With over 85 described species including badgers, otters, weasels, raccoons, skunks, the wolverine, and the red panda, Musteloidea is an incredibly diverse group of animals. Equally impressive is the great ecological disparity, allowing different species to exploit arboreal, fossorial, or aquatic lifestyles as well as a variety of diets ranging from bamboo to insects to rodents. The incredible ecological and phenotypic diversity in Musteloidea is often attributed to adaptive radiation coinciding with two major climate events, the Eocene–Oligocene transition (~33 million years ago) and the Mid-Miocene Climate Transition (~14 million years ago). In both instances, periods of abrupt cooling and drying resulted in a dramatic expansion of grasslands and woodlands, and, in turn, led to the diversification of rodent species, which are the predominate prey of many mustelids. Whether musteloids are an adaptive radiation has never been quantitatively assessed. In this study, tested the hypothesis that patterns of lineage diversification and phenotypic evolution in Musteloidea are consistent with adaptive radiation theory.
Contrary to expectations, we found that musteloids do not exhibit an adaptive radiation as previously hypothesized. Rather, a subclade of “elongate” mustelids exhibit increased lineage carrying capacity as well as increased rates of body length evolution but not body mass evolution. These results suggest that body elongation might be an innovation for the exploitation of novel Mid-Miocene resources, resulting in the radiation of some musteloids. Therefore, we next quantified body elongation in musteloids. We found that several mustelid subfamilies exhibited evolutionary transitions towards relatively longer bodies. Even more exciting, we found that more elongate bodies correlated negatively with body size and forelimb length but not hindlimb length. This relationship between body elongation and forelimb length has not previously been described in mammals but follows the major trend exhibited by other ectothermic vertebrate clades such as snakes and fishes. Almost all elongate ectothermic tetrapods are burrowers, hide under debris, tunnel, or dig in soil. Similarly, extension of the body in weasels may have served as an innovation in body shape by facilitating the ability to enter burrows to exploit subterranean prey.
Overall, these studies reveal that although musteloids did not rapidly diversify under an adaptive radiation as previously thought, the evolution of an elongate body allowed mustelids to escape the constraints of the clade carrying capacity by exploiting novel grassland habitats and subterranean prey.
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Law CJ, Slater G, & Mehta RS. 2018. Lineage diversity and size disparity in Musteloidea: testing patterns of adaptive radiation using molecular and fossil-based methods. Systematic Biology. 67:127–144.
Law CJ, Slater G, & Mehta RS. 2019. Shared extremes by ectotherms and endotherms: body elongation in mustelids is associated with small size and reduced limbs. Evolution. doi.org/10.1111/evo.13702.