Coolers Full Of Toad Eggs: Zoo Scientists Work To Save Endangered Amphibians

Houston Zoo scientists are wrapping up the springtime breeding program for the Houston toad.

Houston toad in a gloved hand
The Houston toad is close to extinction, with an estimated 300 left in the wild, mostly in Bastrop County. The Houston Zoo houses an additional 450, and breeds them each spring in the hopes of re-building the wild population. Photo by Houston Zoo/Beth Moorhead


The Houston toad, once native to this sprawling and humid metropolis, has not been found in the city for decades. Only three hundred are estimated to be left alive in the wild — mostly in the piney woods in and around Bastrop State Park. That leaves more Houston toads living at the Houston Zoo than in the wild.

You can see two Houston toads in the regular zoo exhibit, but the rest — about 450 — live behind closed doors. Their homes are rows of clear tanks filled with pools of water and hillocks of fluffy moss, where the toads nestle quietly.

If the Houston toad has a future, it begins here, in this biological ark. This captive group is the nucleus for a breeding program that has the goal of increasing the outdoors population until it can reach a stable and self-sustaining level.

As water filter tanks hum in the background, Dr. Lauren Howard, a zoo veterinarian, prepares vials of egg-stimulating hormones to inject in the female.

Dr. Lauren Howard, a veterinarian at the Houston Zoo, prepares vials of hormones.
Photo credit: Houston Zoo

“This is day one of five days, where they get injected on day one, day four and day five,” Howard says.

Females get three injections to stimulate egg production, and males also get one that, essentially, gets them in the mood.

“The males can be quite spunky especially this time of year,” Howard says. “If we grab them up to work on them, sometimes they’ll clutch our thumb thinking that they’re going to breed our thumb or whatever. So they’re very animated.”

Up close, each toad has the unique, crinkled beauty of an autumn leaf — grey-brown bumps and ridges mixed with hints of orange, yellow and green. Female toads are about the size of tennis balls, while males are much smaller. 

As Howard wields the syringe, two assistants scan each toad with a microchip reader to confirm its identity. Each male and female is carefully matched for genetic purposes.

“So we hold the toad nice and still and then we do an injection right into their abdominal cavity,” Howard says as she plunges in the needle.

The female toad squirms a bit, but it’s over quickly.

So far this spring the toad pairs have produced almost 300,000 fertilized eggs.

Houston Toad Egg StrandsStrands of newly-fertilized eggs await transportation from the Houston Zoo to ponds in Bastrop County. © Houston Zoo/Stephanie Adams

Each week the toad team puts new eggs into coolers lined with plastic bags and filled with water. They drive them to ponds in and around Bastrop State Park.

A recording of male Houston toads in Bastrop County provides audio evidence of a decimated population, according to Stan Mays, curator of herpetology at the Houston Zoo.

The recording captures the sound of only a few calling males — nothing like the oscillating waves of sound from hundreds of calling males, that he used to hear back in the 1980s.

“I have witnessed the population crash, from times when you would go to Bastrop County and you would hear literally a hundred toads at one spot and where you’d have to actually hold your flashlight down while you were walking to make sure you didn’t step on any,” Mays says.

The habitat of the toads had been shrinking for decades, mostly from development and agriculture. But more recently it was the one-two punch of drought and wildfires that have practically driven the toad to extinction.

“To find seven calling toads in a week is a really big deal now. And just not happening,” Mays says. “And after the fire, for a while we didn’t hear anything.”

Re-introducing the toad is a numbers game. The zoo may dump cooler after cooler of eggs into ponds, but Mays says only one out of every thousand eggs will survive. Fish, birds and other predators all love toads.

“It’s much more difficult to establish an animal like a toad which everything that can eat a toad, does, than it is to establish something like a wolf, where once you quit hunting the wolf, they come back, because nothing eats a wolf,” he explains.

Not to mention that a Houston toad doesn’t have the charisma of a wolf or the popularity of a panda bear. But Dr. Howard says saving them is just as important.

“To be able to do the hormones one day and the walk in the next day and see tank after tank of eggs of these highly endangered amphibians that might be extinct if not for our program it’s a really, a really good feeling,” Howard says.