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

Abstract

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. © 2001 Wiley‐Liss, Inc.

Effects of season, food deprivation and re-feeding on leptin, ghrelin and growth hormone in arctic foxes ( Alopex lagopus ) on Svalbard, Norway

“…Another major drawback is the fact that we did not study acute effects of food withdrawal and can therefore not exclude the possibility that fed animals could have had higher leptin levels during the first days of food deprivation. However, it has previously been observed in captive blue foxes that the plasma leptin levels remain almost unaffected during a 3-week total fast, thus supporting the results of this study (Nieminen et al 2001).…”

“…5 Leptin-ghrelin ratios during food deprivation and refeeding in May (unfilled symbols) and November (filled symbols), in arctic foxes on Svalbard presented as the mean±SD increase in circulating leptin concentrations during the recovery period from food deprivation (Nieminen et al 2002) as the energy reserves (adipose tissue) are replenished. The results of the present study further enforce the hypothesis of leptin not always being an indicator of body adiposity in carnivores (Nieminen et al 2001(Nieminen et al , 2002Arnould et al 2002).…”

“…Leptin protein could be measured in arctic fox plasma at concentrations comparable to leptin levels of other carnivores measured with the same method (Nieminen et al 2001(Nieminen et al , 2002Arnould et al 2002). Similar to the raccoon dog, plasma leptin concentrations were unaffected by fasting (Nieminen et al 2002), but increased during refeeding supporting the model of leptin being, at least at some points of the animalÕs seasonal cycles, an indicator of increasing adipose tissue mass.…”

“…Thereafter blood was sampled on day 7 and day 13 after food deprivation, and again on day 5 and day 9 of re-feeding, with the same protocol in May and in November. The arctic fox encounters long periods of food deprivation in winter in its natural habitat and is quite well adapted to withstand periods of food deprivation (Fuglei and Øritsland 1999;Fuglei 2000;Fuglei et al 2000;Nieminen et al 2001). The blood samples were obtained from the cephalic vein using a syringe (needle 1.2·40 mm); heparin was used as an anticoagulant.…”

“…All these assays were validated such that serial dilutions of the arctic fox plasma showed linear and parallel changes with the standard curve produced with human standards (data not shown). The leptin and GH assays have also previously been used in the blue fox (Alopex lagopus), a variant of the arctic fox reared in the fur industry (Mustonen et al 2001b;Nieminen et al 2001).…”