Morphological variation within Australian populations of the house mouse: an observational and experimental approach

Morphological variation within Australian populations of the house mouse ('Mus domesticus') was evaluated using measurements of head-body length and relative tail length. In order to examine the effects of temperature on these characters, and to attempt to experimentally replicate this variation, mice from one of the wild populations were raised under three different temperature regimes in captivity (13 ± 2°C, 22 ± 1°C and 30 ± 1°C). There was considerable variation between wild populations in both head-body length and relative tail length. Comparisons with estimated values of mean annual temperature at study locations found that mice were smaller in cooler climates (opposing Bergmann's Rule), but no real climatic trends were found for tail length. In contrast, mice bred at different temperatures within the laboratory displayed very large differences in morphology, with those raised at the warmest temperature being the smallest in size and having relatively longer tails. These results corresponded with both Bergmann's Rule and Allen's Rule. The differences between the treatments were more apparent in younger mice (6 weeks of age) than those at 12 weeks and 24 weeks. It is apparent that just a few degrees difference in temperature has a major effect on the morphology of house mice. However, comparison with the wild populations shows that many other factors must have a role in shaping mouse morphology. The research presented in this paper and continuing work on internal organ systems has major implications for taxonomy and adaptation of house mice, and could help explain the success of mice in colonising the wide range of habitats within Australia and other parts of the world.