Testing optimal foraging models for air-breathing divers

Abstract

Models of diving optimality qualitatively predict diving behaviours of aquatic birds and mammals. However, none of them has been empirically tested. We examined the quantitative predictions of optimal diving models by combining cumulative oxygen uptake curves with estimates of power costs during the dives of six tufted ducks, Aythya fuligula. The effects of differing foraging costs on dive duration and rate of oxygen uptake (VO2up) at the surface were measured during bouts of voluntary dives to a food tray. The birds were trained to surface into a respirometer after each dive, so that changes in VO2up over time could be measured. The tray held either just food or closely packed stones on top of the food to make foraging energetically more costly. In contrast to predictions from the Houston & Carbone model, foraging time (tf) increased after dives incorporating higher foraging energy costs but surface time (ts) remained the same. While optimal diving models have assumed that the cumulative oxygen uptake curve is fixed, VO2up increased when the energy cost of the dive increased. The optimal breathing model quantitatively predicted ts in both conditions and oxygen consumption during foraging (m2tf) in the control condition, for the mean of all ducks. This offers evidence that the ducks were diving optimally and supports the fundamentals of optimal diving theory. However, the model did not consistently predictts or m2tf for individual birds. We discuss the limits of optimal foraging models for air-breathing divers caused by individual variation. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.