Costs of group spawning to guarding males in the rainbow darter, Etheostoma caeruleum

The rainbow darter (Etheostoma caeruleum) is a promiscuous fish with a moderate rate of group spawning and sperm competition (1-5 males may simultaneously mate with one female). In any given spawning event, males may spawn as either a guarder or sneaker. Theory indicates that the manner in which individual males behave in sperm competition depends on the expected fitness they receive when spawning in either the guarder or sneaker role. This study uses allozyme markers to compare the fertilization success of males spawning as guarders and sneakers for a common set of eggs. Two points emerge from this study. First, group spawning is costly to guarders in terms of lost paternity. Males spawning as guarders did not achieve 100% paternity. When group spawns involve two males, each male has roughly an equal probability of fertilizing the eggs. Second, male competitive ability is associated with male size. In paired contests, larger males spawn singly with females and spawn as guarding males in group spawnings more often than smaller males. The benefits to competitively superior males may come from single spawns with females where they are assured 100% paternity.