Aquatic and terrestrial locomotory energetics in a toad and a turtle: A search for generalisations among ectotherms

Murray short-necked turtles were trained to walk on a motorised treadmill a nd to swim in a recirculating flume. Through filmed records, the frequency of limb movement and the time that thrust was directed against the substrat e were measured. The animals wore masks when walking and accessed air when swimming from a ventilated capsule placed on top of the water surface. Meas urement of the exhalant O-2 and CO2 levels from these devices enabled the m easurement of metabolic rates. Equivalent data were obtained from swimming and hopping cane toads, although repeatable measures of limb frequency and contact times were not obtained due to the intermittent form of locomotion in this species. Comparing the cost of transport, the energy required to tr ansport a mass of animal over a unit distance, with other animals showed th at toads do not have a cheap form of terrestrial locomotion, but turtles do ; turtles use half the cost predicted from their body mass. This economy of locomotion is consistent with what is known about turtle muscle, the mecha nics of their gait, and the extremely long contact time for a limb with the substrate. Swimming in toads is energetically expensive, whereas turtles, on the basis of mass, use about the same energy to transport a unit mass as an equivalent-size fish. The data were compared with the predictions of th e Kram-Taylor hypothesis for locomotory scaling, and walking turtles were f ound to provide a numerical fit. The data show that both terrestrial and aq uatic locomotory energetics in toads are generally higher than predictions on the basis of mass, whereas in turtles they are lower.