Table 1.

Some commonly proposed costs and benefits of multiple mating.

sex	benefits	costs
male	Darwin–Bateman effect—male fitness increases steeply with number of matings with different females due ultimately to anisogamy	ejaculate expenditure, plus any subsequent paternal care increased risks (e.g. predation, disease) during courtships, copulations, etc. increased time or energetic costs owing to courtship, mating, mate-guarding, etc.
female	increased proportion of eggs fertilized as sperm numbers increase insurance against a prior mate being sterile increased number of offspring produced increased genetic diversity of offspring permits post-copulatory mate choice reduced cost of male coercion: persistence and harassment paternity confusion—protection against infanticide	increased risks (e.g. predation, disease) during courtships, copulations, etc. increased time or energetic costs increased harassment or other costs of male–male competition