Many species of reptiles maintain their body temperature behaviorally in a narrow range, even in the presence of considerable environmental temperature variation, by choosing microhabitats with different temperatures. In freshwater turtles, thermoregulation is generally achieved by aerial basking, even though they perform all other vital activities such as food consumption and reproduction in the water. Therefore, time budgets related to basking should be constrained and individuals should maximize the energy per unit time during basking, potentially by increasing basking frequency at noon during colder months and increasing use of basking when water temperature decreases. We analyzed basking behavior during the austral summer to study the effects of season, water temperature, and time of day in 2 South American freshwater turtles: Trachemys dorbigni (black-bellied slider) and Phrynops hilarii (Hilaire's side-necked turtle). We found that water temperature negatively affected basking frequency in both species differently; basking by T. dorbigni occurred on a diel cycle while basking by P. hilarii occurred on a seasonal level. Both species showed a bell-shaped basking frequency during the day, with more individuals basking at noon than in the morning and afternoon. However, only P. hilarii showed a significant seasonal effect on basking, with basking frequency decreasing in summer. These results suggest the thermoregulatory role of basking behavior in 2 austral turtle species and its trade-off with other vital activities.

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