Members of the family Myrmecophagidae (i.e. anteaters) show a variety of anatomical and behavioural adaptations to deal with their low-energy diet; for example, they all have low body temperatures in comparison with other eutherian mammals. In this study, we investigated the tympanic membrane temperatures ( Tmt) of two giant anteaters and three southern tamanduas, housed in captivity and exposed to natural climatic variations in temperature, using an infrared thermometer. Additionally, we measured external dorsal temperature ( Td), air temperature ( Ta), substrate temperature ( Ts) and whether the subject was active or not. To understand the effect of time of day on these variables, we recorded them, on the hour, over four 24-h cycles for each animal during which the subjects were non-restrained within their enclosures. The results show that both giant anteaters and southern tamanduas allow their Tmt to reduce between 4.0 and 6.5 °C when they are sleeping. Furthermore, linear regressions between Tmt and Ta or Ts showed that the giant anteaters were much more affected by Ta and Ts than the southern tamanduas. Both species also showed higher Tmt when active (comparing subjects active and inactive at the same Ta). Both species appear to use shallow torpor during a normal 24-h cycle probably as a means to economize energy. The torpor in giant anteaters occurred during the night when asleep, whereas in the southern tamanduas it occurred at any time of day when asleep. The giant anteaters appeared to be more directly affected by environmental temperature than the southern tamanduas. [ABSTRACT FROM AUTHOR]