Drivers of female and male body size in the Aegean wall lizard, Podarcis erhardii

Abstract

Biologists have noted since the days of Darwin and Wallace how island populations of vertebrates differ in body size from their mainland relatives. While much progress has been made in understanding global patterns in body size evolution across different species of wildlife (Benítez-López et al., 2021) scientists still don’t have a good understanding of the ecological and environmental processes that generate these patterns. While many studies concerning vertebrate body size evolution have been performed, a hypothesis-driven approach that concerns life history differences in females and males is lacking, and it is not even known whether any putative factors influence female and male body size in the same way. We examined 36 populations of Aegean wall lizards (Podarcis erhardii, Lacertidae), a species widely distributed across the Balkan mainland and hundreds of Aegean islands. The species is unusual in that it not only occurs in a broad range of ecological conditions, but also that it varies substantially in body size (Males: 37%; Females: 33.9%) and that it occurs in numerous, clearly isolated and well-characterized island populations. It therefore constitutes an excellent model for determining the drivers of island body size across varying island environments. Here we evaluate multiple hypotheses ranging from predation pressure (predator species richness), resource availability (Normalized Difference Vegetation Index (NDVI), seabird density, island area), and competition for mates (lizard density) as possible factors affecting lizard body size. We found that the factors affecting both female and male lizard body size depend on the presence of nesting seabird colonies on an island. Female body size on seabird islands was most strongly associated with seabird colony density, while male body size

on seabird islands was only associated with conspecific lizard population density, suggesting a role for seabird subsidies and increased food resources for the evolution of female body size and intraspecific competition in the evolution of male body size. On non-seabird islands, female body size was positively correlated with island size, while male body size was not significantly associated with any of the factors we measured. This suggests that different factors were associated with female and male body size evolution in island populations of P. erhardii, and we propose that a general pattern like the island rule is subject not only to selective forces that are sex-specific, but also that there will important differences dictated by island ecology.