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The most exciting phrase to hear in science, the one that heralds new discoveries, is not “Eureka” but “That’s funny...” –Issac Asimov

What would you do if you discovered something that proved wrong a fact that had been accepted for decades? Would you be excited, skeptical, or think you had just gone crazy?

Image courtesy of Wikimedia Commons.

Some of the most interesting discoveries can come across as "mad science" because they disprove ideas that other scientists had accepted since they were in school. Dr. Karin Pfennig made a discovery that went against everything that most people have learned about the definition of a species, and at first, not many people believed what she had found.

But before we get to that... What is a species?

The usual definition is that a species is a group of living things that can interbreed to produce fertile offspring. That is, their babies will be able to make their own babies in time. Therefore, animals that are two different species cannot produce fertile offspring. All animals want their children to continue passing on their genes, so interbreeding with a different species, or hybridization, would be a bad thing to do.

Remember, populations of animals will evolve to be better able to survive and reproduce in their environment. Because hybridization does not fulfill this goal, a group of animals will specifically evolve not to interbreed with another species.

Some species are more closely "related" than others. Mules are an example of a hybrid of two closely related species. They are the offspring of a female horse and male mule. The horse's and donkey's genes are similar enough that they can produce healthy offspring, but the resulting mule will not be able to make more baby mules. It is sterile. Closely related species can often hybridize, but their offspring will usually be less likely to reproduce or possibly completely sterile. This is a raw deal for the mule's parents. They spent valuable energy making and raising their baby, but it will not be capable of continuing to pass on their genes. This is why we don't have wild herds of mules roaming the country.

Mule
Image courtesy of Wikimedia Commons.
Pictured above: an evolutionary dead-end.

However, Dr. Pfennig found an exception to this rule: a species of spadefoot toad that will purposely mate with another closely related species!

What are spadefoot toads, and why are they called that? In short, they have spades sticking out of their feet! They use these spades to dig burrows in the sand.

Image provided courtesy of Linton and Charles Robertsonand John White

These toads can live in many different places, but they are most well-known for how they survive in the desert. They spend most of their life buried in the sand where they surround themselves with a mucus “shell” to stop themselves from drying out. They can sleep like this for over a year without eating! Once a year when it rains, the toads will dig out of their burrows to eat and mate.

Spadefoot toads mate and lay their eggs in the puddles that form after a rainstorm. Because the rain that they need is not guaranteed to come, it is very important that the female toads choose an appropriate male when she does get the chance to mate.

Once the eggs have been laid, the tadpoles then have to race against the falling water levels in their puddle. They will not survive if they do not change from a tadpole to a toad that can live on land before the water evaporates. Some species of toad can completely change from a tadpole to a toad in a week!

This process can take months or years for other amphibians.

One species of these toads, Plains spadefoot toads or Spea bombifrons, are the species that shows an unusual preference for hybridization. Mating between two different species, or “hybridization” is usually a really bad thing for animals to do. Their offspring will either not be able to reproduce, or it will be deficient in some other way. So why would spadefoot toads hybridize on purpose? And how do they decide when to do it?

Spadefoot toads seem like an obscure species to study. How did Dr. Pfennig first realize that something strange was happening with these toads?