Meeting Abstract
During postcopulatory selection, sperm competition may select for a variety of ejaculate traits, including sperm morphology, count, and sperm function. Because ejaculate production is energetically expensive, tradeoffs (e.g. sperm size vs. sperm number) may generate correlated evolution of these traits. By examining testis size and sperm morphology together in a phylogenetic framework, we can better understand the processes that generate the extreme variation in sperm morphology we see among species. We collected data from the field and the literature on sperm morphology (sperm head, midpiece and tail length), testis size, and body size for 81 species of snakes and lizards. Using phylogenetic least squares regressions, we found a strong positive correlation between testis size and midpiece length, and a marginal negative correlation between body size-corrected testis size and tail length. When we examined snakes and lizards separately, we found differing patterns between the groups, such that snakes exhibited a strong positive correlation between midpiece length and testis size, whereas there were no correlations between sperm morphology and testis size in lizards. To understand how these traits evolve as a suite of traits, we tested for differences in the Brownian rates of evolution of testis size, body size, and sperm morphology in snakes and lizards separately. We found that, relative to other traits, body size-corrected testis size and midpiece size evolve faster in snakes than in lizards. This suggests that the midpiece may be more may be under stronger selection in snakes than in lizards, potentially due to differences in the mating systems and reproductive physiology between the groups.
