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, 281 (1791), 20140836

Gamete Evolution and Sperm Numbers: Sperm Competition Versus Sperm Limitation

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Gamete Evolution and Sperm Numbers: Sperm Competition Versus Sperm Limitation

Geoff A Parker et al. Proc Biol Sci.

Abstract

Both gamete competition and gamete limitation can generate anisogamy from ancestral isogamy, and both sperm competition (SC) and sperm limitation (SL) can increase sperm numbers. Here, we compare the marginal benefits due to these two components at any given population level of sperm production using the risk and intensity models in sperm economics. We show quite generally for the intensity model (where N males compete for each set of eggs) that however severe the degree of SL, if there is at least one competitor for fertilization (N - 1 ≥ 1), the marginal gains through SC exceed those for SL, provided that the relationship between the probability of fertilization (F) and increasing sperm numbers (x) is a concave function. In the risk model, as fertility F increases from 0 to 1.0, the threshold SC risk (the probability q that two males compete for fertilization) for SC to be the dominant force drops from 1.0 to 0. The gamete competition and gamete limitation theories for the evolution of anisogamy rely on very similar considerations: our results imply that gamete limitation could dominate only if ancestral reproduction took place in highly isolated, small spawning groups.

Keywords: anisogamy evolution; evolution of two sexes; sperm competition; sperm limitation.

Figures

Figure 1.
Figure 1.
The threshold number (N − 1) of competing males (dashed line), or risk (p) of a competing male (solid line), for SC to give the same marginal gain as SL in relation to the fertility level in a species, F. For species falling above the line, SC is the dominant pressure increasing sperm numbers, and below the line SL is dominant. These results are derived using a negative exponential fertility function [6,7]. More generally, any concave fertility function ensures that the presence of at least one competing male makes SC stronger than SL (see main text).

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