Forced Copulation

This form of sexual coercion involves the physical restraint and forcible insemination of resistant females. Among primates, forced copulation has been noted occasionally in several species (chimpanzees Tutin 1979; patas monkeys Chism and Rogers 1997; spider monkeys Gibson et al. 2008), but it is regularly observed in only two species, the orangutan (Pongo pygmaeus) (van Schaik and van Hooff 1996) and Homo sapiens (Smuts 1992; Goetz et al. 2008).

Although forced copulation occurs in a number of different taxa (Table 3.1), it is a less common form of sexual conflict than harassment or intimidation. This may be because forced copulation is only possible under a restricted set of conditions, such as when males are much larger than females (Clutton-Brock and Parker 1995) or when females are isolated and unable to obtain social support. However, neither of these factors provides an entirely satisfactory explanation for the distribution of forced copulation in primates. Although the orangutan is a strongly dimorphic

Table 3.1 Potential forms of (interlocus) sexual conflict

Context

Category

Nature of sexual conflict

Example taxa3

Reference3

Precopulatory Forced copulation

Harassment, indirect costs of mating or mate guarding

Catch and physically restrain female followed by forced insemination; incl. anatomical specializations to grasp and prevent escape of female prior to forced insemination Repeated, persistent courtship or copulation (attempts), by single or especially multiple males; physical aspects of courtship or copulation (e.g., posture, inexperienced males)

Male dominance displays Males target females in aggressive

Pongo pygmaeus, Homo sapiens, Anseriform birds, some insects

Insects, Fish, Anurans, Snakes, Artiodactyls

Microcebus murinusl

Papio hamadtyas griseipes, P. h. ursinus

Postcopulatory (Prezygotic)

Sexual intimidation/ punishment

Seminal fluid proteins

Non-fertile sperm Reproductive tract injury

Genital plugs, coagulates displays that function in acquisition and/or maintenance of intragroup dominance status or intergroup spacing

Aggression to (estrus) females that refuse Primates to associate or copulate with male, or that associate or copulate with other male(s)

Proteins beneficially affect outcomes of Insects, Nematodes sperm competition for males, while imposing costs upon female viability and/or reproduction Anucleate sperm reduce female receptivity to subsequent mating Male-induced changes/injury of female genital tract, typically during copulation, results in decreased sexual interaction Seminal coagulates may: improve sperm transport, reduce sperm loss, physically block intromission by other males, and/or physiologically induce female refractory period

Insects

Insects, Rodents, Strepsirrhines?

H. sapiens?

Insects, Rodents, Primates

Thornhill and Alcock 1983; Gowaty and Buschhaus 1998; Bertin and Fairbairn 2005; Siva-Jothy 2006; Vahed and Carron 2008, Knott in press Howard 1980; Clutton-Brock and Parker 1995; Reale et al. 1996; Arnqvist and Nilsson 2000; Shine et al. 2000; Eberle and Kappeler 2004a; Bowcock et al. 2009

Henzi et al. 1998; Kitchen et al. in press

Smuts and Smuts 1993; Clutton-Brock and Parker 1995, see text

Gems and Riddle 1996; Holland and Rice 1999, see text

Cook and Wedell 1999

van der Schoot et al. 1992, Crudgington and Siva-Jothy 2000rBlanckenhorn et al. 2002; Stockley 2002; Low 2005 Matthews and Adler 1978;

Simmons and Siva-Jothy 1998; Dixson and Anderson 2002

CT"

Genital lock

Postcopulatory (Postzygotic)

Sexual intimidation/ punishment (mate guarding)

Egg-sperm interaction

Feticide

Sexually selected infanticide

Parental investment & genomic imprinting

"Policing"

Prevention of penis removal from female reproductive tract for prolonged period following ejaculation; due to genital clasping structures or partial enlargement of penis and vaginal adhesion

Temporary male association with an inseminated female for a prolonged period following ejaculation to aggressively prevent subsequent mating by female Genes of sperm and egg differentially influence processes surrounding capacitation, penetration of egg, and fertilization

Male harassment (or forced copulation) of pregnant female promotes or induces spontaneous abortion of implanted zygote or fetus

