Tiny short-headed frog clumsily jumps through an inner ear that is too small

Too small to jump properly: that’s the bad luck of tiny short-headed frogs that live in the Brazilian rainforest. In Science Advances writes an international team of biologists that the frogs’ vestibular system is so small that they can only make uncontrolled movements prior to landing. Such poor jumping behavior can make them more likely to fall prey to predators.

The clumsy jumping technique of the short-headed frog, filmed from the side and from above by the researchers involved.

Short-headed frogs belong to the genus brachycephalus and live in southeastern Brazil. The dozens of species described are at most a few centimeters in size, and vary in color from bright orange to brown or black. Sometimes they are poisonous. And each one has a tiny inner ear.

Semicircular canals

That inner ear is essential for the sense of balance of organisms. The fragile vestibular system located there includes three ‘semicircular canals’: tubes filled with potassium-rich fluid that register rotational movements – not only in frogs, but also in humans and other vertebrates. If you move your head, the fluid moves past the sensory hair cells in the inner ear, and those cells then pass the information on to the central nervous system, which ensures correct posture.

Of all adult vertebrates, Brazilian short-headed frogs have the smallest semicircular canals, the biologists write Science Advances† This ensures that the potassium-rich fluid or ‘endolymph’ is not sufficiently set in motion when the frogs spin around during a jump. As a result, their nervous system registers only very limited angular acceleration that takes place during rotational movements, which is necessary for correct orientation. Result: the mini-frog does not land nearly as gracefully as larger frogs and toads, unless they have a damaged vestibular system.

Of course, a frog jump is not a gymnastics competition: in terms of aesthetics, nobody cares whether a frog jumps more or less beautifully. But for the frogs themselves, their jump can be vital. After all, a frog that ends up on its back is defenseless against attackers, especially if it is not poisonous or not properly camouflaged.

dead

The short-headed frogs in the experiment do not ‘lay dead’ to fool any predators. Nor do they mimic a withered leaf with stiff outstretched legs, as has been observed in other Brazilian frogs in the same habitat. During the jump, they do extend their hind legs, so that they turn less quickly and thus perhaps slightly reduce the chance of a back landing. If they do land on their backs, they scramble to their feet as quickly as possible. Furthermore, when they escape, they will mainly have to rely on other advantages: camouflage, toxicity, or simply their inconspicuous stature.

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