On the vast Wyoming plains, even the summer air temperature can fluctuate between 2°C and 38°C. It's a tough environment, and a challenge for any animal to keep its body temperature just right; but pronghorn antelope(Antilocapra americana) are right at home in the hot dry summers and cold, wet winters. Graham Mitchell and Amanda Lust from the University of Wyoming and their colleagues wanted to know if pronghorn, which evolved 25 million years ago, rely on the same thermoregulatory tricks as their more modern and better studied southern hemisphere relatives, such as springbok,which don't have to cope with such large summer temperature variations (p. ).
To find out how the pronghorn regulate body and brain temperatures over the course of a summer season, the team anaesthetised and operated on five captured animals to insert the temperature recorders: thermistors attached to data loggers. They put thermistors into the carotid artery and jugular vein,and also under the skin and into the abdomen, placing the data loggers under the skin nearby. To insert a thermistor into the brain, the team made a small opening in the skull and delicately pushed it between the two halves of the brain, nudging it close to the hypothalamus, which controls body temperature. Once the pronghorn had recovered from surgery, they roamed free in the research station for three months over the summer before the team collected the data loggers from the animals, and analysed the recorded temperatures together with climate data from a nearby a weather station.
The team found that pronghorns' body temperatures varied by up to 3–4°C. Taking a closer look at the correlation between the temperature in the carotid artery and environmental temperature, the team found that the maximum environmental temperature occurred around 7 h before the highest recorded carotid temperature. This suggests that, like springbok,the pronghorn allow their bodies to heat up during the day, meaning that they don't lose precious water trying to cool their bodies down. It also suggests that they could use this stored heat to stay warmer at night. The team suspect that an endogenous rhythm controls this fluctuation in body temperature,because the changes weren't closely correlated to sunrise or sunset time, or day length.
When the team looked at brain temperatures, they found that the pronghorn maintained a very constant brain temperature, which only varied by around 2°C. When they compared body and brain temperatures, they found that when the body temperature dropped below 37.5°C, the brain stopped getting colder, and when the body was more than 39°C, the brain stopped getting warmer. This suggests that the pronghorn have a brain warming mechanism which lets the body cool down, conserving energy. Likewise when it gets too hot the pronghorn don't waste water cooling their bodies down, focussing instead on cooling the brain. The team calculated that this brain warming and cooling was not related to changes in brain blood flow, and aren't yet sure of the exact mechanism.
While springbok can cool down their brains, they can't warm them up. So pronghorn can thermoregulate in much the same way as Springbok, but have the additional benefit of a brain warming mechanism which probably evolved in response to the colder temperatures in the pronghorns' environment.
