Environment predicts repeated body size shifts in a recent radiation of Australian mammals*

Abstract

Closely related species that occur across steep environmental gradients often display clear body size differences, and examining this pattern is crucial to understanding how environmental variation shapes diversity. Australian endemic rodents in the Pseudomys Division (Muridae: Murinae) have repeatedly colonized the arid, monsoon, and mesic biomes over the last 5 million years. Using occurrence records, body mass data, and Bayesian phylogenetic models, we test whether body mass of 31 species in the Pseudomys Division can be predicted by their biome association. We also model the effect of eight environmental variables on body mass. Despite high phylogenetic signal in body mass evolution across the phylogeny, we find that mass predictably increases in the mesic biome and decreases in arid and monsoon biomes. As per Bergmann's rule, temperature is strongly correlated with body mass, as well as several other variables. Our results highlight two important findings. First, body size in Australian rodents has tracked with climate through the Pleistocene, likely due to several environmental variables rather than a single factor. Second, support for both Brownian motion and predictable change at different taxonomic levels in the Pseudomys Division phylogeny demonstrates how the level at which we test hypotheses can alter interpretation of evolutionary processes.

Climate, crypsis and Gloger’s rule in a large family of tropical passerine birds (Furnariidae)

“…To determine if the posterior distributions of the mean brightness for each habitat are different from one another, we calculated the distributions of the differences of each habitat’s brightness estimates, i.e . contrasts (nonforest-intermediate, nonforest-forest, intermediate-forest) (McElreath 2016; Roycroft et al 2019) using the compare levels function in the R library tidybayes (Kay 2019). If the 95% credible interval of these difference distributions does not overlap zero, then we can credibly say that brightness is different between those habitats.…”

Digest: The evolutionary relationship between environment and size in Australian rodents

“…In other words, species converge on larger body masses in the cooler mesic regions of southern Australia, and smaller body masses in warmer arid/monsoonal biomes. Interestingly, Roycroft et al (2020) hypothesize that the presence of strong phylogenetic signal in body DIGEST mass, despite nonrandom associations with biome, may be indicative of body mass "tracking" climate during recent climate oscillations, and conclude that changes in body mass (a strong candidate β trait) may be highly responsive to changing climate conditions. This initially seems like a paradox; distinct environmental conditions place constraints on the evolution of body mass, but lineages seem able to shift between environments regularly.…”

“…This initially seems like a paradox; distinct environmental conditions place constraints on the evolution of body mass, but lineages seem able to shift between environments regularly. However, Roycroft et al (2020) suggest reconciling this paradox may be as simple as contextualizing body size evolution against the backdrop of dramatic climatic changes experienced in Australia during the Pliocene and Pleistocene. Roycroft et al (2020) suggest that body size evolution in the Pseudomys Division oscillated in parallel to this period of climatic change, with modern variation in body mass having evolved in response to more recent regional climate change.…”

“…However, Roycroft et al (2020) suggest reconciling this paradox may be as simple as contextualizing body size evolution against the backdrop of dramatic climatic changes experienced in Australia during the Pliocene and Pleistocene. Roycroft et al (2020) suggest that body size evolution in the Pseudomys Division oscillated in parallel to this period of climatic change, with modern variation in body mass having evolved in response to more recent regional climate change. This process would suggest a high capacity for adaptive responses in β traits, while simultaneously providing justification for strong phylogenetic signal.…”

“…Phenotypic evolution and diversification appears to be intimately tied to Mio-Pliocene climate change and biome shifts in a number of Australian taxa, including marsupial mammals, meliphagid birds, agamid dragons, sphenomorphine skinks, pygopodid geckos, myobatrachid frogs, and proteaceous plants (Catullo and Keogh 2014;Cardillo et al 2017;Brennan and Keogh 2018). Roycroft et al (2020) add another case study to the growing list of clades that transition with relative ease among biomes. This study further elucidates how environmental variation shapes diversity in Australian rodents, and in addition, clearly demonstrates the need to consider phylogenetic level when drawing conclusions from evolutionary studies.…”