FITNESS CONSEQUENCES OF FORAGING BEHAVIOR IN THE ZEBRA FINCH

OPTIMAL foraging theory is based on the assumption that natural selection favours animals that forage most efficiently 1-3. But such selection does not act directly on foraging efficiency, but rather indirectly by favouring animals that survive and reproduce most successfully. Studies that use optimal foraging models often assume that maximization of some behavioural currency, such as the animal's net rate of energy gain, maximizes the animal's fitness 4, but rarely is an attempt made to test this assumption 5-8. Most studies of the effects of foraging behaviour on fitness fail to control for the amount of energy gained by the foraging animals 5-8, and lead to the obvious conclusion that animals that eat more reproduce more. Often the studies do not control for characters correlated with foraging behaviour 6 or compare traits assumed to be correlated with fitness 7,8. A better method would be to assign net rates of energy gain to randomly chosen individuals for their entire lifetimes in a controlled environment and measure fitness directly. Variation in the amount of energy consumed would be controlled by using individuals that employ a time-minimizing foraging strategy 9 and would alter the time taken to satisfy their daily energy requirements, while obtaining the same absolute amount of energy. I have now manipulated the net rate of energy gain in four populations of the zebra finch Taeniopygia guttata, and show that fitness, as measured by population growth rate, is indeed positively and significantly correlated with the net rate of energy gain.