From tadpole to bullfrog: the long road of metamorphosis

An American bullfrog tadpole can grow to between 3 and nearly 5 inches long, according to the Missouri Department of Conservation. That is longer than a full-grown spring peeper, which BioKIDS, a University of Michigan site, puts at 19 to 35 millimeters (up to about 1.4 inches). A tadpole that out-measures another species’ adults is already off to an unusual start.

And it is not finished. Metamorphosis, the change from a tadpole with gills and a tail into a froglet, is slow in this species. Missouri says tadpole development takes about 11 to 14 months, including a winter. The Animal Diversity Web, a University of Michigan database, gives a wider range, one to three years before the change begins. Timing varies by source and by place.

This article follows the life cycle one stage at a time, using state, university and charity sources. When the best description of a step comes from a different frog species, we say so. At the end we cover where the bullfrog is native, where sources treat it as invasive, and what the Red List says.

Meet the American bullfrog

Per the Animal Diversity Web, the native range runs from Nova Scotia to central Florida, from the East Coast to Wisconsin, and across the Great Plains to the Rockies. Its natural western edge is unclear because of introductions. It calls the bullfrog the largest true frog in North America, living in warm, still, shallow water.

Stage 1: a floating film of eggs

Breeding runs from May to July in the north and from February to October in the south, according to the Animal Diversity Web. Missouri says it peaks in late June there. The female lays a foamy film of up to 20,000 eggs in quiet water, and fertilization happens outside her body. Missouri adds that typical clutches hold 10,000 to 12,000 eggs, and that some females lay twice in a season.

The eggs hatch in about four to five days, say the Animal Diversity Web and Missouri, and out come spotted tadpoles with gills and a tail.

Stage 2: the tadpole years

Tadpoles mainly eat aquatic plants, according to the Animal Diversity Web, and Missouri says they eat algae. The same page says most fish avoid tadpoles because of an unpleasant taste, and that tadpoles spend a lot of time inactive, which reduces their exposure to predators.

Then they wait. Missouri says development takes about 11 to 14 months, including overwintering. The Animal Diversity Web says one to three years before transformation begins, and does not mention overwintering. Neither page explains why the numbers differ. For comparison, the wood frog tadpoles transform roughly 65 to 130 days after hatching, according to the same database, and spring peeper tadpoles in about two to three months, according to BioKIDS. The bullfrog is the marathon runner of the pond.

Stage 3: the makeover

The best step-by-step description we opened is for a different frog. Froglife describes the common frog, a UK species: about 16 weeks after hatching the tadpoles start to grow back legs, followed by front legs, and froglets leave the pond once they have fully absorbed their tails. The Animal Diversity Web says the bullfrog tadpole’s tail eventually disappears as it becomes a froglet. So the tail is not dropped the way a five-lined skink drops its own. It is taken back in.

A 2018 study in Proceedings of the Royal Society B looked at the gut, using African clawed frog tadpoles, not bullfrogs. At the climax of metamorphosis, feeding declined. By the late stage the intestine had uncoiled and the stomach had developed, and feeding resumed afterward. Wet body weight fell about 50 percent through the climax, mostly from water loss. The authors name thyroxine, a thyroid hormone, as the main controller of metamorphosis, citing earlier work, and they did not measure it themselves. They also say they found no direct evidence that tadpoles stop eating so the gut can be rebuilt, so that link is partly a hypothesis.

Stage 4: froglet to full-grown frog

By late summer, Missouri says, tadpoles have usually transformed, from June through August. Adult size may take another two to three years, and the page puts the lifespan at slightly over seven years. The Animal Diversity Web says sexual maturity arrives at 3 to 5 years, though its own development section says two years after transformation, so its pages do not fully agree.

The diet changes with the body. Adults are predators that eat snakes, worms, insects, crustaceans, frogs, tadpoles and fish, and in a few reported cases bats. According to the same database, they are cannibalistic, and adult males have a yellow throat in the breeding season and a tympanum (eardrum) much larger than the eye.

What this looks like in practice

For example, in Missouri a clutch laid around the late-June peak hatches within days, the tadpoles pass a winter in the pond, and the survivors transform the following June to August.

Common mistakes and myths about metamorphosis

Myth: a tadpole becomes a frog in a few weeks. For this species it is months or years. Even the UK common frog takes about 16 weeks to start growing legs, according to Froglife.

Mistake: the tail falls off. The sources describe absorption. Froglife says froglets leave once the tail is fully absorbed.

Mix-up: a giant tadpole is a different animal. It is the same species, just a big larva. The Missouri figure of 3 to nearly 5 inches is the tadpole, not the frog, and the spring peeper really is smaller as an adult.

Mix-up: all the stages look alike. The tadpole has gills and plant-eating habits, and the adult is a predator. The study on clawed frog tadpoles shows the gut being rebuilt in between.

Mix-up: the bullfrog is invasive everywhere. The U.S. government invasive-species page says it is native to the central and eastern United States and invasive in the West. See below.

Conservation status

The American bullfrog is listed as Least Concern on the IUCN Red List of Threatened Species. That is the category as distributed by GBIF, the Global Biodiversity Information Facility, and we checked it on November 9, 2026. Least Concern means the species was evaluated and does not meet the criteria for a threatened or near-threatened category.

The bigger story is its effect elsewhere, and we report it only as our sources do. The U.S. National Invasive Species Information Center says the bullfrog was first introduced to the West in the early 1900s, that most introductions are linked to escapes from aquaculture, and that it competes with and preys on native species. The Animal Diversity Web says that in places such as California it may be driving native frogs toward extinction, and that its tolerance of warmer water, longer breeding season and higher survival before metamorphosis help it outcompete native species. Froglife says it has bred in the wild in England, where it is a non-native species that has been controlled. A 2017 study in Korea found that sites with bullfrogs had more chytrid infection in treefrogs and fewer endangered Suweon treefrogs, but the authors could not say whether bullfrogs caused it.

We have no confirmed population trend, because none of our sources states one. Categories are reviewed and can change, and the copy GBIF distributes can trail the IUCN website by some months, which is why we print the date we checked. GBIF’s copy does not give the year of the assessment, so we do not state one.