On a hot day on the African savanna, many animals seek out shade to escape the sweltering heat. But do some animals also use a thermal trick to save their limited water supplies? Some inhabitants of the world's hot, dry places are thought to allow their bodies to heat up during the day, limiting water loss by evaporation, and then let their body temperature plummet at night to be able to store heat again when the sun rises. Captive creatures appear to use this so-called adaptive heterothermy, but do free-living animals use it too? Andrea Fuller and her colleagues at the University of the Witwatersrand in South Africa set out to see if free-roaming springbok use adaptive heterothermy to survive the soaring temperatures of the South African plains(p. 2855).

First, the team had to find free-living springbok. Luckily, AndréMatthee at South Africa's Lichtenburg Game Breeding Centre, a 4500 ha reserve teeming with African animals, offered to help. He caught eight adult springbok for Fuller's team. Graham Mitchell, the team's vet, surgically implanted data loggers in each animal's abdomen, which would record the animal's body temperature every 30 minutes for an entire year. The team released the springbok in an enclosed part of the reserve, where they lived undisturbed for a year. To record the thermal challenges that the animals encountered over the year, the team continuously measured air temperature, wind speed and humidity using an on-site weather station connected to a data logger. A year later, the team retrieved the loggers. But were they still intact?

Fortunately, the loggers had not only survived the year but had also collected interesting data. Analysing the body temperature measurements, the team looked for tell-tale signs of adaptive heterothermy: a daily pattern of wide swings in body temperature. They discovered that daily body temperature fluctuations didn't correlate with the 24-h range of air temperatures. But the most striking finding was that, despite experiencing outside temperatures ranging from –6°C to 34°C over the year, springbok body temperatures never fluctuated by more than 1.2°C on a daily basis,regardless of the season; the animals clearly didn't exhibit adaptive heterothermy. While the team expected to find some evidence for homeothermy,they were startled by springbok's remarkable homeothermy in the face of seasonal temperature swings and food shortages, as well as thermally stressful physiological events like giving birth.

`It was surprising to find that these animals don't use adaptive heterothermy, as some people suspected', Fuller admits. But, she says, the results do highlight an important point: when animals are allowed to roam free in their natural home, they might use behavioural rather than physiological adjustments to cope with thermal challenges, such as seeking out shade during the day or huddling together at night. In this study, `we didn't know what the animals were up to', Fuller says, because the team deliberately limited human interactions with the animals to measure body temperature fluctuations in natural conditions. But in future work Fuller will record animals' behaviour while taking physiological measurements, which she hopes will produce a much clearer picture of the relationship between behaviour and body temperature.

Fuller, A., Kamerman, P. R., Maloney, S. K., Matthee, A.,Mitchell, G. and Mitchell, D. (
). A year in the thermal life of a free-ranging herd of springbok Antidorcas marsupialis.
J. Exp. Biol.