Degree Name

Master of Science (MS)

Semester of Degree Completion

1984

Thesis Director

Kipp C. Kruse

Abstract

Tadpoles of the American toad (Bufo americanus) have been shown to be distasteful to largemouth bass (Micropterus salmoides) predators who learn, on the short term, to avoid them. How long bass retain this learned avoidance of Bufo larvae is unknown so an experimental design was constructed in an attempt to answer this question.

Largemouth bass were divided into four treatment groups. Two groups were initially fed exclusively larvae of Bufo americanus (one group was fed large tadpoles, the other group fed small larvae). Similarily, the other two groups were initially fed spring peeper (Hyla crucifer) tadpoles (one group was fed large tadpoles, the other group fed small larvae). A multi-criteria rejection index based on tadpole survival, number of rejections and the time to engulf for bass fed tadpoles was constructed.

In the first feeding phase, all four treatment groups were starved for 48 hours before offering single tadpoles of the appropriate species and size at 10-12 minute intervals. In general, when bass were hungry, they consumed all tadpoles irrespective of size or species.

In feeding phase 2, one hour before tadpole treatments, fish were fed until satiated on small crayfish. Under this feeding regimen, bass avoided feeding on Bufo americanus tadpoles but readily consumed Hyla crucifer larvae.

After a 30 day non-tadpole feeing period, bass in the four treatment groups were fed exclusively on tadpoles of Woodhousei toad (Bufo woodhousei), which have also been shown to be unpalatable to bass. Bass that had previously been fed Bufo americanus tadpoles and learned to avoid them (on the short term), retained this learned avoidance condition. Bass that had previously been fed Hyla crucifer larvae apparently retain a preference for palatable tadpoles because they almost exclusively avoided feeding on Bufo woodhousei tadpoles. These results suggest that bass are capable of discriminating brown palatable Hyla larvae from black unpalatable Bufo tadpoles and retain this learned association over an extended period of time.

Experimental results indicate that large Bufo tadpoles (Gasner developmental stages 38-40) were consumed more often than small Bufo tadpoles (stages 28-30) which were almost totally avoided as food items.

Bass were often observed engulfing a Bufo tadpole, mouthing it for a short period of time, and ultimately rejecting (spitting) it as a food item. This spitting behavior was never observed with Hyla larvae. I observed 684 such rejections for 213 Bufo larvae and 201 (94%) survived uninjured. The significance of these observations to the evolution of aposematism in Bufo larvae is discussed.

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