Tiny pumpkin toadlet frogs are very clumsy jumpers and now we know why

Pumpkin toadlets are only 1 centimetre long – and the minuscule size of their balance organs might explain why they jump so haphazardly

By Jake Buehler

Pumpkin toadlets — miniature frogs native to Brazil — are woefully clumsy jumpers, unable to control their landings. New research suggests that this coordination deficit is rooted in the tiny inner ear canals that guide their balance.

Some relatively primitive frog families — like New Zealand frogs and tailed frogs — don’t have controlled landings when jumping, says Richard Essner at Southern Illinois University Edwardsville. But when his collaborators in Brazil found that 1-centimetre-long pumpkin toadlets (Brachycephalus sp.) were similarly ungainly, it came as a surprise, since they aren’t closely related to the other frogs.

“Given that pumpkin toadlets are among the world’s smallest vertebrates, it seemed likely that this was a size effect rather than ancestral behaviour,” says Essner.

As the semicircular canals of the inner ear get smaller, the fluid inside doesn’t flow as easily through the tubes. This causes reduced sensitivity to changes in the body’s rotation.

To see how this might affect landing posture, Essner and his colleagues made three-dimensional X-ray scans of the semicircular canals of 147 frog and toad species, across the full size range of living species. They found that Brachycephalus frogs have the smallest canals known from any vertebrate.

High-speed video analysis of jumping pumpkin toadlets revealed that the change in rotation speed was lowest in the “flight” stage of the jump. This means that, with their insensitive ear canals, the toadlets may find it hardest to track and control how their bodies are moving in this phase, and end up in the wrong orientation when they get to the landing. In previous work, toads with surgically impaired ear canals landed with Brachycephalus-like inelegance.

The toadlets may compensate for their graceless vaulting with enhanced defences against predators, says Essner. The amphibians are often toxic, camouflaged or fortified with thickened bone in the skull and back.

Molly Womack at Utah State University says the results may hint at the lower limit for ear size in vertebrates, illustrating a consequence of miniaturisation. “Maybe you have to give up your balance if you want to get your inner ear that small,” she says.

Womack points out that other frogs have evolved bodies just as minuscule as the toadlets, but whether they have similarly poor aerial balance is so far unknown.

“We don’t know if it’s a necessary cost,” she says. “Maybe other [frog groups] have found a way to not pay that cost and still get just as tiny.”

Journal reference: Science Advances, DOI: 10.1126/sciadv.abn1104

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