Unravelling the determinants of insular body size shifts

Abstract

The island rule, a pattern of size shifts on islands, is an oft-cited but little understood phenomenon of evolutionary biology. Here, we explore the evolutionary mechanisms behind the rule in 184 mammal species, testing climatic, ecological and phylogenetic hypotheses in a robust quantitative framework. Our findings confirm the importance of species' ecological traits in determining both the strength and the direction of body size changes on islands. Although the island rule pattern appears relatively weak overall, we find strongest support for models incorporating trait, climatic and geographical factors in a phylogenetic context, lending support to the idea that the island rule is a complex phenomenon driven by interacting intrinsic and extrinsic mechanisms. Overall, we find that different clades may be evolutionarily predisposed to dwarfism or gigantism, but the magnitude of size changes depends more on adaptation to the novel island environment.

Figure 1.

Model-averaged effect sizes for parameters included in strongly supported models (ΔAIC_c ≤ 2) explaining normalized (a) and absolute (b) size ratios. Open and closed symbols reflect estimates from models with and without phylogeny respectively. (c–f) Phylogenetically uncorrected data, plus trend lines for continuous variables with strong effects on the normalized size ratio (blue) and absolute size ratio (black). Horizontal grey lines at 1.0 (c–e) and zero (e–f) reflect no change in size between insular and closest mainland population. (f) Factor 5 trendline is for illustrative purposes only, as the final average model controlled for phylogeny.