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Nicola Stead

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For large endothermic animals that produce their own body heat from metabolism, bouts of exercise under the sweltering sun in tropical conditions could be potentially lethal. Hefty endotherms, for example elephants or even the extinct endothermic Edmontosaurus dinosaur, have decreased skin surface-area-to-body-mass ratios compared with other endotherms, and this likely means they find it more difficult to get rid of excess heat produced during exercise. However, no one has ever conclusively proved this, and so Michael Rowe from Indiana State University, USA, and his colleagues set out to test how two elephants, which are also similar in size and have the same habitat as Edmontosaurus, would cope with exercising in hot conditions (p. 1774).

The team exercised the two elephants by walking them around a closed circuit for about 20 min under full sun, over a range of temperatures from 8 to 34.5°C. By measuring their core and skin temperatures, as well as walking speed and environmental factors such as air temperature and solar radiation, the team could then calculate how much heat they produced from increased metabolism and how much heat they lost to (or gained from) the environment. The team found that the elephants' metabolic rate increased 2- to 2.5-fold regardless of whether the animal exercised in winter, spring or summer. However, in the hot summer, radiation from the sun and the environment meant that the elephants' skin reached the same temperature as their core body (35.3°C, compared with 24.9°C in November), meaning that 100% of the heat generated by exercising was stored in core tissues, with body temperature increasing by up to 1.6°C.

From their findings, the authors could predict the limitations this put on diurnal activity in both elephants and Edmontosaurus dinosaurs. For elephants, 4 h of exercising in the sun would see their body temperature steadily increase to a lethal 43°C. Similarly, Edmontosaurus would likely only last 3.5 h. At night, without solar radiation heating up their skin, both the elephants and the endothermic dinosaur could exercise for up to 8 h without overheating. So it seems nocturnal exercise is best for gigantic endotherms.