Lipid Programmes and Life Strategies in Hibernators

Several mammalian hibernators have annual cycles of fat deposition and usage. However, at any time a particular level of fat is defended. The cycles can therefore be described as ones of set-points for body fat. A programmed change in set-points accounts for prehibernation fattening not proceeding at maximal possible rates. During the winter phase there is anorexia even when food is available; reasons are proposed why there must be a lowering of set-points if there is to be prolonged hibernation. Control mechanisms underlying some instances of weight loss and gain in birds are analysed in similar terms. In hibernators cyclic programmes are such that the animals do not usually operate close to physiological limits. In a quasi-predictable environment they derive advantages from a combined use of both homeostatic and rhythmic control of their fat levels.