Foraging Tactics of a Terrestrial Salamander: Judging Prey Profitability

Abstract

(1) Optimal foraging theory predicts that predators will rank all potential types of prey by their profitability and then specialize on the most profitable type when it is abundant. We designed an experiment to test whether the red-backed salamander, Plethodon cinereus (Green), determines the caloric profitability of prey by innate recognition, by relative sizes of the prey or by learning through previous experience with the prey types. (2) Salamanders were trained for 3 months under four conditions: biweekly feedings of (I) both large and small flies, (II) large flies only, (III) small flies only or (IV) a non-dipteran prey. Each group was then given a high density of both large and small flies and choice of diet was determined. (3) Only after Condition I did the salamanders appear to forage 'optimally': expressing a high degree of specialization on large flies, rejecting more encountered small than large flies, pursuing more large than small flies, having shorter intercapture intervals and higher positive rates of net energy. (4) The salamanders did not rely on an inherited ability to assess profitability, because they could specialize on the more profitable prey even though it is not in their natural diet. They did not use relative sizes of prey to judge profitability, because they failed to specialize after Conditions III and IV. (5) We conclude that they learned through foraging experience to assess profitabilities of the prey types. Such experience is necessary to balance off the gross caloric value of the prey types (which can be estimated by size) against the rate at which those prey types can be assimilated once ingested (which is a variable depending on chitinous content of the arthropods).