It’s all relative: the interpretation of Allen’s rule and its consequences for understanding morphological responses to climatic warming

Across animals, and particularly endotherms, there is a broad tendency for individuals living in warmer climates to have longer appendages (legs, tails, bill, ears etc.) than those in colder climates. This phenomenon is known as Allen’s rule, after the naturalist who first described it. In more recent times, interest in the rule has arisen in the context of understanding morphological responses to climatic warming, with theoretical predictions and empirical evidence suggesting that animals are evolving longer appendages. Studies of Allen’s rule, though, have differed on how appendage size is analysed. Does it apply to absolute appendage size, or does it refer to appendage size relative to the overall size of the body? Allen himself discussed both but was not clear on which he considered more biologically relevant. In this perspective I argue that, in the context of understanding the effect of climate on appendage size specifically - the ultimate purpose of Allen’s rule - then it is essential to use relative appendage size. Although absolute appendage can influence the size of the effect of Allen’s rule, evaluation of the existence of an effect of climate (and by extrapolation climatic warming) on body shape requires use of relative appendage size due to the confounding simultaneous effects on body size. I also demonstrate that different allometries of appendage size on body size exacerbate this confusion, meaning that only relative appendage size provides a direct evaluation of impacts of climate change on animal appendages.