All amphibians start as fully aquatic creatures, equipped with gills and tails for surviving underwater. However, they transition to life on land as they develop limbs and other physical tools for semi-aquatic lives. Although time spans vary between frog species and habitats, tadpoles generally go through the same developmental milestones.
Most tadpoles start growing hind legs within 6 weeks after birth, and their font legs soon follow. On average, tadpoles can efficiently use their legs 20 to 25 weeks after birth, or once they reach the froglet stage.
While all amphibians grow legs and transition to life on land in ideal circumstances, the time frames and overall development can vary based on numerous influences. Frog tadpoles generally begin developing hind legs at around 6 weeks old, as their aquatic features give way to the physical features needed for living on land.
While the time frame can vary based on numerous factors, below is an outline of the average time it takes for most tadpoles to develop legs:
|Stage||Leg Development||Time Frame||Habitat|
|Early Tadpole||No legs||3 Days||Aquatic|
|Mid-Tadpole||Hind Legs||6 weeks||Aquatic|
|Late Tadpole||Hind legs, Front legs||14 to 16 weeks||Semi-aquatic|
|Froglet||Developed legs||6 to 9 weeks||Semi-aquatic|
|Adult Frog||Fully developed legs||2 to 4 years||Species-dependent|
Although tadpoles may begin developing legs during the formative phases, they are mostly still reliant on their gills and aquatic habits for survival. Their tiny legs are still extremely fragile and are not yet ready for proper usage.
After the initial growth spurt, it typically takes an additional 6 to 9 weeks to refine their legs enough to freely explore the surrounding land. Limb formation generally indicates the onset of further metamorphic changes necessary for living on land.
Tadpoles will only be able to use their legs efficiently once they are well into the froglet phase, and they will only be able to remain on land for long time frames once they are adult frogs.
How Do Tadpoles Grow Legs?
Tadpoles undergo a wide range of metamorphic changes from birth, and the process is triggered by the Thyroid hormone (TH). Tadpoles begin developing limbs, bones and muscles as they age and will start using their legs in moderation as they slowly transition to life on land.
Tadpoles start their metamorphic journey as fully aquatic creatures. During this time, tadpoles go through many changes, and the tadpole phase has been divided into three primary sub-phases:
- Early Tadpole Phase: Tadpoles are aquatic for the first few weeks after birth, equipped with tails, internal gills, and other physical features necessary to survive underwater. They remain in this state around 6 weeks on average, and their metamorphic changes primarily involve their respiratory and digestive systems.
- Mid-Tadpole Phase: During the middle of the tadpole phase at around 6 weeks, these little creatures begin growing tiny hind legs, feet, limb bones, muscles, and have internal gills. The tadpoles generally still use their physical features such as their tails and gills for swimming during this time.
- Late Tadpole Phase: As the tadpole phase draws to a close after 14 to 16 weeks and the froglet phase draws near, they have developed front legs, and are developing lungs. After their legs have developed, they become stronger and more refined. Further development is aided by a process known as apoptosis, or programmed cell death (Ishizuya-Oka, 2010).
The cells used to support tail movement and functionality gently fade away as more resources are used on further metamorphic changes. While their limbs are still fragile, the tadpoles may begin trying out their new legs in moderation by climbing aquatic vegetation above the water’s surface for brief time frames.
Once their limbs are strong enough to use, these creatures will begin pushing the boundaries of their legs’ capabilities. This phase typically occurs for around 6 to 9 weeks after the tadpole phase. Their hind and front limbs generally continue refining in size, shape, and overall strength.
Froglets cannot survive out of water for long time frames, and they usually return to their initial aquatic habitat frequently throughout this phase.
During this time, the froglets will continue losing their tails as their legs become more refined, and they may have remaining tails stubs or no tails left at all.
The young amphibians will only be able to use their legs proficiently to hop around and survive on land for longer time frames once all of the metamorphic changes are completed successfully. This final shift to adulthood where frogs are fully formed and can reproduce can take 2 to 3 years.
Factors Affecting Tadpole Growth Rates
Most tadpoles start to grow legs after they are around 6 weeks old, and the process is generally complete after they are froglets. However, some tadpoles may develop faster metamorph quicker.
The following factors contribute to tadpole legs growing faster than average:
- Climate: Tadpoles generally develop faster when they live in warmer waters with higher humidity levels, as cold temperatures often cause slower growth rates. This influence means that frogs living in warmer or tropical climates are more likely to grow legs sooner than expected.
- Species: There are over 7,400 known frog species so exceptions to the general rule we covered in this article apply. Some frog species are genetically programmed to have faster growth rates, which generally comes down to specific DNA and inherent qualities. These species will generally go through the relevant metamorphic phases faster than others, provided that the environmental conditions are supportive.
- Health: Although all tadpoles go through similar metamorphic changes, not all tadpoles are equipped with the same nutrition and hydration provisions. Since nourishment affects the body’s ability to perform vital functions, tadpoles with access to better food and water sources will typically gain advantages concerning their growth rates. Tadpoles often die in captivity due to these factors (CTNF)
Tadpoles are not initially equipped with the necessary physical features for living on land, and they can only survive on land once they have developed strong and healthy legs. Frogs are unique in their design, and their bodily components greatly impact their capacity to thrive in various inland habitats.
Ishizuya-Oka A, Hasebe T, Shi YB. Apoptosis in amphibian organs during metamorphosis. Apoptosis. 2010;15(3):350-364. doi:10.1007/s10495-009-0422-y