Effects of seasonality and fasting on the plasma leptin and thyroxin levels of the raccoon dog (Nyctereutes procyonoides) and the blue fox (Alopex lagopus).

Plasma leptin and thyroxin concentrations of eleven raccoon dogs and eleven blue foxes were monitored for 6 months. Half of the animals were placed on a 3-week fast in November. Leptin levels were low in summer, but in October they rose significantly. In November, leptin concentrations decreased rapidly within a week although the body mass of the animals remained stable. There were no significant differences between experimental groups for raccoon dogs, but in blue foxes the fasting group had lower leptin levels than the control group. High thyroxin levels in summer decreased as autumn progressed, but thyroxin concentrations of the fasting groups increased at the end of the fast. Leptin levels of the raccoon dog and the blue fox are not determined only by the fat reserves of the animals, but they seem to reflect the autumnal deposition of fat at the onset of winter. Blue foxes have metabolic rates of active animals during the winter and higher leptin levels in December than raccoon dogs. The superficially hibernating raccoon dogs have low leptin levels after the onset of winter perhaps as an adaptation to fasting. J. Exp. Zool. 289:109-118, 2001. Copyright 2001 Wiley-Liss, Inc.