Maximal Aerobic and Anaerobic Power Generation in Large Crocodiles versus Mammals: Implications for Dinosaur Gigantothermy

Roger S Seymour,Peter Dodson

doi:10.1371/journal.pone.0069361

Abstract

Inertial homeothermy, the maintenance of a relatively constant body temperature that occurs simply because of large size, is often applied to large dinosaurs. Moreover, biophysical modelling and actual measurements show that large crocodiles can behaviourally achieve body temperatures above 30°C. Therefore it is possible that some dinosaurs could achieve high and stable body temperatures without the high energy cost of typical endotherms. However it is not known whether an ectothermic dinosaur could produce the equivalent amount of muscular power as an endothermic one. To address this question, this study analyses maximal power output from measured aerobic and anaerobic metabolism in burst exercising estuarine crocodiles, Crocodylusporosus , weighing up to 200 kg. These results are compared with similar data from endothermic mammals. A 1 kg crocodile at 30°C produces about 16 watts from aerobic and anaerobic energy sources during the first 10% of exhaustive activity, which is 57% of that expected for a similarly sized mammal. A 200 kg crocodile produces about 400 watts, or only 14% of that for a mammal. Phosphocreatine is a minor energy source, used only in the first seconds of exercise and of similar concentrations in reptiles and mammals. Ectothermic crocodiles lack not only the absolute power for exercise, but also the endurance, that are evident in endothermic mammals. Despite the ability to achieve high and fairly constant body temperatures, therefore, large, ectothermic, crocodile-like dinosaurs would have been competitively inferior to endothermic, mammal-like dinosaurs with high aerobic power. Endothermy in dinosaurs is likely to explain their dominance over mammals in terrestrial ecosystems throughout the Mesozoic.

Highlights

The phylogeny of the archosaurs began in the Late Permian and diversified into two main lineages in the Middle Triassic, the Crurotarsi and the Ornithodira [1]
This is impressive, because the anaerobic pathway produces only about 10% of the adenosine triphosphate (ATP) energy as the aerobic one from the same amount of substrate. All of these conclusions come from small reptiles and mammals and may not represent the situation in crocodiles or dinosaurs that weighed 3–5 orders of magnitude more. This paper addresses this question in the estuarine crocodiles, Crocodylus porosus, weighing up to 200 kg, because this is approximately the mass range available from an earlier study of anaerobic metabolism of this species [25]
Muscular power may be calculated on the basis of the energy content change of 30.5 kJ mol-1 when adenosine triphosphate (ATP) is converted to adenosine diphosphate (ADP) during muscle contraction [26]

Summary

Introduction

The phylogeny of the archosaurs began in the Late Permian and diversified into two main lineages in the Middle Triassic, the Crurotarsi (crocodilians and their relatives) and the Ornithodira (dinosaurs, pterosaurs, birds and their relatives) [1]. It would be more interesting to determine the total power output, including both aerobic and anaerobic sources, to assess how ectothermic, crocodile-like dinosaurs would compare to endothermic, mammal-like dinosaurs. It shows that total power generation in maximally active crocodiles is low compared to mammals of the same size and that the disparity increases greatly in larger animals.

Results

Conclusion