Maintaining a constant warm body temperature has major advantages: it’s easier to get going in the morning and your activity levels aren’t prey to the vagaries of the climate. However, there is one colossal down side: it costs a metabolic fortune, so some small creatures drop their body temperature when resources are scarce to conserve energy. Lea Brinkmann and colleagues from the University of Göttingen, Germany, explain that some ungulates also take advantage of the energy savings incurred by low body temperatures, yet most were thought to have lost this ability when they became domesticated. However, recent studies show that the ancient wild predecessor of modern horses, the Przewalski horse, appears to have retained this ability. Curious to find out whether domesticated horses were also capable of regulating their body temperature, Brinkmann and colleagues decided to find out whether Shetland ponies, one of the earliest domesticated horse breeds, drop their body temperature when food is scarce (p. 1061).
Monitoring the activity levels, subcutaneous and rectal temperatures, heart rate and general body condition of a group of ponies over a year, the team noticed that the animals’ subcutaneous temperatures dropped over night and rose again during the day in the summer. ‘This is consistent with a daily shallow hypometabolism,’ the team says. Then, as the winter set in, the team fed half of the group full rations while simulating food shortages by cutting the other half’s diet by 30%. This time, the ponies experiencing the harsher winter conditions lowered their body temperature and were as much as 1.1°C cooler than the well-fed animals. In addition, they had much slower heart rates, suggesting that they also had a lower metabolic rate. ‘Domesticated Shetland ponies showed similar seasonal adjustment mechanisms described for the wild counterpart, the Przewalski horse,’ say Brinkman and colleagues.
So, the domesticated animals have retained their wild ancestors’ ability to conserve energy when resources are scarce, while improving their metabolic efficiency to lay down fat during times of plenty.