These Australian frogs get absolutely covered in seeds

It is a rare documented instance of seed dispersal by an amphibian.
An Eastern dwarf tree frog covered in seeds.
An Eastern dwarf tree frog covered in seeds. Amphibians are not known for their seed dispersal the way that birds or mammals are, but some pod seeds may hitch rides on frogs’ wet skin. John Gould

John Gould, an amphibian scientist at the University of Newcastle in Australia, had wandered into a dense thicket of vegetation around a pond when he first spotted the fluffy frogs. Frogs are not usually fluffy, so Gould took a closer look and discovered that the frogs were covered in the wispy seeds of Typha orientalis, a common pond plant also known as the broad-leaved cumbungi.

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“It immediately reminded me of a recent science story about a frog that was found to carry pollen on its skin,” Gould told PopSci via email. “Which made me think it might be possible for frogs to do the same thing but with seeds.”

Plants have evolved many different ways of spreading their seeds around the planet. Some seeds have adapted to float on the wind, some can float through water, and others simply drop on the ground and hope for the best. But one of the most ingenious ways plants spread is through animals. They can coax wildlife to eat fruits containing their seeds or even cover their seeds in bristles that stick to the fur of an animal, who drops those seeds far away.

But while this phenomenon has been widely studied in animals like birds and mammals, this new observation could be one of the first documented instances of seed dispersal in an amphibian.

“I think we often have a simplistic view of the environment and how species interact within these systems,” Gould said.

The frogs that Gould spotted were Litoria fallax, or Eastern dwarf tree frogs, a small green species native to Australia’s east coast. After the initial spotting, he and his colleague collected some data to investigate how common it was for these tree frogs to end up covered in Typha seeds and published their results on January 3 in the journal Ethology.

Amphibian skin could help disperse Typha seeds that easily stick to the Eastern dwarf tree frog’s wet skin. CREDIT: John Gould.

The team found that around 30 percent of the tree frogs at their pond had Typha seeds stuck to them, with seeds found all over their feet, legs, backs, and bellies. Some of these frogs were carrying just one seed, but at least one individual they observed had up to 14 individual seeds all over its body.

Gould said that he believes the frogs probably picked up the seeds by climbing through the thicket of plants where the seeds had fallen. But just because you brush up against something doesn’t mean you’ll start carrying it around. The Typha seeds seemed to attach themselves to the frogs specifically because the seeds’ thin, hair-like tufts can easily stick to the frogs’ wet skin. Scientists believe the seeds’ wispy tufts evolved to help them catch the wind like a kite—but Gould said that this trait could also come with the secondary, or even unintentional, advantage of attaching the seeds to sticky frog skin.

“One trait may have several benefits, some of which have not been directly selected for but happen to be advantageous,” he said.

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Their study notes a couple of instances where amphibians are known to act as seed dispersers, like Izecksohn’s Brazilian tree frog, which has been documented carrying the seeds of a couple of different plants after eating their fruit. These examples are rare—most frogs are carnivorous, Gould said, meaning they don’t eat fruit and carry seeds around. But the role of amphibians in seed and pollen dispersal may also be understudied, he added.

That being said, it’s unclear how important these frogs are to the Typha’s seed dispersal—or even whether the frogs contribute to the plant’s seed dispersal at all. Just because the seeds stick to the frogs doesn’t mean that the frogs often carry them to places where they might sprout into new plants. To study this, researchers could follow seed-covered frogs to see where they bring the seeds and how the seeds fall off, Gould said. Additionally, he added, researchers could place seeds on different frogs and track them that way.

But dispersing their seeds through sticky frog skin might be advantageous to the Typha plants, Gould noted. Seeds carried on the wind are dispersed essentially randomly, wherever the wind happens to take them. But, as he pointed out, frogs might carry those seeds directly to another pond, or some other optimal location where the plants can grow.