Meeting Abstract
Animals are constrained by their abilities and by interactions with various environmental factors. Constraints range from physical impossibilities to energetic inefficiencies, and may entail trade-offs. Considering locomotion and activity metabolism, allometric comparisons of mammals and lizards (as representative terrestrial vertebrate endotherms and ectotherms) can illustrate those perspectives because the two groups differ greatly in metabolic capacities, energetic costs, and thermoregulatory patterns. Such allometric comparisons are useful and unavoidable, but comparisons of “outliers” (species unusual for their clade) can inform evolutionary scenarios, as they help to indicate extremes of possible adaptation within mammalian and saurian levels of organization (grades). We compare mammals and lizards for standard metabolic rate (SMR) at 35-40 C, maximal oxygen consumption during forced exercise (VO2max), net (incremental) cost of locomotion, maximal aerobic speed (MAS), maximal sprint speed, daily movement distance, daily energy expenditure (DEE) during the active season, and the ecological cost of transport (ECT = % of DEE attributable to locomotion: Garland 1983 Am. Nat. 121:571-587). On average, MAS is ~8-fold higher in mammals, while SMR and VO2max are ~6-fold greater, but overlap can occur. Maximal sprint speeds are similar for smaller mammals and lizards. On average for both lineages, the ECT is surprisingly low, somewhat higher for lizards, and positively allometric. Ecological-energetic constraints related to locomotion are most likely in large, carnivorous lizards. Overall, these comparisons suggest that the evolution of mammalian endothermy did not require major changes in locomotor energetics, performance or associated behaviors.
