Proximate and evolutionary causes of sexual size dimorphism in the crab spider Mecaphesa celer

Animal species’ body sizes result from the balance between selection for survival and selection for reproduction. In species with sexual size dimorphism (SSD), this balance differs between females and males, resulting in distinct sizes despite similar constraints. I used an integrative approach to understand how sexual section, and differences in developmental trajectories and metabolic physiology, resulted in the female biased SSD of the crab spider Mecaphesa celer (Thomisidae). SSD in spiders is often assumed to be a consequence of selection for early male maturation, which should provide males with additional mating opportunities. My results allow us to discard mate choice and differential fitness benefits as sexually selected drivers of M. celer’s SSD. Interestingly, I found evidence that M. celer females may mate with multiple males, and that, in such instances, eggs are fertilized by a mix of the males’ sperm. Such sperm mixing contradicts the hypothesis that M. celer males benefit from early-male maturation, as early-matured males do not necessarily fertilize most of a females’ eggs. To gain a better understanding of the relation between M. celer’s SSD and early male maturation, I identified the proximate mechanisms underlying differences in size between females and males, as well as the effects of the environment on the degree of SSD. Female M. celer reach larger sizes by growing faster and for longer than males, although both sexes have the same metabolism at rest. Also, female, but not male size, may be influenced by the interaction between diet and temperature. Integrating all of these results, I followed the reproductive season of a population of M. celer in the wild and found evidences that early maturation does indeed grant males with increased mating opportunities. Interestingly, I also found that the timing of male maturation is not proportional to female and male size, challenging the relationship between early maturation and SSD. My research offers a new perspective to the study of sexual dimorphism evolution, highlighting the importance of studying both sexes from an integrative perspective and shedding light on the developmental processes underlying SSD