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Treebeard's Stumper
16 March 2001

Too Many Toads!

We have a little frog pond at Dunn Middle School. It's just a ditch that will soon dry up, but it's enough for the critters that depend on it. The Western Toads (Bufo boreas halophilus) are remarkable. They lay their eggs in twin clear jelly tubes that are 5-10 feet long, packed with 40+ tiny black eggs per inch. That's way too many toads for a "pond" that's only a few feet across! Why do our toads lay so many eggs? One must admire their optimism, but wouldn't they do better producing fewer tadpoles rather than hopelessly overpopulating a limited habitat that can't possibly support them all?

The sign says "Frog Habitat, Don't Enter." This little pond was originally dug for a tug-of-war contest, then it was something to jump over in a mountain bike challenge course. It's only a few inches deep, and it's regularly disturbed by kids and lawn mowers and tennis balls that go over the fence. The old tire was already there, and we decided to leave it. It makes a nice little aquarium and warms the water, so the tadpoles within are a bit ahead. One species' waste is another's habitat!

Western Toads and Pacific Tree Frogs lay eggs every year, but this "vernal pond" usually dries up before they mature, despite our help with the hose. Our recent El Niño year (1997-98) with record rainfall was different, and many frogs and toads matured to lay their eggs this year.

Western Toads lay their eggs in long clear jelly tubes. Look close, and you can see that the egg tubes are laid in pairs, though they are intertwined in interesting ways like bicycle tracks in the mud. What is the toad anatomy to do this? This picture shows just part of one toad's egg mass!

Western Toad eggs hatch very quickly, in a week or less, much faster than the Tree Frogs in the same pond. The young tadpoles look like planaria at first. They aggregate on their egg mass and seem to eat it. After another week, they'll venture away into the pond and mingle with the Tree Frog tadpoles.

These toad eggs were laid in a small puddle on the other side of the tennis courts. They have already dried up a week later.

I took these eggs as the puddle disappeared. They failed to hatch in their petrie dish by the kitchen window.

I took this 10x view of the same toad eggs with my Intel Play QX3 Digital Microscope. This remarkable "toy" from Intel and Mattel gives you a live microscope on your computer screen via a USB connection for under $100. What fun! I highly recommend this gadget for kids and teachers and naturalists on a budget. This was my Christmas present for myself this year. I'm working on modifying mine to make it even better.

My question remains: why do Western Toads lay so many eggs?

Answer


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Copyright © 2001 by Marc Kummel / mkummel@rain.org