Sperm Competition in a Viviparous Fish

We investigated multiple paternity and sperm precedence in the Amarillo fish, Girardinichthys multiradiatus (Goodeidae). We allowed females to mate with two different-sized males consecutively and assessed the paternity of the ensuing broods using allozyme electrophoresis. We presented one-half of the females the larger, and the other half the smaller, male first. Allozyme variation among individuals was low, yielding conservative estimates of multiple paternity. Half the broods were of mixed paternity, but one male always sired more than 70% of the embryos in each brood. The proportion of the brood sired was not related to mating sequence, but when we classified males by relative size, the larger male of each pair usually fathered greater proportions of offspring than the smaller male. This association disappeared when we used the actual size of the males in the analysis. Instead, for any pair of males, the difference in number of offspring sired was correlated to differences in the rate of courtship displays, rather than size differences, suggesting that courtship intensity is a better predictor of paternity than male size. Within a pair, the larger male usually displayed more than the smaller one, but there was no correlation between male size and display rate across all males. Parsimony suggests a correlation between courtship rate and sperm production, but we cannot rule out the possibility that females allocate paternity according to the relative merits of the males.