Biomechanics of Running Indicates Endothermy in Bipedal Dinosaurs

Abstract

BackgroundOne of the great unresolved controversies in paleobiology is whether extinct dinosaurs were endothermic, ectothermic, or some combination thereof, and when endothermy first evolved in the lineage leading to birds. Although it is well established that high, sustained growth rates and, presumably, high activity levels are ancestral for dinosaurs and pterosaurs (clade Ornithodira), other independent lines of evidence for high metabolic rates, locomotor costs, or endothermy are needed. For example, some studies have suggested that, because large dinosaurs may have been homeothermic due to their size alone and could have had heat loss problems, ectothermy would be a more plausible metabolic strategy for such animals.Methodology/Principal FindingsHere we describe two new biomechanical approaches for reconstructing the metabolic rate of 14 extinct bipedal dinosauriforms during walking and running. These methods, well validated for extant animals, indicate that during walking and slow running the metabolic rate of at least the larger extinct dinosaurs exceeded the maximum aerobic capabilities of modern ectotherms, falling instead within the range of modern birds and mammals. Estimated metabolic rates for smaller dinosaurs are more ambiguous, but generally approach or exceed the ectotherm boundary.Conclusions/SignificanceOur results support the hypothesis that endothermy was widespread in at least larger non-avian dinosaurs. It was plausibly ancestral for all dinosauriforms (perhaps Ornithodira), but this is perhaps more strongly indicated by high growth rates than by locomotor costs. The polarity of the evolution of endothermy indicates that rapid growth, insulation, erect postures, and perhaps aerobic power predated advanced “avian” lung structure and high locomotor costs.

Terrestrial movement energetics: current knowledge and its application to the optimising animal

“…Interestingly, this appears to hold for dinosaurs as well; estimates of their power requirements to walk (modelled from measurements of hip height) fall consistently close to the line of best fit for extant species, across a great range of dinosaur body masses including the 6-tonne Tyrannosaurus (Fig. 2B) (Pontzer et al, 2009).…”

Gait-specific energetics contributes to economical walking and running in emus and ostriches

“…Recently, E cot has been found to vary with gait in the ostrich [23]. Despite these exceptions, the assumption that metabolism varies linearly across gaits is still widely used in treatments of the locomotor costs in extant birds and mammals, and in predicting the locomotor energy use by extinct species [24,25]. Our focus here is on walking and running gaits and we do not consider the unusual pattern of locomotor energetics in hopping macropod marsupials [26,27].…”

Adaptations for economical bipedal running: the effect of limb structure on three-dimensional joint mechanics

“…What are the specific relationships between cursorial musculoskeletal specializations and gait mechanics that allow these species to run with a low metabolic energy cost? This question has been of interest to biologists trying to understand structurefunction relationships among both extant species [3 -5] and extinct species such as dinosaurs [1,6] or early hominids [7 -9]. The relationship between limb structure and locomotor economy is also pertinent to several bioengineering fields.…”