Determinants of body-size variation within Australian mammal species

Author: Rachel Correll

Correll, Rachel, 2016 Determinants of body-size variation within Australian mammal species, Flinders University, School of Biological Sciences

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Body size is arguably the most important character affecting the morphology, life history, physiology, behaviour, ecology, evolution and extinction probability of animal species. Spatial and temporal patterns of body-size variation have been well documented in many endotherms, particularly mammals. The most familiar pattern of body-size variation is Bergmann’s rule, which posits that, within endotherms, larger-bodied forms are found in cooler regions. Several hypotheses centring around thermoregulatory response, primary productivity (food availability) and seasonality, have been advanced to explain geographic body-size patterns, including Bergmann’s rule. The relative importance of various drivers within different taxa and regions has been the topic of much debate. To a significant degree this is due to biases in sampling, comparisons of varying geographic scales, and a lack of consideration of the potential impacts of, and interactions between, different potential drivers.

This study uses measurements from around 5,000 specimens representing seven Australian mammal species with continental distributions to examine spatial body-size patterns and explicitly test each of the major hypotheses typically advanced to explain such patterns. Highly correlated environmental variables were dealt with, in part, a) by comparing ‘aspatial’ regression models as well as spatial autoregression models that accommodate both single-cause and multi-causal explanations for spatial body-size variation within a species, b) using model selection procedure based on information criteria (AIC/DIC) to tease apart the best-supported body-size drivers, and c) excluding highly correlated environmental variables from the same fitted model. Moran’s I spatial autocorrelation coefficients showed that spatial models were less spatially autocorrelated than the aspatial models. Thus, focus was placed on the spatial models. Spatial autoregression coefficients also indicated that environmental drivers included in those studies could only account for some of the spatial pattern in the data. Bayesian models were used to impute missing sex data, fit non-linear growth models and account for non-random spatial sampling methods.

Bergmann’s rule, as a generalised pattern, is revealed within Australian marsupials that occupy a broad latitudinal / temperature range. Primary productivity and thermoregulatory requirements are evidently key counterparts in driving body-size evolution in the Common Brushtail Possum, Trichosurus vulpecula, and species of kangaroo (genus Macropus). These findings are consistent with a recent review (Yom-Tov & Geffen 2011), which concludes that food availability and ambient temperature are the principal predictors of body size. This thesis highlights the importance of multi-causal variables responsible for spatial body-size variation; i.e., body size is not driven by a single mechanism. Thus, hypotheses explaining body-size variation in endotherms should not be viewed as mutually exclusive.

Life-history theory predicts the progressive dwarfing of animal populations that are subjected to chronic mortality stress, but the evolutionary impact of harvesting terrestrial herbivores has seldom been tested. In Australia, marsupials of the genus Macropus (kangaroos and wallabies) are subjected to size-selective commercial harvesting. Contrary to expectation, skull measurements from wildlife collections spanning the last 150 years demonstrate a slight increase in body size in the Eastern and Western Grey Kangaroos, Macropus giganteus and M. fuliginosus, Red-necked Wallaby, M. rufogriseus, and Common Wallaroo, M. robustus. This may be in response to increased food and water availability due to European pastoral activities. Little evidence of temporal body-size change occurs in the Red Kangaroo, M. rufus, over this time period.

The island rule is a recognised pattern of body-size evolution, most often noted in mammals, where larger species are held to become smaller bodied and smaller species larger on islands. However, recent research has suggested that the island rule may not hold in most mammal groups. Using body-size measurements from mainland and island populations of T. vulpecula and the Australian Bush Rat, Rattus fuscipes, two fundamental features of the rule are refuted by showing that a) size shifts within a species are not unidirectional, and b) species with a larger initial (mainland) body mass can actually increase in size instead of decreasing in size, contrary to a core prediction of the island rule. Moreover, the absence of any detectable overall global island effect shows that the island rule is upheld in neither species. Island area, distance from mainland, interval of isolation, and numbers of competitors or predators exert no influence on island body-size patterns in these species. Rather, temperature is the best predictor of T. vulpecula island body size followed by productivity. However, predictors of R. fuscipes body size were unable to be demonstrated. Including covariates in model analyses and sampling from islands within distinct climatic zones likely has a critical bearing on the identification of island body-size patterns and their determinants.

In summary, productivity and thermoregulatory requirements appear to be the key drivers of spatial (including island–mainland) and temporal body-size evolution in several Australian marsupials and a rodent. Future studies might fruitfully extend the approaches employed here to other species within Australia and elsewhere. It would also be worth explicitly testing the degree to which Bergmann’s rule, as a pattern, is scale dependent through space and in relation to temp¬erature, and whether environmental determinants of body size vary in their relative influence between regions. Studies of body-size patterns through the late Quaternary hold potential for exploring past influences of climatic and pre-European hunting practises on Australian mammals, which could provide an improved platform for refined predictions of the likely future impacts of changes in rainfall and temperature. Exploring the degree to which body-size patterns are phenotypically plastic or genetically coded should be another target of future studies. All in all, despite two millennia of pondering why animals are the size they are, it is clear that body-size studies still hold great potential for addressing key questions in ecology and evolution.

Keywords: body size, Bergmann's rule, Australian mammals, Australian marsupial, geographic variation, environmental variables, human-induced evolution, island rule

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2016
School: School of Biological Sciences
Supervisor: Dr Michael Gardner