Multiple mating by females is a natural outcome of a null model of mate encounters

Why do females of so many species mate multiply? The question makes use of an implicit null model that females by default should be monandrous and that polyandry requires an explanation. Here, we make the simple point that females encounter mates over their lifetime in a stochastic manner, and as they should accept at least one male, acceptance of all males may be a better null model than the more advanced strategy of accepting the first satisfactory one and rejecting all others. The advantage of this view is that it makes it explicit that females must accept and reject mates without precise knowledge of future mate encounters. In insects, for example, limitations of cognitive and sensory capabilities make it hard for females to compare many potential mates simultaneously. It is then not always possible for a female to be very choosy (i.e., to reject a large proportion of encounters) without simultaneously increasing the expected time spent as a virgin and decreasing the overall expected number of mates she accrues during her lifetime. We show that this fact easily leads to a pattern where choosiness is reduced and most females mate with more males than their optimal mate number. Our results suggest that monandry and polyandry may be less distinct strategies than they first appear as they may, to a large extent, reflect chance events influencing mate encounters. Polyandry can arise as a side effect of avoiding the risk of encountering too few acceptable mates - a viewpoint that is easily missed if females that have remained unmated are not included in datasets.