Eco-geographic variation in size and sexual dimorphism in sugar gliders and squirrel gliders (Marsupialia: Petauridae)

Geographic variation iri body size and sexual dimorphism, as determined by measurements of condylo-basal length, was investigated in the sugar glider (Petaurus breviceps) and the squirrel glider (P. norfolcensis). Correlation and multiple regression analyses were employed to determine whether geographic or climatic variables accounted for more of this size variation. The effects of age and sex were removed from analyses prior to applying statistical techniques. Numerous geographic and climatic variables were correlated with size variation in both species. Both species followed a clinal change in body size consistent with Bergmann's rule (i.e. both species were larger in the south of their ranges where temperatures are colder). One geographic variable, latitude, and three climatic variables representing temperature, precipitation and seasonality, were then selected for multiple regression analyses. Latitude accounted for more of this size variation (20-28%) in P. breviceps than climatic variables in four multiple regression models (considering two age and two sex classes). This result indicated that an isolation-by-distance model was operating in this species which was attributed to the oceanic barriers between the Australian mainland and New Guinea and Tasmania, causing genetic differentiation between isolated populations. Once latitude was removed from the analyses, temperature accounted for more of the variation (18-24%) in body size in three regressions, whilst precipitation (11%) contributed significantly to the remaining model. This result was interpreted as an adaptation to ambient temperature following Bergmann's rule. When using both geographic and climatic variables, latitude accounted for more size variation (47-69%) than climatic variables in two regressions for P. norfolcensis, whilst seasonality accounted for more variation (26-46%) in the remaining two regressions. When latitude was excluded From the analyses, seasonality (body size decreases with increasing seasonality) accounted for more variation in size in three of four regressions (26-46%), whilst precipitation (60%) accounted for the most variation in the fourth regression. These findings for P. norfolcensis suggested that large body size may be an advantage in aseasonal environments where climates and therefore foods are less predictable. Latitude and precipitation both contributed significantly to the degree of sexual dimorphism exhibited across the range of P. breviceps, suggesting that an isolation-by-distance model and primary productivity account for some of the change in sexual dimorphism in this species. Both these variables were more important than temperature and average sexual dimorphism was greater in the tropics. The relationship with primary productivity implies that in areas where food is more abundant, males attempt to grow larger in order to enhance fighting ability for access to food and mates. In contrast, females channel extra energy towards offspring production, not body size, in order to minimise energy costs during reproduction. Character displacement did not appear to influence body size variation in the two Petaurus species.