Sperm length variation in House Wrens Troglodytes aedon

It is well documented that sperm size and structure varies considerably among avian species, but we know much less about the extent of intraspecific variation in sperm morphometry and its possible co-variation with somatic traits like body size and condition. Here, we investigate patterns of sperm length variation and co-variation in a population of House Wrens (Troglodytes aedon). Total sperm length showed considerable between-male variation, with high repeatability between seasons indicative of a strong genetic basis for this trait. However, we also detected a seasonal increase in the flagellum:head length ratio, which might indicate phenotypic plasticity or adjustment in the relative size of sperm components. The variation in total sperm length within an ejaculate sample was higher for males sampled very early in the season, which may reflect more heterogeneity in the size of seminiferous tubules when testes are growing. None of the studied sperm morphometry traits correlated significantly with any measures of male body size or physiological condition. Further studies are needed to reveal if the observed individual variation in sperm morphology plays any functional or adaptive role.