Keith Wilhelmi's Self-Paced Frog Dissection Guide
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Why I'm promoting bullfrogs for dissection

I certainly do not want teachers' support of frog dissecting to result in harm to frog populations. The staff people of two well known biological supply companies have assured me that their grass frogs are collected according to government regulations which prohibit damage to wild populations.

Nevertheless, I've become convinced that bullfrogs are the much better choice.

First, bullfrogs sold by biological supply companies are farm raised (produced primarily for the food market) negating the fear of harming wild populations. In fact, as the following links give evidence, in large areas of North America bullfrogs are now considered an invasive species!

Columbia University Introduced Species Summary Project: North American Bullfrog

United States Department of Agriculture Invasive Species: Aquatic Species - Bullfrog (Rana catesbeiana)

National Wildlife Federation National Wildlife Magazine: A Plague of Aliens

International Union for Conservation of Nature Global Invasive Species Database: Ecology of Rana catesbeiana

Enviromental Protection Agency Invasive Species in the Pacific Northwest: Bullfrogs

Second, the added size of bullfrogs makes for a remarkably better dissection experience! This is particularly true for younger adolescents, many of whom are still developing their fine motor skills. My first experience with bullfrogs (a few years ago) left me so impressed that, if my budget is ever reduced, I will opt to increase the size of my student teams rather than return to grass frogs. Bullfrogs’ hearts are big enough to see each chamber clearly and their stomachs often yield crawdads in addition to insects. Dissection of a bullfrogs’ brain and eyes reveals subdivisions large enough to clearly discern, and inflation of the large lungs and urinary bladder is something students will never forget!

Comparison of the largest available sizes
of grass frogs and bullfrogs

5½" Grass frog
grass frog size comparison
Grass frog photography thanks to David Miller
6-7" Bullfrog
bullfrog size comparison
Bullfrog photography thanks to Gayle Wilhelmi

What about cost? It pays to shop around. If purchased in quantities of at least ten, the largest bullfrogs (6-7") range from $6.75 to about $10.00 each, while the largest grass frogs (5½") range from $5.10 to about $6.00 apiece. In lean budget years, assigning students to teams of 3 instead of 2 will nearly offset the cost difference.

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