Killing of dependent infants to prematurely end lactational amenorrhea and return females to fertilizable (estrus) state Activity of genes depends upon sex of parent from which inherited (e.g., paternally derived genes induce disproportionately greater maternal investment in offspring) Male intervenes to curtail female-female aggression, mitigating or eliminating benefits a "winner" could derive via individual or coalitionary competitive superiority

Insects, Galago crassicaudatus. Macaca arc toi des

Insects, Primates

Invertebrates, Fish

Equids, Primates

Primates, Fissiped carnivores, Toothed whales Rodents

Gorilla gorilla subspecies, Pan troglodytes

Thornhill and Alcock 1983; Dixson 1998; Werner and Simmons 2008

Eberle and Kappeler 2004b; Sato and Kohama 2007

Rice and Holland 1997; Levitan 2008; Martin-Coello et al. 2009

Berger 1983; Pereira 1983; Sommer 1987; Agoramoorthy et al. 1988; Pluhácek and Bartos 2000

van Schaik 2000a

Keverne 2001; Roulin and Hager 2003

Boehm 1994; Watts 1997; Sicotte 2002; Stokes 2004; Harcourt and Stewart 2007

aTaxa and references list are not exhaustive, but rather represent illustrative examples species, forced copulation is frequently done by small males, who are either subadults or "unflanged" adults with arrested development of secondary sexual characters (Knott 2009). Moreover, in many strongly dimorphic monkeys, males do not exhibit the behavior at all. Social isolation may increase vulnerability to forced copulation. In contrast to the vast majority of highly gregarious anthropoid primates, female orangutans are often alone (Rodman and Mitani 1987). Humans are not solitary, but Emery-Thompson (in press: 361) argues that college-age women experience the highest rate of rape in the United States partly because "they are the group most likely to be living away from natal kin but not yet with a domestic partner." However, social vulnerability does not explain why forced copulations are so rare in chimpanzees (0.2% of the copulations observed by Tutin (1979)) even though females typically disperse from their natal communities and spend much time alone. Possible explanations for the rarity of forced copulation in chimpanzees are female influence on male dominance relations (Stumpf and Boesch 2006) or simply the effectiveness of male sexual coercion in generating mating opportunities (see below), which reduces the benefits of physical restraint and forcible insemination.

Forced copulation in orangutans is commonly considered part of an alternative reproductive strategy of unflanged adult males. The males avoid direct mating competition with large, flanged males by retarding development of secondary sexual traits and relying on force to copulate with uncooperative females that generally prefer flanged males as mates (van Schaik and van Hooff 1996; Atmoko and van Hooff 2004; Maggioncalda et al. 1999). Knott (2009) argues, however, that since forced copulation is not restricted to unflanged males, it is better viewed as a general male orangutan strategy to overcome female resistance. Both models are consistent with sexual conflict arguments that forced copulation in nonhuman animals is an alternative mating strategy (Table 3.1).

Thornhill and Palmer (2000) have similarly proposed the controversial hypothesis that human rape reflects an alternative strategy of low-status, socially disadvan-taged males to obtain conceptions. Emery-Thompson (2009) rejects this argument on several grounds, including observations that a substantial majority of rapes are perpetrated by men casually or intimately known to their victims (acquaintance rape) and that women often continue their relationships with these attackers. Thus, she contends instead that rape is one of several forms of sexual aggression used by men to maintain long-term reproductive access to female mates. Emery-Thompson has shifted the functional focus from immediate copulatory benefits (as in orangutans) to prospective reproductive gains via intimidation and punishment (see below). Again, both hypotheses are based on sexual conflict.

It is important to recognize that forced copulation in humans is an extremely heterogeneous phenomenon (Travis 2003). Some cases of rape may originate in pathological behavior (such as "stranger rape") (Emery-Thompson 2009) or in male tactics of terror and control (e.g., violent rape in the context of warfare; Swiss and Giller 1993). Thus, although a comprehensive understanding of rape in humans will no doubt involve an array of processes and factors, sexual conflict theory seems likely to improve understanding of some forms of the behavior (Emery-Thompson 2009).

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