The evolution of sexual size dimorphism in squirrels

Abstract

Difference between the sizes of the sexes in a species (sexual size dimorphism SSD) is a common phenomenon, and most mammals are male-biased in size. Female-biased size dimorphism (FBSD) is uncommon in mammals. Flying squirrels exhibit a unique lifestyle (gliding), which could require females to evolve larger size to attain functional or reproductive advantages. In this dissertation I examined the evolution of FBSD in flying squirrels using comparative approaches. I began by examining all squirrels and their patterns of dimorphism to determine if FBSD evolved in association with gliding. FBSD was seen to evolve multiple times across squirrels, and the ancestor to flying squirrels was male-biased, implying that FBSD evolved in association with a gliding lifestyle. An analysis of ecological predictor variables revealed that FBSD was predicted by arboreal habits, nocturnality and tropical latitudes, and that extreme MBSD was predicted by open habitats and sociality. In a second analyses, I examined body size partitioning across two sympatric squirrel communities, temporal and diurnal, in Borneo. Species and sexes partitioned themselves along a non-overlapping size spectrum, implying that species evolved specific sizes to minimize competition. SSD does not seem to enhance partitioning of resources. Finally, I examined patterns of scaling (Rensch’s rule) of FBSD across 27 flying squirrel species based on morphometric measurements from museum specimens. Flying squirrels were seen to scale in accordance with Rensch’s rule, which predicts that FBSD decreases with increasing body size across related species. Females had relatively longer tails and larger heads than males, indicating that selection for enhanced gliding ability may have resulted in FBSD. Within flying squirrels, small-bodied gliders with reduced patagia had more compensatory morphological adaptations than large bodied gliders with extended patagia. Comparative phylogenetic analyses revealed that patterns of gliding morphostructure and dimorphism are deeply rooted within the phylogenetic history of this tribe.