitatively (do less harassed females achieve their preferences more often?) as well as tested against the alternative that female resistance functions as mate choice (see below). Moreover, the mating benefits of harassment for the males remain obscure.

A quasi-experimental anecdote concerning ring-tailed lemurs (Lemur catta) further supports the notion that female dominance limits sexual intimidation (Parga and Henry 2008). Partly due to the effects of provisioning, a young female reached sexual maturity at an earlier age than usual, but before she had developed social dominance over males. This young estrous female subsequently became the target of direct aggression and even forced copulation attempts by a particular adult male.

Data on the diurnal, group-living strepsirrhines also provide a relatively rare primate example of support for the mate choice hypothesis for coercion. In ruffed lemurs (Varecia variegata), female conspicuously resist male sexual overtures, even resorting to physical aggression against them. Although males do not typically retaliate with aggression of their own, both Foerg (1982: 119) and Morland (1993) suggest that this sexual antagonism ensures that a female copulates with higher quality ("strong") males who "are more likely to overcome her beating" long enough to achieve insemination.

Studies of anthropoid primates have made little effort to test between indirect and direct costs to females. Japanese macaques (Macaca fuscata) were among the first primates to provide data on sexual coercion, primarily in the form of chases of estrous females or "possessive following" (Carpenter 1942; Itani 1982; Enomoto 1981). As Huffman (1987) points out, these patterns were often interpreted as incidental components of male courtship or "precourtship" behavior (Itani 1982: 362), thereby implicating sexual harassment. Likewise, a key form of sexual harassment - the costs of mating with multiple males - is reflected in the decreased foraging efficiency of females on days they mated polyandrously, compared with days they consorted with the alpha male only (Matsubara and Sprague 2004). Soltis et al. (1997, p 725; 2001, p 486) conclude that male aggression to estrous females is primarily a "side effect" of a general mating season increase in overall male aggressiveness and female-maintained proximity to males. Although sexual intimidation does occur, it accounts for a minority of instances of sexual coercion. Subsequent studies of mating-related aggression in this species, however, have emphasized sexual coercion in Clutton-Brock and Parker's (1995) sense of intimidation (Jack and Pavelka 1997; Soltis 1999; Soltis et al. 2001).

Indeed, this interpretation tends to emerge from many recent studies of male aggression over mating in primates (e.g., Kuester et al. 1994; Perry 1997; Reed et al. 1997; Boinski 2000; Colmenares et al. 2002; Arlet et al. 2008; Table 3.1 and references above). This is partly because comparatively few investigations have addressed the Clutton-Brock and Parker (1995) distinction between harassment and intimidation (Soltis et al. (1997) being a notable exception) and have focused on the processes of intimidetion implicit (or explicit) in (Smuts and Smuts 1993). But this emphasis may also reflect the fact that many of the species studied are characterized by gregariousness and male contest competition, which are conditions especially likely to promote sexual intimidation.

One of the more compelling demonstrations of intimidation is provided by the 10-year study of the Kanyawara population of chimpanzees, Kibale, Uganda. It is striking - as well as suggestive of the biological significance of sexual intimidation - that in a species well-known for male-male aggression, male-female aggression occurs at roughly the same rate at Kanyawara (Muller et al. 2009). The majority of this aggression involves male charging displays and chases, but approximately 35% of it entails physical attacks on females (often in coalition with other males). Muller et al. (2007) provide data by directly testing three predictions of the Smuts and Smuts (1993) sexual coercion hypothesis:

Prediction 1: Sexual coercion is costly to females. The intensity of male aggression is difficult to quantify, but assaults on females can involve flailing with branches, pummeling with fists, pulling of hair, and inflicting injuries (Goodall 1986). These attacks are typically assumed to carry costs, such as risk of infection from wounds, but Muller et al. (2007) clarify potential costs with evidence that cycling parous females, who are the primary targets of male coercion, have elevated cortisol levels. The data cannot demonstrate that male coercion directly causes hormonally mediated stress in females. A causal connection is suggested, however, by the fact that, compared with parous females, nulliparous females copulated at equivalent rates, spent similar (if not more) time in the company of males, but received relatively less coercion from them (as less preferred sexual partners) and had cortisol levels that were not only lower but that did not differ significantly on estrous versus nonestrous days.

Prediction 2: Male mating success is improved by sexual coercion. Previous primate studies had rejected this prediction based on the lack of a positive correlation between overall rates of male aggression to females and male mating success (Bercovitch et al. 1987; Soltis 1999; Stumpf and Boesch 2006). Muller et al. (2009) provide a more direct assay of the selective impact of sexual coercion by demonstrating that male chimpanzees copulated at significantly higher rates with females that they were more aggressive to, than with females that they were less aggressive to (Fig. 3.2).

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