Abstract

1. Sexual size dimorphism (SSD) can allow males and females of the same species to specialize
on different sized food items and therefore minimize intraspecific competition.
2. Interspecific competition, however, is thought to limit sexual dimorphism, as larger competitors
in the community will prevent the larger sex from evolving larger size, and smaller
species may prevent the smaller sex from becoming even smaller.
3. We tested this prediction using data on the sexual size dimorphism of lizards, and mammalian
carnivores, on islands world-wide.
4. Because insular communities are depauperate, and guilds are species-poor, it is often
assumed that enhanced sexual size dimorphism is common on islands. The intensity of interspecific
competition, hindering enhanced dimorphism, is thought to increase with competitor
richness.
5. We tested whether intraspecific sexual size dimorphism of mammalian carnivores and lizards
decreases with increasing island species richness. We further computed the average sexual
dimorphism of species on islands and tested whether species-rich islands are inhabited by relatively
monomorphic species. Within families and guilds across carnivores and lizards, and
with both intraspecific and interspecific approaches, we consistently failed to find support for
the notion that species-poor islands harbour more sexually dimorphic individuals or species.
6. We conclude that either interspecific competition does not affect the sexual size dimorphism
of insular lizards and carnivores (i.e. character displacement and species sorting are
rare in these taxa), or that the number of species in an assemblage or guild is a poor proxy
for the intensity of interspecific competition in insular assemblages.