Effects of surface activity patterns and dive depth on thermal substitution in fasted and fed lesser scaup (Aythya affinis) ducks

Abstract

High thermoregulation costs incurred by aquatic endotherms can be reduced if heat from digestion and exercising muscles can substitute for thermogenesis. Costs (VO2) of surface activities and diving were measured in lesser scaup (Aythya affinis (Eyton, 1838)) ducks while fasted and when feeding on mixed grain at depths of 1.2 and 2 m at thermoneutral (23 °C) and sub-thermoneutral (8 °C) water temperatures. Substitution occurred if the heat increment of feeding (HIF) or costs of activities above resting metabolic rate (RMR) were lower at 8 °C than at 23 °C, indicating that the thermoregulatory part of RMR at 8 °C had been reduced. Changes in body core temperature were not significant, and there was no substitution of HIF. At the surface at 8 °C, fasted birds that were active (nonresting) >75% of the time had lower RMR, and lower cost of activities above RMR, than less active birds. Dive recovery cost after dive bouts (DRC) constituted 33%–35% of total dive costs at 23 °C and 48%–55% at 8 °C. Dive costs above RMR (including DRC) were lower at 8 °C than at 23 °C by 14% at 1.2 m and 25% at 2 m, suggesting appreciable substitution that increased with longer, deeper dives; however, results varied widely and were not statistically significant. Our experiments indicate that thermal substitution varies in importance depending on levels of activity and heat loss.