Gamete evolution and sperm numbers: sperm competition versus sperm limitation

Area of Science:

• Evolutionary biology
• Reproductive strategies
• Sexual selection

Background:

• Anisogamy (differing gamete sizes) may evolve from isogamy (equal gamete sizes) due to gamete competition or limitation.
• Both sperm competition (SC) and sperm limitation (SL) can drive increases in sperm production.

Purpose of the Study:

• To compare the marginal benefits of sperm competition (SC) versus sperm limitation (SL) across different population sperm production levels.
• To utilize sperm economics models, specifically risk and intensity models, to analyze these benefits.

Main Methods:

• Analysis using the intensity model, where N males compete for a clutch of eggs.
• Analysis using the risk model, examining the probability of sperm competition.
• Mathematical modeling of fertilization probability (F) as a function of sperm numbers (x).

Main Results:

• In the intensity model, marginal gains from SC exceed those from SL when at least one competitor is present (N-1 ≥ 1) and fertilization probability is a concave function of sperm number.
• In the risk model, the threshold probability of SC for it to be dominant decreases from 1.0 to 0 as fertility increases from 0 to 1.0.
• Gamete limitation is unlikely to be the primary driver of anisogamy evolution except in highly isolated, small spawning groups.

Conclusions:

• Sperm competition is generally a stronger evolutionary force than sperm limitation in driving sperm production.
• The conditions favoring gamete limitation as the dominant factor in anisogamy evolution are highly restrictive, requiring small, isolated reproductive groups.