Scared Eggs Hatch Smarter Tadpoles

Researchers at Oregon State recently published findings suggesting American bullfrogs are capable of learning to avoid predators while still in the egg. This newly identified ability adds to our understanding of how the bullfrog has managed to become so successfully invasive.

The American bullfrog, Lithobates catesbeianus, is listed in the top 100 worst invasive species in the world by the Invasive Species Specialist Group, a global network of scientific and policy experts on invasive species operating in over 40 countries. It’s earned this reputation by invading over 40 countries on four continents, laying up to 20,000 eggs at once, and outcompeting many native amphibians in the process.

According to an ODFW bullfrog fact sheet, adults can reach up to eight inches, thrive in a variety of habitats, and tolerate a range of temperatures. In some ways, the most damaging aspect of the bullfrog is its desire and ability to eat anything that fits in its mouth. This includes native frogs, toads, snakes, fish, and insects.

Though native to central and eastern United States and southern Canada, the bullfrog spans every western continental state, as well as Hawaii, Europe, Asia, the Caribbean, and South America. It reached Oregon in the early 1900s. In a stroke of brilliance, it was decided that because you can eat bullfrog legs, they should be farmed. Long story short, the industry collapsed, prompting many frog ranchers to dump their inventory into the wild.

Continuing the long and tedious quest to figure out how to stop the bullfrog, OSU researchers in the Garcia Lab conducted a study on their eggs. After collecting eggs from a pond known to be free of predators, the eggs were placed into three separate categories: a group exposed to fish odor, a group exposed to fish odor and signals of injured tadpoles, and an unexposed control group.

They found that when exposed to the greatest risk at the embryonic level – fish and wounded tadpole odor – longer-bodied tadpoles developed. Furthermore they found that when exposed to variable predation risk over the course of their embryonic development, tadpoles would develop a more flexible anti-predator strategy. In essence, they learned to identify and adapt to their predators before they themselves were the target of predation.

The study does not suggest how we might better manage American bullfrogs, but it does contribute to our understanding of how and why they have become such successful invaders. The good news is that while more research is being conducted, you can do your part to help out. That’s right, the bullfrog is still edible, you don’t need a license to harvest, and it’s open season all year. Wheeeew doggie!

Check out the top 100 worst invaders in the world at www.issg.org – and notice humans aren’t on it. 😉 Also see the lab doing this research at garcia.fw.oregonstate.edu.

By Anthony Vitale

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