THE EFFECT OF SPERM COMPETITION ON MALE GAIN CURVES IN A COLONIAL MARINE INVERTEBRATE

Abstract

The shape of the function that describes the relationship between resource allocation to sperm production and resulting male fertilization success (the male gain curve) plays a central role in models of sex allocation in hermaphrodites. A variety of ecological factors are expected to affect the shape of this function. Although male gain curves and male allocation patterns have been assayed in a variety of different organisms, there have been few attempts to determine whether the shape of the gain curve changes with ecological conditions. I assayed male gain curves in experimental populations of a colonial ascidian in which I manipulated the intensity of sperm competition to test the hypothesis that gain curves would saturate in the absence of competitors, but remain linear when males competed for fertilizations. Allozyme markers were utilized for paternity assignment. Results supported theoretical predictions, with the gain curve becoming progressively more linear as the intensity of sperm competition increased. Increased levels of sperm competition also had the effect of decreasing male fertilization success at all sperm production levels. These results have a number of evolutionary implications. First, the existence of a saturating gain curve under some experimental conditions provides support for the hypothesis that diminishing gains in fertilization success via male function may favor the evolution of hermaphroditism. Secondly, the changing shape of this function with ecological conditions suggests that increased allocation to sperm production should be favored in high-density populations, where sperm competition is more intense. Finally, since male fertilization success only continued to increase with sperm production when competitors were present, this suggests that high levels of sperm production in some free-spawning marine invertebrates may be in part due to the selective pressures imposed on allocation patterns by sperm competition.