Multiple morphological pathways underlie climatic adaptation in North American mustelids

Abstract

Ecogeographic rules predict that endotherms inhabiting colder environments exhibit larger body sizes and shorter appendages to reduce heat loss. However, these hypotheses have largely been tested using size-based traits, leaving it unclear whether climatic adaptation also alters body proportions that more directly influence surface-area-to-volume ratio. Here, I examined ecogeographic variation in body shape, body size, and relative limb lengths across climatic gradients in three western North American mustelids: American ermine (Mustela richardsonii), American mink (Neogale vison), and Pacific marten (Martes caurina). I then modeled how variation in body shape and size affected surface-area-to-volume ratio. Body size increased in colder climates in all three species, consistently supporting Bergmanns rule. In contrast, only ermines became stouter in colder climates, whereas body shape remained unchanged in minks and martens. Patterns of appendage variation also differed: ermines and martens exhibited relatively longer limbs in colder climates, contrary to Allens rule, whereas only minks exhibited relatively shorter hindlimbs. Modeling showed that increased body size reduced surface-area-to- volume ratio in all species, whereas body shape contributed only in ermines. These findings demonstrate that body shape, body size, and appendage lengths respond independently to climatic gradients, with species-specific ecological and functional constraints shaping thermoregulatory adaptation.