Growing apart together: The development of contrasting sexual size dimorphisms in sympatric Sceloporus lizards

Comparative biologists often conceptualize sexual size dimorphism (SSD) as a static characteristic of adult populations, but recent work has emphasized that SSD reflects a developmental process in which males and females "grow apart" in body size. This ontogenetic perspective requires knowledge of (1) the demographic factors that give rise to SSD (e.g., differential survival, migration, or growth) and (2) the ontogenetic timing of these sexual differences (e.g., juvenile, maturational, or adult divergence). Together, such data help formulate testable hypotheses concerning proximate physiological mechanisms responsible for the development of SSD. To illustrate this approach, we present a case study of two sympatric lizard congeners with opposite patterns of SSD (Sceloporus virgatus: female-larger; S. jarrovii: male-larger). Using mark-recapture data, we show that (1) sex differences in survival and migration cannot account for SSD, and (2) both nonlinear growth models and age-specific linear growth rates identify sexually dimorphic growth as the cause of SSD in each species. SSD develops in S. virgatus because females grow more quickly than males, particularly during the spring mating season. By contrast, SSD develops in S. jarrovii because males grow more quickly than females throughout the first year of life, particularly in association with vitellogenesis and gestation in females. Thus, opposite developmental patterns of SSD in these species may reflect underlying differences in energetic trade-offs between reproduction and growth.