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Six bat species in northeastern North America spend winter hibernating in caves/mines. Hibernation is characterized by multi-day periods of torpor, when metabolic rates are reduced. Body fat levels increase from 7 to 27% in the fall, just prior to the onset of hibernation by bats. Some groups of hibernating bats in northeastern North America display abnormal behaviors and increased overwinter mortality called white nose syndrome (WNS). Large numbers of dead bats are found in hibernation sites affected by WNS. Previous research has demonstrated that the increased over-mortality is due to the depletion of depot fat reserves by February, which is in turn caused by greatly shortened torpor bouts. Published studies on hibernating mammals have revealed that moderately high levels of polyunsaturated fatty acids (PUFAs) are required in the diet during the fall for torpor. We thus predicted that bats suffering from WNS have lower levels of PUFAs in their fall diets than those that do not suffer from WNS. We tested this hypothesis by analyzing white adipose tissue (WAT) samples from: (a) little brown bats (Myotis lucifugus) collected from different hibernation sites and (b) big brown bats (Eptesicus fuscus), a species not greatly affected by WNS. Our analyses reveal that M. lucifugus populations prone to WNS have significantly less of the PUFA α-linolenic acid in their fall diets than those where WNS does not occur. The fall diets of E. fuscus contain significantly more of the PUFA linoleic acid than those of M. lucifugus from the same site. Our findings thus support the hypothesis that bats suffering from WNS have lower levels of PUFAs in their fall diets.

Tocopherols (Vitamin E) are antioxidants that intercept the reactive oxygen species that initiate lipid peroxidation. Plants produce four tocopherol isomers, with α-tocopherol being the most biologically active. All mammals require tocopherols in their diets. Field and laboratory studies were conducted to determine the influence of diet α-tocopherol content on torpor by ground squirrels (Spermophilus lateralis). The diets of free-ranging ground squirrels contained both α and γ-tocopherols, with α-tocopherol being the most abundant. Natural diet atocopherol levels ranged from 1.5 to 34.1 µg/g. Laboratory experiments, however, revealed that squirrels fed diets with α-tocopherol contents above or below 12 µg/g were: 1) were less likely to enter torpor, and, 2) were less likely to survive torpor than those fed a diet containing 12 µg atocopherol/g. A low α-tocopherol diet is thus actually required for proper torpor.

Changes in blood flow during torpor-arousal cycles in mammalian hibernators could increase the risk of oxidative stress to sensitive tissues. We used thirteen-lined ground squirrels (Spermophilus tridecemlineatus) to determine the effect of hibernation on glutathione (GSH) redox balance and immune cell populations in the intestine, a tissue highly susceptible to oxidative stress secondary to ischemia-reperfusion injury. Ratios of reduced:oxidized GSH were lower in intestine of hibernators compared with summer-active squirrels, an effect most pronounced for torpid animals. Numbers of intraepithelial lymphocytes were increased in torpid hibernators compared with summer squirrels or hibernators undergoing arousal or interbout euthermia. These results suggest that intestinal GSH redox balance is altered during the hibernation season to a more oxidized state, and this is accompanied by changes in intestinal mucosal immune cells that may play a protective role for stressed enterocytes.

White-tailed prairie dogs (Cynomys leucurus) are spontaneous hibernators that enter torpor each fall, whereas black-tailed prairie dogs (C. ludovicianus) hibernate facultatively only when food- or water-stressed during the winter. The body masses of both species greatly increase during the fall feeding period, with most of this gain in the form of depot fat. Body fat is utilized during winter fasting and/or hibernation. We measured the activities of fatty acid synthase (FAS), ATP-citrate lyase (ACL), malic enzyme (ME), glucose-6-phosphate dehydrogenase (G6PDH), and hormone-sensitive lipase (HSL) in the tissues of both C.leucurus (hibernating and euthermic) and C. ludovicianus (euthermic only) under controlled conditions. The activities of FAS, ACL, and G6PDH in the liver all decreased during hibernation. The activities of ME and G6PDH in white adipose tissue (WAT) were also reduced during hibernation. Euthermic C. leucurus and euthermic C. ludovicianus differed only in brown adipose (BAT) ACL and WAT G6PDH activities. No significant differences in HSL activities were found between these two species or between euthermic and hibernating animals. These results suggest that this seasonal body fat cycle is due, at least in part, to seasonal variations in the activities of FAS, ME, ACL, and G6PDH that affect the rate of fatty acid synthesis. This study also demonstrates that spontaneous hibernators do not have a greater capacity to synthesize fatty acids during the fall than facultative hibernators, as previously suggested.