These miniature frogs have lost the ability to leap straight because they’re just too tiny

A group of miniaturised frogs known as pumpkin toadlets, or flea toads, have become so small they’re no longer able to maintain their balance during quick manoeuvres like jumping, according to new research in the journal Science Advances.

International researchers have studied four species in the frog genus Brachycephalus – a clade of miniaturised frogs from Brazil – and shown that though they are capable of jumping (when prodded) they’re not very good at it.

The frogs lose control of their trajectory mid-leap, and ungracefully pirouette through the air before coming in for a crash-landing.

“They’re not great jumpers, and they’re not particularly good walkers either. They sort of stomp around in a stilted, peg-like version of walking,” says co-author Edward Stanley, director of the Florida Museum of Natural History’s Digital Discovery and Dissemination Laboratory in the US.

The problem? These frogs are just too small for the fluid needed for balance to move around sufficiently in their heads.

In fact, adults range in size from only 7.4-19.7mm from snout to rear.

Brachycephalus brunneus jumping. Credit: Essner et al., (2022).

These frogs’ heads are just too small

Vertebrates (animals with spines) maintain their balance though the vestibular system – a fluid-filled network of spiralling chambers and canals within the inner ears.

When an animal moves its head, the liquid in these chambers moves with it, brushing against tiny sensory hair cells that send electrical impulses to the brain that allow it to maintain balance, tell up from down, and detect acceleration – allowing animals to control their posture and movement.

The semicircular canals in particular are responsible for sensing how an animal’s orientation changes (angular acceleration) and are remarkably similar in size across different species with widely varying body mass.

For example, baleen whales – a group that includes the largest mammals on Earth – have semicircular canals that are only of average size for mammals and slightly larger than in humans.

But while this works for larger animals, the vestibular system takes up proportionally more and more space inside the skull for animals on the other end of the size spectrum.

Three-dimensional models of the pumpkin toadlets’ vestibular system were created by stitching together computed tomography (CT) scans, and it turns out that they actually have the smallest vestibular system in any vertebrate organism measured yet.

Their fluid-filled chambers look like overinflated balloons.

“Even though the canals are as big as they can possibly be relative to their heads, they’re still not big enough for the liquid to move at a rate that would allow them to maintain balance,” Stanley explains.

As a result, they just can’t maintain their posture at all when jumping.

Brachycephalus coloratus jumping. Credit: Essner et al., (2022).

Why jump when you can just sit there?

But this lack of grace hasn’t spelled doom for Brachycephalus frogs just yet, because their main tactic to avoid being eaten involves staying in one place and being as unappetising or as inconspicuous as possible.

“They’re peculiar frogs,” says co-author André Confetti, a PhD candidate at the Federal University of Paraná, Brazil. “They can’t swim, they don’t have tadpoles, and they don’t seem to get around much either.

“We’ve monitored the acoustic behaviour of these frogs and have been able to record the same individual at the same spot over the course of a year.

“When they do jump around, they’re probably not that worried about landing, because they’re doing it out of desperation,” he adds. “They get more benefits from being small than they lose from their inability to stick a landing.”

Brachycephalus pernix jumping. Credit: Essner et al., (2022).

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