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

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 The Myth of Monogamy

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

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

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

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

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

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 Female Choice

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

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

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