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ONTARIO, Canada -- Nature has many tricks to protect its weaker creatures against more aggressive prey. One of these is being so unpalatable that predators have no desire to eat you. By being unpalatable to fish, bullfrog tadpoles have a better chance to grow up to become American Bullfrogs.
A study in the current issue of the journal Herpetologica examines this unpalatability in larvae, which is well known in some kinds of adult amphibians as well as plants and other animals. Researchers sought to effectively control other factors that might influence the interaction between tadpoles and Pumpkinseed Sunfish. Among these factors are predator hunger levels, prey behavior and prey appearance, and the possibility of a range of taste preferences or palatability.
By training sunfish to consume a standard food ration, researchers were then able to add the skin of tadpoles to their diet and study the effect. The daily food ration ensures that the fish are not "hungry enough to eat anything." Three different species of tadpoles were presented to the sunfish, those of the American Bullfrog, the Wood Frog, and the Green Frog.
Fish are a visually oriented predator and even subtle differences in activity or body coloration of prey might influence their prey-seeking decisions. The current study offered fish the skin of the tadpoles within their food ration, eliminating outside influences on their consumption.
The results of the study indicate that sunfish can discriminate between food pellets that look identical but have different tastes, depending on which type of tadpole skin was added. A range of palatability does exist—the sunfish showed distaste for American Bullfrog tadpoles while finding Green Frog tadpoles highly palatable. Adding Wood Frog tadpoles to the diet had little effect on the fish, whose consumption rate remained steady.
This unpalatability of American Bullfrog tadpoles to sunfish is curious in that these tadpoles often are found at low densities with fish or in ponds without fish at all. It is not known whether this is a passive trait, maintained at no real cost to the animal, or is actively maintained to allow the species to exist with fish if required.
Full text of the article "Palatability in the Larvae of Three Species of Lithobates," Herpetologica, Volume 67, Issue 3, September 2011, is available at http://allenpress.com/publications/journals/herp