Locomotion into New Niches: Limbs, Ecology, and Evolution in Mustelid Mammals

• Applicant: Brandon Kilbourne, Ph.D.
• Subject Area: Systematics and Morphology (Zoology) ; Nuclear Medicine, Radiotherapy, Radiobiology ; Ecology and Biodiversity of Animals and Ecosystems, Organismic Interactions
• Term: from 2015 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 269215105

Limb morphology is often tied to the mechanical demands of locomotion for a species’ ecological niche, making limb morphology ideal to understand adaptive evolution of integrated traits. Using mustelid mammals, a functionally rich lineage of carnivorans, I tested whether bone external, bone internal, and muscle traits differ according to locomotor habit and whether similar dynamics of trait evolution have governed all of these traits. With a sample of 42 mustelid species (70% of mustelid extant diversity), I found that the external dimensions of the limb skeleton in mustelids distinguish climbing and swimming mustelids from those with other locomotor habits, with these traits likely evolving under distinct selective regimes for climbing and swimming habits. From µCT-scans of 240 long bones of 22 mustelid species, I with my collaborators are the first to discover that long bone cross-sectional area (CSA; i.e., a bone’s resistance to axial compression) and second moment of area (SMA; i.e., a bone’s resistance to bending) vary according to locomotor habit, with digging and swimming mustelids having long bones with significantly greater CSA and SMA than those with climbing and generalized habits. Performing dissections in eight countries, we are building a globally sampled dataset on limb muscle architecture, finding among currently 13 species that swimming mustelids have longer fascicle lengths, greater muscle belly mass, and stronger muscles compared to those with climbing and generalized locomotor habits. With the proposed funds, I will we next determine if locomotor habit also influences bone microanatomy –trabeculae – to understand how locomotor habit influences and adaptive diversification governs the evolution of traits from the macroscopic to microscopic level. I will also test whether similar evolutionary dynamics govern limb morphology and body size – one of the most influential traits in organismal biology. To do this, I will sample fossil mustelids, which uniquely exhibit gigantism and specializations for running, and, using models of selective regimes and rates of evolution, test whether the evolution of body size is tied to shifts to new adaptive peaks for limb morphology. Lastly, we will conduct a novel study to test whether the postnatal ontogenetic development of limb morphology is influenced by locomotor adaptations. Analyzing the postnatal ontogenetic scaling of bone external and internal (e.g., CSA, SMA) traits and muscle length and mass, we will test whether a given locomotor adaptation is associated with a distinct postnatal growth strategy or whether a ubiquitous growth pattern occurs across adaptations and whether the degree of specialization influences postnatal growth. This research will result in unprecedented insight into how integrated traits (i.e., bones and muscles) – including macro- and microstructural traits – contribute to the evolution of adaptations and how adaptations influence ontogenetic development.

• DFG Programme: Research Grants