Certain Tadpoles Delay Defecation for Weeks to Maintain Clean Pools

Some tadpoles don’t poop for the first weeks of their lives. At least, that’s the case for Eiffinger’s tree frogs (Kurixalus eiffingeri), scientists report September 22 in Ecology.

Eiffinger’s tree frogs are tiny frogs that live in Taiwan and on two Japanese islands: Ishigaki and Iriomote. The tree-dwelling amphibians lay their eggs in puny puddles, which are often nestled in plant stems, tree hollows, and bamboo stumps.

Once the tadpoles hatch, they spend their early lives in these puddles. However, in pools as small as these, there’s not a lot of water to dilute ammonia — a toxic chemical animals release when they pee or poop.

Bun Ito and Yasukazu Okada, biologists from Nagoya University in Japan, now have uncovered the tadpoles’ secret sanitation strategy — self-induced constipation. The tadpoles store their poop in an intestinal pouch until they start to metamorphize into full-fledged frogs.

Ito and Okada raised tadpoles from four different frog species in makeshift nurseries. Once the experiment began, they moved the tadpoles to smaller cribs, plastic cases with a little more than a tablespoon of water. The team measured and compared how much ammonia each species released. They also measured the amount of ammonia each species stored in their guts.

Eiffinger’s tree frog tadpoles released less than half as much ammonia on average than the species that released the most. And compared with two of the other species, the tadpoles kept more ammonia in their guts. The researchers note that unlike Eiffinger’s tree frogs, the other species typically lay their eggs in open ponds where ammonia is easily diluted.

“The behavior likely serves to prevent contamination of small water bodies,” Ito says. Some ammonia still seeped into the tree frogs’ water, potentially through their pee.

It turns out Eiffinger’s tree frog tadpoles have another superpower: Experiments showed that they can survive in higher ammonia concentrations than one of the other species included in the study, Dryophytes japonicus, better known as the Japanese tree frog. While that might seem counterintuitive, given the tadpoles’ poopless period, Ito notes that sometimes the tadpoles share their cribs with other animals, such as mosquito larvae, which also release ammonia.

“We hypothesize that the tadpoles have developed a tolerance to ammonia as a dual defense mechanism,” says Ito, “both against ammonia produced by other organisms and the ammonia they generate themselves.”