Testing hypotheses of marsupial brain size variation using phylogenetic multiple imputations and a Bayesian comparative framework

Abstract

Considerable controversy exists about which hypotheses and variables best explain mammalian brain size variation. We use a new, high-coverage dataset of marsupial brain and body sizes, and the first phylogenetically imputed full datasets of 16 predictor variables, to model the prevalent hypotheses explaining brain size evolution using phylogenetically corrected Bayesian generalized linear mixed-effects modelling. Despite this comprehensive analysis, litter size emerges as the only significant predictor. Marsupials differ from the more frequently studied placentals in displaying a much lower diversity of reproductive traits, which are known to interact extensively with many behavioural and ecological predictors of brain size. Our results therefore suggest that studies of relative brain size evolution in placental mammals may require targeted co-analysis or adjustment of reproductive parameters like litter size, weaning age or gestation length. This supports suggestions that significant associations between behavioural or ecological variables with relative brain size may be due to a confounding influence of the extensive reproductive diversity of placental mammals.

Radiating pain: venom has contributed to the diversification of the largest radiations of vertebrate and invertebrate animals

“…We follow recent studies (e.g. [77]) in addressing this by adding a small value to branch lengths (0.5my) and then adjusting the branch lengths to ensure the tree remains ultrametric using the force.ultrametric function in the R package 'phytools' [57]. The impact of polytomies in most comparative methods is still poorly understood, but arbitrarily resolving them tends to lead to inflation of rates of phenotypic evolution [78].…”

Savviness of prey to introduced predators

“…Animals long thought of as "notably less intelligent" and relatively "simple" (Britannica, 2021) have turned out to be smart (Todorov et al, 2021), fast (Garland et al, 1988), and savvy (this study). Where does this leave the notion that introduced species, particularly predators, are a categorical threat?…”

The brain’s bootstrapping problem and its consequences: Parental provisioning and variation in vertebrate brain sizes

“…These findings are therefore also fully consistent with the parental provisioning hypothesis. In a recent analysis of marsupial brain size variation, Todorov et al [101] found that the only variable with explanatory value was litter size, which, given that size at birth and growth rates are rather uniform in this lineage, is consistent with the parental provisioning hypothesis.…”