The focus on sexual selection in reptiles continued yesterday when Ariel Kahrl, a graduate student in Bob Cox’s lab at the University of Virginia, gave her presentation on investment tradeoffs between pre- and post-copulatory sexual selection. She predicted that investment in male-male competition (an important component of pre-copulatory sexual selection) constrains an organism’s ability to invest in post-copulatory selection such as sperm competition. To test this hypothesis across squamates, Ariel performed a meta-analysis using data from 143 species (111 lizards and 32 snakes), and she reported her results in the DCPB (Division of Comparative Physiology and Biochemistry) Wake Award competition for Best Student Presentation. Using male-biased sexual size dimorphism (SSD) as a proxy for pre-copulatory selection and relative testis size as a proxy for sperm competition, Ariel found that both these traits were negatively correlated across the group – that is, species with relatively larger male body size had relatively smaller testes. In other words, in species where males invest more in access to females, the less they invest in sperm! This pattern was strongest among territorial lizards (like anoles). So, it appears that there are significant trade-offs between pre- and post-copulatory selection in squamates.
Ariel is following up on this work with a large-scale comparative analysis of sperm morphology in anoles, to determine if the interaction between pre- and post-copulatory selection shapes sperm evolution. She ended her talk showing how impressively variable sperm traits are among anoles – perhaps a sneak peek for her talk next year?