ADAPTATIONS FOR HIBERNATION IN THE DEPOT FATS OF A GROUND-SQUIRREL (SPERMOPHILUS-BELDINGI)

Ground squirrels are small herbivores that hibernate during winter. The ecological-nutritional limitations on hibernation are virtually unknown, but one constraint may be the melting point of stored fat. Lipids must be fluid to be metabolizable, and body temperatures maintained during hibernation are usually 30-degrees-C below the melting point of typical mammalian fats. Fats containing greater amounts of unsaturated fatty acids, however, have correspondingly lower melting points. White adipose tissue was sampled from free-ranging Belding's ground squirrels, Spermophilus beldingi, during both the summer and fall. The lipids were twice as unsaturated as those of other rodent species, most of the increased unsaturation being due to the accumulation of plant-produced polyunsaturated fatty acids derived from the animals' diet. The melting points of S. beldingi fats were consequently 25-degrees-C lower than those of other mammals. These results suggest that ground squirrels may depend upon their plant diet for the polyunsaturates necessary to produce the lipids with low melting points that are needed for hibernation.