Sexual harassment refers broadly to the fitness costs of mating to females (sensu Daly 1978), particularly costs connected with persistent male courtship, repeated intromission attempts, or the nature of copulation itself. Sexual intimidation refers to situations in which "males punish females that refuse to associate with them or that associate with other males" and is thus designed to reduce female resistance or promiscuity (Clutton-Brock and Parker 1995, p 1353). When males use sexual intimidation tactics, females learn to modify their behavior to minimize the costs of male aggression. This definition is directly similar to Smuts and Smuts's (1993) original definition of sexual coercion. To illustrate the distinction between sexual harassment and sexual intimidation, consider the following examples:
1. During the rut, female sheep (Ovis spp.) may be pursued by up to 11 rams at a time, whose repeated attempts to charge, sniff, and mount result in exhaustion and injury to females (Riale et al. 1996) as well as increased mortality, as females evade male suitors on precipitous terrain (Festa-Bianchet 1987).
2. When a female dung fly (Scatophaga stercoraria) lands on a dropping occupied by several males, their struggles to copulate and exclude rivals from mating may trample her into the patty, covering her with dung that impairs her ability to fly and sometimes even drowns her (Parker 1970).
3. A male chimpanzee severely attacks an estrous female for "no obvious reason," i.e., in circumstances unrelated to ongoing mating, and when the female's sexual swelling is small or flat; many days later, during the period of maximal swelling and mating, she restricts copulations to this male (Goodall 1986: 341).
The various costs imposed on female sheep and dung flies are classified as sexual harassment because they are the indirect by-product of female mate discrimination and male competition, which are particularly relevant when mating attempts are made repeatedly or by multiple males (or both). The chimpanzee example highlights aggression designed to reduce female resistance or promiscuity, in this case, to promote future female mating compliance. Harassment and intimidation can operate in either pre- or postcopulatory contexts. For example, mate guarding is a common manifestation of coercion that can precede or follow copulation. It may comprise threats and attacks on the female herself (sensu intimidation) or aggression directed at rival males, thereby imposing indirect mating costs on females (sensu harassment).
Harassment and intimidation are behavioral examples of a general distinction in sexual conflict theory between negative pleiotropic side effects and adaptive harm to females, respectively (Partridge and Hurst, 1998). Many students of sexual conflict maintain that the costs accrued by females are incidental (pleiotropic) byproducts of male mating strategies, selected for not because of, but in spite of the harm to females (Hosken et al. 2003; Morrow et al. 2003; Arnqvist 2004). Conversely, proponents of the adaptive harm hypothesis posit that males benefit from directly harming females, if an existing system of phenotypic plasticity promotes female responses that benefit males (e.g., a female injured by a male may increase her resting time, thereby limiting copulation with other males, or she may invest more in her current offspring due to the harm-induced reduction in her reproductive value) (Lessells 1999; Lessells 2005; Johnstone and Keller 2000).
Of crucial importance for understanding these forms of sexual coercion are data addressing not just the magnitude of costs to females, but also the nature of those costs. There are few such data for primates, but studies of other animals reveal costs in the form of reduced foraging efficiency (Rubenstein 1986; Magurran and Seghers 1994; Pilastro et al. 2003; Erez et al. 2005; Heubel and Plath 2008), increased exposure to predation (Magellan and Magurran 2006), injury and associated increased mortality (Hiruki et al. 1993; Miller et al. 1996; Reale et al. 1996; Mühlhauser and Blanckenhorn 2002), and physiological stress (Moore and Jessop 2003; Shine et al. 2004). These costs are in addition to those associated with suboptimal reproduction due to fertilization by a lower quality male or to lost opportunities for polyandry.
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