Partial fertility and polyandry: a benefit of multiple mating hiding in plain sight?

Abstract

Multiple mating is common in many socially monogamous birds and mammals, but why is still unclear. Fertility insurance is one the posited genetic benefits. However, this mechanism fails if females cannot detect male infertility before mating, and infertile sperm are equally capable of fertilizing eggs (e.g., fertilization occurs but is followed by early developmental arrest): multiple mating simply exchanges whole brood loss for partial brood loss with no difference in the overall proportion of failed offspring. However, I note here that these two options will rarely result in equivalent reproductive success: partial brood loss will generally result in more surviving offspring than whole brood loss vs. whole brood survival due to increasing per capita survival of offspring in smaller broods. A partial fertility strategy can yield benefits even in the extreme case where fertility is cryptic and defective sperm are capable of fertilizing eggs. Moreover, partial fertility will yield an added benefit to fertility insurance when the precedence of fertile over infertile sperm is incomplete.