Proximate causes of sexual size dimorphism in the iguanian lizard Microlophus occipitalis

Among iguanian lizards, males are often larger than females. As a result, much attention has been directed toward understanding the evolution of sexual size dimorphism (SSD) in this group. SSD can result from a number of developmental, physiological, and demographic processes, which may cause sex differences in growth trajectories, timing of maturity, and adult age distributions. Knowledge of these proximate mechanisms can improve our understanding of the evolution of SSD. Only recently, however, have efforts been directed toward understanding proximate mechanisms. I examined the proximate causes of SSD in a population of a South American iguanian lizard, Microlophus occipitalis. The majority of the SSD in this population resulted from continued postmaturity growth in males and reduced postmaturity growth in females. Additionally, adult males tended to live longer than females, which affected adult age and size distributions. The proportion of young individuals in samples had strong effects on temporal fluctuations in SSD in the population. I discuss the importance of studying proximate mechanisms for evolutionary analyses of SSD in organisms that continue to grow after reaching maturity.