The frog life cycle involves dramatic transformations – a process known as metamorphosis. Learn about the intricate stages of frog metamorphosis from the egg to tadpole into the adult frog. Also explore variations in the life cycle among different frog species.
Stages of Frog Metamorphosis
The frog life cycle has three basic steps: eggs → tadpole → adult. However, a lot goes on during each step.
Stage 1: Eggs
The life cycle of a frog begins with eggs, often laid in a cluster or string format, usually in still water or damp places. Females deposit hundreds or thousands of eggs to increase the chances of survival. The eggs are vulnerable to various predators and environmental conditions. Females of some species retain fertilized eggs and release them just before hatching (ovoviviparous) or after they hatch within her body (live-bearing or viviparous).
Stage 2: Embryo
The frog embryo begins forming within a protective jelly-like substance. After a few days, it resembles a tiny fish. The frog’s heart starts beating.
Stage 3: Tadpole
Soon, the embryos hatch as tadpoles. Tadpoles are the larval stage of frog development. These creatures somewhat resemble fish, with gills for breathing underwater and tails for swimming. They eat algae, leaves, and detritus in the water to fuel their growth. But, the tail also serves as a nutrient supply.
Stage 4: Metamorph Stage
After a few weeks, tadpoles undergo remarkable physical transformations in a process called metamorphosis. The tadpole grows hind legs, followed by front legs, while its tail slowly shortens and disappears. It develops lungs to replace gills and its diet shifts from being an herbivore to an omnivore or carnivore, depending on the species. The final transition from tadpole into froglet or frog is very fast (about a day) and the frog is vulnerable to predators and other threats.
Stage 5: Froglet
Once the tadpole grows legs, it is called a froglet. While it still may have a bit of tail left, it leaves the water and explores the terrestrial environment.
Stage 6: Adult Frog
Eventually, the froglet transforms into a mature frog. The frog’s development doesn’t stop here, though. Juveniles continue growing and developing features such as skin coloration, vocal sacs in males, and the ability to reproduce.
Frog Breeding Behavior
Frog breeding behaviors are diverse and fascinating. They usually return to the same bodies of water year after year for breeding purposes. The males attract females with distinctive calls, which also serve to ward off rival males.
Once a male and female pair up, the male clasps the female in a position known as amplexus. As the female releases her eggs, the male fertilizes them externally in the water. In a few species, the female retains her eggs and internal fertilization occurs. The breeding season varies depending on the species and the geographical location.
Variations in Metamorphosis across Frog Species
While the stages outlined above are the norm for most frog species, there are some variations. For example, the members of the family Leptodactylidae (the “marsupial frog”) carry their eggs in a pouch on their back. The eggs hatch into froglets, bypassing the tadpole stage entirely.
In the case of the Surinam toad, eggs embed in the female’s back and develop into froglets. Poison dart frogs lay their eggs on land, and the males carry the hatched tadpoles to nearby water bodies.
Direct development occurs in some species. This is where frogs hatch directly from eggs as small adults (no tadpole stage). The terrestrial-breeding Eleutherodactylus coqui or coqui frog reproduces via direct development. This is an adaptation to a terrestrial lifestyle, where water bodies may be scarce.
Ten Fascinating Frog Life Cycle Facts
- Frogs lay a vast number of eggs because the survival rate is remarkably low; only about five out of 1,000 eggs become adult frogs. But, some females lay only part of their eggs at one time, in case conditions become more favorable later.
- Tadpoles don’t immediately look like adult frogs. They lack arms or legs but have a tail and they breathe through gills instead of lungs.
- The transformation process from tadpole to frog usually takes between 6 to 12 weeks, but in some species, it can last up to 2 years.
- Some frogs, like the glass frog, lay their eggs on the underside of leaves to protect them from predators.
- The wood frog (Rana sylvatica) has a symbiotic relationship with green algae. It includes the algae in the jelly mass with the eggs so photosynthesis helps boost oxygen for the larvae.
- The African Bullfrog lays up to 4,000 eggs at a time.
- A frog’s lifecycle is an indicator of an ecosystem’s health. They are bio-indicators, meaning their health directly relates to the overall health of the ecosystem.
- In cold climates, some species of frogs hibernate in winter. They bury themselves in the mud and their bodies produce a natural anti-freeze that protects their vital organs from damage.
- The gastric-brooding or platypus frogs from Australia swallow their eggs and brood them in their stomachs. The babies give birth through their mouth. Both species of these frogs are extinct, but scientists are using cloning to try to bring them back.
- Frogs use one of two methods of breeding: prolonged breeding or explosive breeding. In prolonged breeding, adults meet up at a pond or other location at a certain time of year. This is the usual method. In explosive breeding, the whole process of attracting mates, breeding, laying eggs, and progression to adulthood happens very quickly. Explosive breeding occurs where conditions are only favorable for a brief window of time.
References
- McDonald, K.R. (1990). “Rheobatrachus Liem and Taudactylus Straughan & Lee (Anura: Leptodactylidae) in Eungella National Park, Queensland: distribution and decline”. Transactions of the Royal Society of South Australia. 114 (4): 187–194.