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Post-Copulatory Sexual Selection

Post-Copulatory Sexual Selection. The Coevolutionary Battle of the Sexes (Part II). The Myth of Monogamy. Monogamy, polygyny, polyandry Sexual vs. social monogamy Aristotle was among the first to write about polyandry Darwin generally assumed that females were monogamous

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Post-Copulatory Sexual Selection

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  1. Post-Copulatory Sexual Selection The Coevolutionary Battle of the Sexes (Part II)

  2. The Myth of Monogamy • Monogamy, polygyny, polyandry • Sexual vs. social monogamy • Aristotle was among the first to write about polyandry • Darwin generally assumed that females were monogamous • In the 1960s-1970s, it was realized that polyandry abounded in nature

  3. Cooperation or Competition? • Males and females have disparate goals • Males attempt to fertilize as many ova as possible; they are constrained mainly by the number of available females • Females attempt to maintain choice over which male fertilizes their ova • This leads to sexual conflict and coevolution of reproductive traits

  4. Genitalia • Females of many species store sperm • Increases female choice • Relative testis size • Penile-vaginal coevolution (e.g., labia) • “Accessories” • Male bean weevil intromittent organ

  5. Sperm Variation • Sperm length has been selected (direct benefits, intermale competition, counter-adaptations) • Constrained by sperm length vs. numbers • Sperm motility predicts successful fertilization • Speed and duration of “swimming” • Non-nucleate sperm enhance ejaculate “bulk” • Sperm “train” for faster access

  6. Kamikaze Sperm • As noted in your text, Baker & Bellis (1988) report “kamikaze sperm”; however • When sperm were observed killing each other, how do we know that it was between two different ejaculates? • Supposedly good sperm morphs (e.g., “egg getters”) had chromosomal errors • Results could not be replicated

  7. Last-Male Sperm Precedence • The last male to mate with a female tends to secure a disproportionate number of fertilizations • How do several matings affect paternity? • Fair raffle • Loaded raffle • Second ejaculate displacing first • Female internal “pump” displaces first ejaculate

  8. Female Choice • Early conceptualizations of post-copulatory sperm competition assumed female passivity • There has recently been research on cryptic female choice • Female comb jelly (Beroë) nucleus selects among several male nuclei • Diploid ova select for compatibility • Female feral fowl differentially eject sperm from lower-status males

  9. Benefits of Polyandry • Direct benefits • Sperm shortage • Fertility insurance • Gifts (e.g., nutrients) • Parental care • Indirect benefits • Compatibility (e.g., MHC) • Parasite resistance

  10. The Wrap-Up • Myth of monogamy • Competition and coevolution • Variation in genitalia and sperm • Problems with “kamikaze sperm” theory • Last-male sperm precedence • Female choice • Benefits of polyandry

  11. Things to Come • The evolutionary origins and impact of human language • Memetics • Gene-culture coevolutionary models

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