Anisogamy explains why males benefit more from additional matings

Abstract

Why do males typically compete more intensely for mating opportunities than do females and how does this relate to sex differences in gamete size? A new study provides a formal evolutionary link between gamete size dimorphism and ‘Bateman gradients’, which describe how much individuals of each sex benefit from additional matings.

Fig. 1. Bateman gradients of mutant males (blue) and females (yellow) in the three models of Lehtonen.

The structure of each model is outlined in Fig. 2. Fertilisation is either efficient (A, C, E) or inefficient (B, D, F). The resident number of matings is indicated by the dashed red lines. Under most circumstances, a mutant male’s fitness increases more steeply than a mutant female’s fitness with its number of matings (A–E). However, under inefficient internal fertilisation with a low anisogamy ratio (i.e. few sperm for each egg), Bateman gradients can theoretically reverse, so that female fitness increases more steeply with the number of matings (F). In this illustration, females produce ten eggs each and males produce one hundred sperm. Resident individuals are monogamous (Model 1), participate in two spawning groups (Model 2), or mate twice (Model 3). Fertilisation efficiency is given by a = 1 (‘efficient fertilisation’) or a = 0.01 (‘inefficient fertilisation’) (see ref. for parameter definitions).

Fig. 2. Structure of the three models of Lehtonen, showing differences in mating behaviour between resident males (green), resident females (blue) and mutant males and females (both yellow).

For illustration, we suppose that females produce four eggs each and males produce eight sperm (the anisogamy ratio in nature is typically much higher). In Model 1, resident individuals spawn monogamously in a ‘nest’ (black outline), whereas mutant males and females can bring additional partners to their nest to spawn in a group. In Model 2, resident individuals divide their gametes equally among m spawning groups, each consisting of m individuals of each sex (shown here with m = 2). Mutant males and females instead divide their gametes among a larger or smaller number of groups, m_mutant (shown here with m_mutant = 4). In Model 3, there is a further sex asymmetry in addition to anisogamy: Fertilisation takes place inside the female’s body. Resident individuals mate with m partners (shown here with m = 2), whereas mutant males and females mate with a larger or smaller number of partners, m_mutant (shown here with m_mutant = 4).