In strepsirrhines, keratinized penile spines, plates, or papillae are so conspicuous, widespread, and variable as to have long informed taxonomy (Bearder et al. 1996). Similar, but generally simpler, anatomical features are also found in a few platyr-rhines and catarrhines (Dixson 1998). Spines develop upon sexual maturity (Perkin 2007), suggesting testosterone mediation and a mating-related function, but the precise nature of that function remains obscure. Adaptive hypotheses include tactile facilitation of ejaculation, removal of sperm or copulatory plugs, genital locking of partners, stimulation of reproductive readiness in females or of synchrony between partners, and Fisherian female choice (Dixson 1989; Eberhard 1990; Harcourt and Gardiner 1994).
Comparative data from insects suggest an alternative explanation: sexual conflict. In the cowpea weevil (Callosobruchus maculatus), the penis is equipped with spines that damage the female genital tract during copulation, reducing her likelihood of subsequent mating, and thereby enhancing sperm competition outcomes for the male (Crudgington and Siva-Jothy 2000; Hotzy and Arnqvist 2009). In primates, the magnitude of spinosity is negatively correlated with the duration of female sexual receptivity during the ovarian cycle (Stockley 2002), suggesting that penile spines similarly improve male sperm competition success by restricting female mating. The precise mechanism underlying this association is unclear, however. Penile spines could stimulate ovulation or associated neuroendocrine reflexes, but they could also cause "short-term local damage to the female genital tract, making continued sexual activity painful or aversive" (Stockley 2002, p 130).
Correspondingly, sexual conflict theory may shed light on the function of human practices of genital modification (e.g., Wilson 2008). The patterns and frequency of female genital cutting vary substantially across populations, and the effects on female (and male) sexual behavior and reproduction are strongly debated (Gruenbaum 2001). Reason (2004) argues that in one West African population, the practice enhances female reproductive success because it is a virtual prerequisite for marriage and because men invest significantly more in the offspring of wives who are circumcised. Both patterns are consistent with a sexual conflict interpretation, but clearly more study of human behavioral ecology in the context of relevant cultural influences is needed to test this hypothesis against alternative explanations. As Low (2005, p 76) concludes, although current data on genital modification "may not prove [sexual] conflicts of interests, they are suggestive."
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