Swimming Kinematics and Energetics of the American Alligator (Alligator mississippiensis)

Abstract

espite the importance of coupling the mechanics of movement with power consumption, relatively few studies have attempted to do so with crocodylians. None have tried to marry these two elements with a focus on swimming, limiting our understanding of the anatomical and behavioural factors that affect the cost of transport and comparisons between terrestrial and aquatic locomotion. In addition, current estimates for the optimal swimming speeds (U_opt) of extinct marine reptiles have been made using the kinematic efficiencies of mammals without quantitative comparisons with swimming speeds used by reptiles in the wild. Here, we combine kinematic and respirometry data from a swimming crocodylian: Alligator mississippiensis to measure thrust, power consumption and the cost of transport, and to inform predictions of U_opt for extinct vertebrates. Alligators swam in a flume at different speeds while oxygen consumption and carbon dioxide production were recorded. The frequency of tail waves modulates swimming velocity in crocodylians, but although thrust and power consumption appear to increase with tail beat frequency, they were not significantly related in A. mississippiensis. Thrust and power outputs were very close to those observed in other crocodylians, however. Better morphometrics data are needed to demonstrate the relationships between thrust, power consumption and tail beat frequency more conclusively. The cost of transport when swimming was less than values reported for alligators when walking. Oxygen consumption and the cost of transport were comparable to other reptiles of similar sizes swimming at similar speeds and decreased with body mass. This may be because stride length increases with body size, causing a concomitant increase in kinematic efficiency with stride length. The cost of transport was unrelated to swimming speed, but kinematic efficiency increased when alligators swam at higher speeds than those typically used by wild alligators. Data for alligators swimming over a greater range of speeds is needed so that the effect of velocity on the cost of transport and efficiency can be identified. Respiratory quotients (R_Q) both at rest and when swimming were characteristically low, despite accounting for carbon dioxide excretion via the urine as ammonium bicarbonate. The decrease in R_Q between resting states and when swimming suggests that anaerobic glycolysis and/or carbon dioxide sequestration in the tissues increased with activity. Despite its practical limitations, direct calorimetry may be the only viable solution to acquiring accurate energetics data in crocodylians. U_opt for alligators aligns more closely with those of extinct marine reptiles that use axial swimming than fusiform swimmers or underwater flyers. Determining metabolic rate accurately in swimming crocodylians would further support the development of models to predict the optimal swimming speeds of extinct archosaurs.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Phillip Manning (Supervisor) & Jonathan Codd (Supervisor)