Body size and growth in tropical small mammals: examining variation using non-linear mixed effects models

Abstract

A diverse small mammal fauna inhabits the tropical savannas of Kakadu National Park in northern Australia. A detailed long-term capture–mark–recapture study was conducted to estimate body growth parameters in relation to intrinsic and extrinsic factors that may influence them. Three non-linear growth increment functions were compared to determine which one best represented the general growth patterns of nine small mammal species at Kapalga. The von Bertalanffy growth increment function provided the best fit to a pooled species dataset. Hierarchical mixed-effects models were then used to examine variation in body size asymptote and growth and account for non-independence of observations. Model selection was based on the Kullback–Leibler information theoretic approach. As expected, the body size growth constant was correlated to body mass at Kapalga, and body size sexual dimorphism was evident in most species. Difference in body growth constant was only evident in Isoodon macrourus, where females showed accelerated growth. Three of the four marsupial species exhibited either temporal (annual) and spatial (catchment level) variation in body size asymptote, whereas rodent species showed none. There was no clear influence of other extrinsic factors such as fire treatment, rainfall or habitat on body size asymptote or growth rate constant. Our results suggest that body growth is largely controlled by intrinsic factors for each species such as gender in small mammal species at Kapalga. Change in body size and growth caused by human-induced changes to the landscape from factors such as altered fire regimes is unlikely to be substantial for tropical small mammals in northern Australia.