The Spitzbergen reindeer--a winter-dormant ungulate?

Seasonal changes in serum levels of growth hormone, cortisol and thyroxine in calves and adult Spitzbergen reindeer (Rangifer tarandus platyrynchus) were measured and compared to those previously found in Norwegian reindeer (R.t. tarandus). Cortisol did not differ significantly between summer and winter, or between the subspecies. Growth hormone and thyroxine exhibited highly significant seasonal changes and subspecific differences: winter levels of growth hormone were much higher than summer levels in the Spitzbergen reindeer, while Norwegian reindeer differed very little from season to season. Both Spitzbergen and Norwegian reindeer had markedly reduced thyroxine levels during winter, but the values from the former were much lower than those from the latter. In summer, however, the levels were equal. The high levels of growth hormone and low levels of thyroxine in the Spitzbergen reindeer during winter are indicative of high lipolytic activity and a reduced metabolic rate, respectively.