Meeting Abstract
Theory predicts that males should invest more in ejaculate production when the likelihood of sperm competition is high, thereby increasing the chance of fertilization. However, ejaculates can be energetically costly, and increased investment into sperm production should only occur if there are fitness benefits associated with that increased investment. Growing experimental evidence suggests that sperm traits respond plastically to social environment. However, it is not known whether fine-scale spatial variation in the local density of male competitors or potential female mates corresponds to individual variation in ejaculate production. Using a wild population of brown anoles (Anolis sagrei), we tested the prediction that, as the risk of sperm competition increases (i.e., higher local density of male competitors), males will increase their total investment in their ejaculates (sperm size * sperm count). We also tested for correlations between sperm morphology and local density. We estimated the local density of male competitors and potential female mates for 202 individual males by taking into account all conspecific adults that were captured within a 5.8 m radius of an individual’s own capture location. We found that length of the sperm midpiece increased with local density, whereas length of the sperm head and sperm count decreased with local density. Contrary to our predictions, we found that total investment in ejaculates decreased with local density. Our findings indicate that fine-scale differences in local density within a wild population can affect sperm count and various sperm phenotypes.
