THE EVOLUTION OF SPERM SIZE IN BIRDS

Sperm size varies enormously among species, but the reasons for this v ariation remain obscure. Since it has been suggested that swimming vel ocity increases with sperm length, earlier studies proposed longer (an d therefore faster) sperm are advantageous under conditions of intense sperm competition. Nonetheless, previous work has been equivocal, per haps because the intensity of sperm competition was measured indirectl y. DNA profiling now provides a more direct measure of the number of o ffspring sired by extrapair males, and thus a more direct method of as sessing the potential for sperm competition. Using a sample of 21 spec ies of passerine birds for which DNA profiling data were available, we found a positive relation between sperm length and the degree of extr apair paternity. A path analysis, however, revealed that this relation ship arises only indirectly through the positive relationship between the rate of extrapair paternity and length of sperm storage tubules (S STs) in the female. As sperm length is correlated positively with SST length, an increase in the intensity of sperm competition leads to an increase in sperm length only through its effect on SST length. Why fe males vary SST length with the intensity of sperm competition is not c lear, but one possibility is that it increases female control over how sperm are used in fertilization. Males, in turn, may respond on an ev olutionary time scale to changes in SST size by increasing sperm lengt h to prevent displacement from rival sperm. Previous theoretical analy ses predicting that sperm size should decrease as sperm competition be comes more intense were not supported by our findings. We suggest that future models of sperm-size evolution consider not only the role of s perm competition, but also how female control and manipulation of ejac ulates after insemination selects for different sperm morphologies.