BEHAVIORAL-CHANGES IN FASTING EMPEROR PENGUINS - EVIDENCE FOR A REFEEDING SIGNAL LINKED TO A METABOLIC SHIFT

This study examines the relationships between metabolic status and beh avior in spontaneously fasting birds in the context of long-term regul ation of body mass and feeding. Locomotor activity, escape behavior, d isplay songs, body mass, and metabolic and endocrine status of captive male emperor penguins were recorded during a breeding fast. We also e xamined whether body mass at the end of the fast affected further surv ival. The major part of the fast (phase II) was characterized by the m aintenance of a very low level of locomotor activity, with almost no a ttempt to escape, by an almost constant rate of body mass loss, and by steady plasma levels of uric acid, beta-hydroxybutyrate, and corticos terone. This indicates behavioral and metabolic adjustments directed t oward sparing energy and body protein. Below a body mass of similar to 24 kg (phase III), spontaneous locomotor activity and attempts to esc ape increased by up to 8- and 15-fold, respectively, and display songs were resumed. This probably reflected an increase in the drive to ref eed. Simultaneously, daily body mass loss and plasma levels of uric ac id and corticosterone increased, whereas plasma levels of P-hydroxybut yrate decreased. Some experimental birds were seen again in following years. These findings suggest that at a threshold of body mass, a meta bolic and endocrine shift, possibly related to a limited availability of fat stores, acts as a ''refeeding signal'' that improves the surviv al of penguins to fasting.