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Treebeard's Stumper Answer
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?


To maintain a stable population, each Western Toad must produce just one offspring to replace it during its 20-year lifetime. So why do they lay so many eggs? Unlike frogs, our toads have dry skin and only need standing water for breeding. Maybe this isn't coincidence. With our unpredictable climate swings between deluge and drought, the temporary ponds and ditches that these toads prefer must usually fail. The toads' strategy of laying tens of thousands of eggs that won't survive seems brutal, but it must be effective in the long run since the toads are still here!

These tiny worm-like toad tadpoles are starting to leave their egg mass and wander around the pond, though they will continue to congregate more than the tree frog tadpoles. They face many predators including Backswimmer beetles, Damselfly nymphs, Pacific Tree Frogs, and maybe even their own parents. Crows and other birds also take a toll, especially when the water level is low.

The very dark tadpoles tend to congregate on the windward edge of our little pond. Maybe their dark color and the warm water in the shallows accelerates their growth? Note the pond is full of grass clippings after being mowed. Life is full of hazards for these little critters, but the biggest hazard is drying out. We've already turned on the hose to maintain the water level despite the heavy rain this year.

Our toads are California Toads (Bufo boreas halophilus) which have lighter markings than the true Western Toads (Bufo boreas boreas) found further north. The subspecies name suggests they are more alkali-tolerant. I grabbed this photo from the USGS Northern Prairie Wildlife Research Center.

Toads doing it in the tight embrace known as amplexus. The male fertilizes the eggs externally as they emerge. Note the twin tubes of eggs. What is their anatomy that can do this? I found this photo on the Web, but I can't remember where.

Notes:

Two tubes of eggs 10 feet long with 40 eggs per inch come to:

     eggs        inch        foot       tube         eggs
  40 ----  x  12 ----  x  10 ----  x  2 ---- =  9600 ----
     inch        foot        tube       toad         toad
That's a conservative estimate. Other sources estimate the fecundity of these Western Toads at 12,000 or 16,500 eggs per laying. One of my books, Doris Cochrans's Living Amphibians of the World (Doubleday, 1961) says that the Cane Toad (Bufo marinus) can produce 35,000 eggs. The author also says that "toads are probably the most prolific of all amphibians."

Of course our toads face many predators, especially as tadpoles. Backswimmer beetles seem especially deadly. I've watched them catch tadpoles and suck them dry as they swim with their straw-like mouthparts. As the water level drops to a little puddle swarming with tadpoles and almost-toadlets, the crows show up. But frog tadpoles also face many predators, and they don't lay nearly as many eggs. Adult toads of the genus Bufo even have poison glands as extra protection. So why do toads lay so many eggs?

Most years, our little pond dries up before any toads mature, despite our attempts to keep it going with the hose. I've only seen mature toadlets leave the pond during our 1997-8 El Niño year with record (and well-spaced) rainfall that kept the pond level high all spring. Nevertheless, we still have toads at school.

As a naturalist, my working assumption is always that there's a reason for everything in nature. I'm struck by two facts about our Western Toads: they lay more eggs than other amphibians, and they are better adapted to dry conditions in several ways. Toads are actually less likely than frogs to "die of light". My guess is that our toads lay so many eggs in marginal water sources because their eggs will fail in most years. Toads can't predict the weather any better than we can, so they make the most of every year. But if their young only mature one year in twenty, that's good enough. It's brutal for the kids, but effective in the long run for the species.  
 
A Toad can die of Light!
Death is the common right
     Of toads and men,--
Of earl and midge
The privilege.
     Why swagger then?
The gnat's supremacy
Is large as thine.

     -- Emily Dickinson

 

Special dry-climate adaptations of Western Toads include:

Our weather in Central California is especially unpredictable, with erratic swings between drought and deluge. (See my Rain Or Shine (5 Jan 2001) stumper.) We had an extended drought in the 1980s that brought water rationing and threatened to empty the city water supply at Lake Cachuma. Santa Barbara voters decided to build a desalination plant and import California State Water as a result. These decisions have had a big effect on local land use policy. Recently we've had a series of wet years. Toads have to deal with these climate swings more stoically.

The problem with my guess is that toads are found around the world, in wet climates as well as dry. If I'm right that laying so many eggs is a evolutionary response to a changeable dry environment, then I would expect three results about our California subspecies of the Western Toad (Bufo boreas halophilus):

I don't know if these claims are true, but they are surely verifiable, and that would settle it.

Julie and I attended a meeting of the Santa Ynez Valley Natural History Society last week by Dr. Craig Fusaro (UCSB and CalTrout) about Steelhead Trout Restoration on the Santa Ynez River. With toads on my mind, that's what I learned. Steelhead Trout are anadromous fish like salmon that live in the river and the ocean. I think of them as belonging in the Pacific Northwest, like my folks' place on the North Umpqua River in Oregon. But there's evidence that those northern rivers were too low during the Pleistocene Ice Ages, and the species retreated south. As the climate warmed over the last 12,000 years, they again moved north to recolonize the northern coastal rivers. But the endangered Southern Steelhead Trout retain greater genetic diversity and special adaptations, for example, they mature faster and they tolerate warmer water. Southern Steelhead can survive 20-year droughts because adults run as far up the river as they can, and they get land-locked in perennial pools in the upper reaches of the river. I believe there are still landlocked steelhead/trout in Los Laurales Canyon below my house on San Marcos Pass. Those fish even survived our extended drought in the '80s.

Climate change has been on-going since the Pleistocene, so our toads at school could also be a genetic resource for future evolution. Our Western Toads seem to be doing ok, but with a 20-year lifetime, it might take that long to notice a problem. I'll keep watering our pond at school, and I wish them well. These are the survivors!

Here are some links for your own research:

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