Sperm-limited males save ejaculates for future matings when competing with superior rivals

Abstract

Adjusting ejaculates to sperm competition can lead to sperm limitation. Particularly in polygynous species, males may face a trade-off between investing sperm in current or future mating opportunities. The optimal sperm allocation decision should depend on the relative intensity of sperm competition experienced in a mating sequence. Here we asked how males respond to this trade-off in polygynous fish with alternative male mating tactics, intense sperm competition and sperm limitation. Large bourgeois males of the shell-brooding cichlid Lamprologus callipterus build nests consisting of empty snail shells, in which females spawn and raise offspring. During spawning, nest males release ejaculates into the shell opening. Genetically distinct, parasitic dwarf males enter shells during spawning to fertilize the eggs from inside the shell. These dwarf males were previously shown to be superior sperm competitors to nest males. Here we showed that when spawning with several females simultaneously, nest males reduced the spawning duration for each clutch and the number of ejaculations per female tended to decrease, reflecting sperm limitation. Experimental exposure of nest males to sperm competition with dwarf males reduced the number and duration of ejaculations by roughly half. Hence, when exposed to competition with a superior rival, nest males did not increase their sperm expenditure as predicted by sperm competition risk models, but in fact saved sperm for future mating opportunities as predicted by sperm competition intensity theory. This seems to be adaptive because of the considerable sperm demands in this species, which is partly due to their high degree of polygyny.