Cardiac responses to first ever submergence in double-crested cormorant chicks (Phalacrocorax auritus)

Heart rates were recorded from double-crested cormorant chicks during their first ever and subsequent voluntary head submergences and dives, as well a s during longer dives made after the chicks were accustomed to diving. Desp ite variation between chicks, the cardiac response to first ever and subseq uent voluntary submergence (head submergences and dives) was similar to the response observed in adult cormorants. Upon submersion the heart rate fell rapidly when pre-submersion heart rate was high (325-350 beats min(-1)). T he heart rate established within the first second of voluntary submergence was between 230 and 285 beats min(-1), well above resting heart rate (143 b eats min(-1)). The same initial cardiac response occurred during longer div es performed after the chicks were accustomed to diving. In these dives the heart rate remained at the level established on submersion, unlike the res ponse observed in shallow diving adult cormorants in which the heart rate d eclined throughout the dive. The heart rate was also monitored in a separat e group of chicks in which the first exposure to water was during whole bod y forced submergence. Again, the observed response was similar to the adult response, although the cardiac response of chicks to forced submergence wa s more extreme than to voluntary submergence. Our results do not support th e hypothesis that learning (by conditioning or habituation) is involved in the cardiac adjustments to voluntary submergence. It is suggested that the initial cardiac adjustments are reflex in nature and this reflex is fully d eveloped by the first submergence event. Although the nature of this reflex pathway is obscure, cessation of breathing before submersion and the close linkage between breathing and heart rate might provide a plausible mechani sm. (C) 1999 Elsevier Science Inc. All rights reserved.