Seasonal variation in metabolic rate in ponies

Shetland ponies can drop their body temperature to save energy when food is scarce, according to a study carried out in Germany.

Warmblooded animals can keep functioning in cold conditions, but to do so they expend much energy maintaining their body temperature. Some primitive species seemed able to allow their temperature to fall to conserve energy. It was thought that animals lost this ability as a consequence of domestication. But recent studies have shown that the Przewalski horse, the primitive relative of the modern day horse, seems to have retained the ability to control its body temperature according to the environmental conditions.

Lea Brinkmann and colleagues at the University of Göttingen, Germany, wanted to see if this characteristic was still present in domesticated animals.

They conducted a study to see if Shetland ponies, one of the earliest domesticated breeds, retained the ability to drop their body temperature when food is scarce.

The research team studied a group of ten Shetland ponies throughout the year, monitoring subcutaneous and rectal temperatures, heart rate, general body condition and activity levels.

They noticed that, during the summer, the animals’ subcutaneous temperatures dropped over night, being lowest around dawn, and rose to a peak around mid-day. “This is consistent with a daily shallow hypometabolism,” the team says.

Then at the onset of winter, the researchers divided the ponies into two groups. One group received full rations; the other ponies were fed a restricted diet providing only 30% that of the control group.

The feed-restricted group had significantly lower daily subcutaneous temperatures compared with the control group on cold winter days, when the ambient temperature fell below 0°C. Mean heart rate and locomotor activity closely followed the ambient temperature.

Feed-restricted ponies showed a significant drop in average heart rate (from 52.8 beats per minute in summer) to 29bpm in winter. This response differed significantly from that of the ponies on full rations, suggesting that the feed restricted ponies had a lower metabolic rate.

Ponies were significantly less active in the winter than in the summer.

“Our results show that Shetland ponies exhibit signs of a winter hypometabolism indicated by reduced heart rate and subcutaneous temperature” the team conclude. “Thus, domesticated horses seem to have maintained the capacity for seasonal adaptation to environmental conditions by seasonal fluctuations in their metabolic rate.”

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