If I were to write a children’s book, it might start like this: Hop, hop, hop, went the frog. Bound, bound, bound, goes the toad.
Toads also hop, but they’re more likely to bound like a squirrel, says a new study published January 29 in Functional Ecology. That bounding method of locomotion gives toads a leg up when it comes to dispersal, helping species like cane toads become the invading menaces we know today.
Stephen Reilly of Ohio University in Athens and colleagues studied the hopping behavior of three species of bufonid toads: American toads (Anaxyrus americanus), Fowler’s toads (A. fowleri) and cane toads (Rhinella marina). Cane toads are a particular concern because they are an incredibly destructive invasive species in Australia.
When making a single hop, a toad would take off, legs extended, and land first with its hands extended. A moment later, the folded-up legs followed. During a sequence of hops, takeoff was similar to the single hop but the landing differed — the hands and rear legs hit the ground almost simultaneously, and the rear legs were extended. That’s similar to the bounding stride that is common among small mammals such as mice and squirrels. It’s also a method of locomotion that has never before been seen outside mammals.
“Bounding strides maintain some speed, conserve momentum, [let toads] jump farther and appear to use the landing on the legs to store energy for the next take-off,” says Reilly. “All of which give the toads about an 18 percent cost savings compared to using repeating hops to cover the same distance.” With those benefits, it’s perhaps not all that surprising to find that toads bound slightly more often than hop.
The hop evolved among early frogs as an escape mechanism, a quick way to get away from something dangerous. But that kind of locomotion isn’t all that useful for the rapid expansion of a species, something that is characteristic of bufonid toads and that cane toads in particular have mastered, the researchers note.
Cane toads arrived in Australia a mere 80 years ago and have quickly spread across the northern part of the continent. Since their arrival, the toads have evolved new characteristics, such as longer legs and straighter travel, that have helped their spread. But their natural preference for bounding across the landscape appears to have optimized them for rapid dispersal.