EVOLUTION OF DAILY TORPOR AND HIBERNATION IN BIRDS AND MAMMALS - IMPORTANCE OF BODY-SIZE

1, The evolution of hibernation and daily torpor in mammals and birds remains a controversial subject. The original view was that use of tor por reflects a primitive thermoregulation, as it occurs in ancestral g roups of mammals, 2, This view is no longer widely supported. However, the interpretation of a polyphyletic derivation of torpor also has be en challenged because of the astonishing similarity of torpor patterns among various orders and even the two classes. 3, A recent argument i s that mutations required for torpor and hibernation are unlikely to o ccur simultaneously and that torpor must be plesiomorphic (ancestral), although it is not functionally primitive. Homeothermy is interpreted as a loss of the ability to enter torpor in those groups that could s urvive without the requirement of heterothermic periods for energy con servation, 4, Interestingly, while torpor in mammals occurs in the phy logenetically old groups, lending support to the hypothesis of an ance stral derivation of torpor, the opposite is the case for birds. Modern bird groups and ancestral mammal groups contain mainly small species that often rely on fluctuating food supply, whereas modern mammalian o rders and ancient bird orders contain the largest species with low ene rgy requirements for maintenance and thermoregulation, 5, It is, there fore, possible that not phylogenetic position but size and diet determ ine the occurrence of heterothermy. Moreover, because endothermy and t orpor in birds has apparently evolved separately from that in mammals and because it is possible that daily torpor and hibernation represent two distinct torpor patterns that evolved separately, a convergent ev olution of torpor in endotherms cannot be excluded.