Foraging Tactics of a Terrestrial Salamander: Choice of Diet in Structurally Simple Environments

Abstract

Plethodon cinereus was tested in a structurally simple environment in the laboratory to determine whether the foraging tactics of the salamanders, when given different densities of prey, adhered to predictions of optimal foraging theories. Two morphologically similar species of flies, differing in size and caloric content, were used as prey. As predicted by theory, the salamanders had an indiscriminate diet at low prey densities but specialized on large flies at high densities. Specialization occurred by increasingly "ignoring" small flies that were encountered; large flies were not similarly ignored. Contrary to theory, salamanders showed partial preferences at high densities, since small flies were not eliminated from their diet. Choice of diet was not based strictly on the absolute abundance of large flies, although specialization on them occurred even when small flies were twice as abundant. This may have been the result of mistakes in judging the profitability of small flies at high densities, when flies were being captured rapidly. Such mistakes were indicated by a higher proportion of aborted attacks on small flies than on large flies. Salamanders switched from pursuit to ambush tactics with increasing prey density, contrary to Norberg's (1977) optimal food-searching model. Norberg's assumption that the highest energy-consuming search method is the most efficient one proved incorrect for salamanders: at high density, capture rates were higher by ambush than by pursuit, reflecting the wariness of prey to moving predators. Evidence suggests that salamanders did not use capture rates to monitor prey densities but instead used encounter rates. There is no evidence that salamanders, at high densities, increasingly specialized on large flies along the sequence of prey captures. Net energy while foraging increased from low to high prey density as a result of concurrent increases in specialization (increasing assimilated energy) and ambushing (decreasing expended energy). The observed behavioral tactics at both low density (pursuit and indiscriminate diet) and high density (ambush and specialized diet) yielded higher values of net energy than would be expected if those tactics had been reversed between densities. Consequently the salamanders chose the appropriate set of tactics, in terms of net energy, at both densities. However, the salamanders could have increased net energy while foraging at high density by not ignoring small flies that had been encountered, because of the trivial cost of time and energy to attack them. Small flies took longer to digest than large flies because of their proportionally larger chitinous exoskeleton. We conclude that the salamanders attempted to compromise between maximizing net energy while foraging and minimizing the passage time of prey through the digestive tract. We suggest that foraging theories that ignore differential digestion rates of prey types can lead to false predictions about optimal choice of diet.