You probably know that jumping out of a moving car is a bad idea. You probably also don't find it surprising that this is a common opinion among other people as well.

People who habitually jump out of cars will be unlikely to survive long enough to have children. This means that any genes that contribute to this risky behavior will not be passed on to any offspring. Therefore, the “jumping out of cars” behavior has not become common in the human species. In the same way that helpful traits are passed on, unhealthy traits are weeded out. The so-called "Darwin Awards" are based on a tongue-in-cheek interpretation of this fact.

Usually, animals will behave in a way that will allow them to survive and reproduce. Those animals that are able to eat the most food and produce the most offspring will have the most influence over what the next generation looks like and how it behaves.

In this way, populations will change over time as more and more individuals are born with the traits that benefited their ancestors. Destructive traits like jumping off of fast-moving objects would not be very common because the individuals carrying those traits would die before they could reproduce.

Image courtesy of Wikimedia Commons

But, going back to the car example, what if your car is speeding towards a cliff? In that case, jumping out of the car would be better than showing your normal behavior. Whether or not a certain behavior is beneficial depends on your current situation. You receive certain stimuli (like an approaching cliff), which provokes a reaction in your body (considerable amounts of adrenaline), and you respond in the way that is most appropriate (making your escape). This is a context-dependent behavior, or a behavior that changes depending on what is going on in your environment.

Very unwise.
Image courtesy of Denis Frolov

Image courtesy of Wikimedia Commons

Scientists who study context-dependent behavior look at how different situations affect the decisions of animals. Dr. Pfennig discovered a very interesting example of context-dependent behavior when she observed the conditions under which a species of spadefoot toads called Spea bombifrons, or Plains spadefoot toads, will mate with a different species, Spea multiplicata, or New Mexico spadefoot toads.

You have probably learned that two different species cannot produce fertile offspring. While this is usually true, new discoveries are built on finding exceptions to every rule! For example, fertile grizzly/polar bear hybrids have been recorded in captivity and in the wild.

Polar/grizzly bear hybrid

Image courtesy of Alexandra Preub
Top: Female and male hybrids. Bottom left: Grizzly bear. Bottom right: Polar bear.

What normally stops different species from producing fertile offspring? Well, what causes one species to be different from another? It all comes down to DNA. If two species’ genes are very similar, they are more likely to be capable of producing offspring. However, the babies may not be as fertile as their parents when they grow up, so their genes won't have as much influence on the next generation.

There are many ways that animals ensure that they only mate with their own species. For example, they might separate themselves geographically or mate at different times of the year. However, a major exception to this rule has been discovered in spadefoot toads.

Plains spadefoot toads will choose to mate with New Mexico spadefoot toads when they meet in shallow ponds.

S. bombifrons

Plains spadefoot toad (Spea bombifrons): Courtesy of Stanley Trauth through Calphotos

S. multiplicata

New Mexico spadefoot toad (Spea Multiplicata): Courtesy of Sarah Beckwith through National Park Service


Hybrid toad: Courtesy of Richard Zweifel

Spadefoot toads tend to live in dry, desert conditions. Plains and New Mexico toads will often share a habitat. They only mate once a year when rain has produced small, temporary ponds. The female will choose a male to fertilize her eggs, and the eggs will hatch within 24 hours into tadpoles. From there, tadpoles have to race against the falling water levels to metamorphose into their toad form that can live on land.

Have you ever heard of 'The Dating Game'? It was a game show from the 1970s where a woman would choose from three eligible bachelors who were hiding behind a screen. She would ask them questions and then choose the man she thought would be the best match for a date.

Image courtesy of ABC

Every mating season, female spadefoot toads will choose one mate from a group of male toads. Just like in the TV show, she has to choose based on only sound. Different species of toads have different calls, and the female will choose the one that indicates the best match. The sound clips below are recordings of the male calls of the two species. You will need to have Quicktime installed to hear them.

Click play to hear the call of a male Plains toad.

Click play to hear the call of a male New Mexico toad.

But what makes a good match? In this case, let's talk about a Plains spadefoot toad. The mother Plains toad wants to make sure that her babies will be healthy! She will choose a mate that will give her tadpoles the best genes possible.

Since two different species of toads are living together, you would think that she would be very careful to only choose her own species. After all, hybridizing is a lot like jumping out of a car; it is almost never a good idea! All animals want their DNA to be represented in the next generation. She wants her babies to be able to reproduce and continue passing on her genes. Choosing a mate who will give her less fertile offspring is usually a very bad choice.

But here's the "cliff" in the toad's scenario. She is in a shallow pond that will evaporate very quickly. This is when she needs to make a tough choice. If she wants her offspring to live at all, she has to make sure that they will metamorphose, or change from a tadpole into a toad, very quickly. But how will she do this?

New Mexico and Plains spadefoot toads are different, but closely related species. One of the unique characteristic of the New Mexico toads is that they metamorphose very quickly! If the mother Plains toad decides to mate with a toad of that species, her babies will be less fertile, but they will mature fast enough to survive!

So now we know why the toads make this unusual choice. But how do they do it? What parts of their brain and what hormones are responsible for making this choice? How did this behavior evolve?

But there is one other question that may be on your mind. Why are we even studying toads?

Photo courtesy Texas Parks and Wildlife Department © 2006.