Pumpkin garlands are just a centimeter long — and the tiny size of their well-balanced organs might explain why they jump so randomly.
Pumpkin toads - miniature frogs native to Brazil - are woefully clumsy jumpers, unable to control their landing. New research suggests that this coordination deficit is rooted in the delicate inner ear canals that direct its balance.
Some relatively primitive families of frogs — such as New Zealand frogs and tailed frogs — don't control their jumping landings, says Richard Isner of Southern Illinois University Edwardsville. But when his collaborators in Brazil found that centimeter-long pumpkin leeches (Brachycephalus sp.) were similarly tough, it was a surprise, as they are not closely related to other frogs.
"Given that pumpkin eaters are among the world's smallest vertebrates, it appears that this was an effect of size rather than an inherited behavior," Eisner says.
As the semicircular canals of the inner ear become smaller, the fluid inside them does not flow easily through the tubes. This leads to a reduced sensitivity to changes in the body's circulation.
To see how this might affect landing posture, Eisner and colleagues performed a 3-D X-ray scan of the semicircular canals of 147 species of frogs and toads, across the full-size range of living species. They found that Brachycephalus frogs have the smallest canals known of any vertebrates.
High-speed video analysis of the pumpkin toad's jump showed that the change in rotational speed was minimal in the "fly" phase of the jump. This means that with insensitive ear canals, frogs may find it difficult to track and control how their bodies move at this point and end up in the wrong direction when they reach the landing. In previous work, frogs with impaired ear canal surgically prolapsed with humeral-like disorganization.
Eisner says the frogs may be compensating for their ruthless basement with reinforced defenses against predators. Amphibians are often venomous, camouflaged, or fortified with thick bones in the skull and back.
Molly Womack of Utah State University says the results may indicate minimal ear size in vertebrates, illustrating the result of miniaturization. "You'll probably have to let go of your balance if you want to get your inner ear this small," she says.
Womack points out that other frogs have developed very small bodies like frogs, but whether they have a similarly poor air balance is not yet known.
"We're not sure if this is a necessary cost," she admits. "Other [frog groups] may have found a way to avoid paying that cost, but it is still insufficient."
R. Essner, Jr., et al. Semicircular canal size constraints vestibular function in miniaturized frogs. Science Advances. Published online June 15, 2022. doi: 10.1126/sciadv.abn1104.