Comparative analysis of physiological and morphological adaptations of sooty shearwaters, with non-diving seabirds

Abstract

Sooty shearwaters (Puffinus griseus, Procellariidae family) are exceptional athletes, annually flying thousands of kilometres and foraging to depths of up to 93 m. The aim of this research was to elucidate some of the physiological and anatomical adaptations that allow this seabird to dive to depth and compare these traits to those for a sympatric species and a non-diving species from the same family, all three species being endemic to the Auckland region of New Zealand. To study the physiological adaptations, total oxygen stores were calculated from the measurement of blood, muscle and respiratory oxygen stores, and the theoretical aerobic dive limit (tADL) was calculated, using a diving metabolic rate obtained from the literature. The blood oxygen store was estimated from the haematological profile of the study species at the time of laying/incubation. The muscle oxygen store was measured from the muscles of by-catch birds and the respiratory oxygen store was measured using Magnetic Resonance Imaging of the lungs and air sacs of by-catch sooty shearwaters. The buffering capacity of the muscles was analysed to assess the tissues’ ability to maintain homeostasis under acidotic conditions. The wing surface area for each species was measured and the wing-loading (WL) was calculated. The WL were compared against reported diving variables and against muscle oxygen stores. A review of literature was performed, looking for correlations between wing-loading and diving parameters for a number of Procellariidae species. Total oxygen stores and tADL were 28.4 ± 3.1 mL.kg-1 and 29.0 s respectively and were lower than those of the other study species. The respiratory volume of the sooty shearwater was measured, using MRI, rather than using allometric scaling and this may account for the low values observed for the sooty shearwater. The tADL was lower than that observed for mean dive duration (39 s) and less than the maximum recorded duration of 103 s and it is possible that other physiological mechanisms help to conserve oxygen during a dive as no additional resilience to metabolic acidosis was found. A linear relationship was seen between muscle oxygen stores and WL and muscle oxygen stores and dive depth, correlating both physiology and anatomy with diving ability. The literature review reinforced the positive relationship between WL and diving ability for Procellariidae species. Sooty shearwaters have evolved with physiological and morphological adaptations that support their diving ability and that differ from those found in other flying, diving avian families.