Tiptoeing through the trophics: Geographic variation in carnivoran locomotor ecomorphology in relation to environment

That ankle posture in Carnivora is linked to substrate is well known. Large species from open settings like grasslands are often cursorial with digitigrade stance, whereas smaller species from closed woodlands are often arboreal with plantigrade or semidigitigrade stance. Digitigrade species typically have feet that are proportionally longer than plantigrade species. It is less well known that the proportional length of the foot varies within mammalian species according to substrate, as demonstrated by previous authors. Taken together, these two observations suggest that a simple index of digitigrady calculated either for a single species or an entire fauna across a broad geographic range could reveal variation in palaeocommunity types. Here, 135 terrestrial carnivoran species were studied to determine whether three index ratios were associated with locomotor habit and posture: (1) the femur/metatarsal III; (2) the in-lever to the out-lever of the calcaneum; and (3) the position of the sustentacular facet (measured from the proximal end of the calcaneum) to the total length of the calcaneum. All three indices were correlated with posture, all but the second were correlated with locomotor habit, and all were correlated with taxonomic family. The third index was associated with posture and is easily measured in fossil speciemens and so was chosen as an ecomorphological “gear ratio” index. The calcaneal gear ratio was measured in 45 out of 49 North American carnivoran species and used to estimate the mean gear ratio at points spaced 50 km apart across the whole continent. Mean gear ratio in carnivorans was highly correlated with ecological province (the latter explained 70% of the geographic variance in mean gear ratio), mean annual temperature (which explained 48% of variance), and vegetation cover (49% of variance). Mean calcaneal gear ratio was not correlated with number of carnivoran species (which explained 5% of variance), elevation (7% of variance), or mean annual precipitation (1% of variance). The potential for mean gear ratio in carnivorans to be a proxy for ecological province, vegetation cover, or palaeotemperature is strong.