How a Tadpole Becomes a Frog

How does a tadpole become a frog? The metamorphosis of the tadpole larva into an adult frog is one of the most striking transformations in all of biology. 

Frogs lay eggs without a protective calcareous shell, and they need to do so in a body of water, because the eggs do not have a shell that can keep them from drying out. Larvae hatch from these eggs, which breathe through gills and often look completely different to their parents. This frog offspring is called a tadpole. Only after metamorphosis do they take on the appearance of the adults and become lung-breathing animals. 

Tadpoles don’t have any limbs or fins, and swim propelled by only their tail. They only breathe through their gills, and feed on algae and aquatic plants in the water. After about ten weeks, the hind legs of tadpoles start to develop. After about three months, the front legs begin to develop as well and the tail slowly starts to shrink into the body. The lungs also start to develop in preparation for the transition onto land. 

Metamorphosis is initiated by hormones from the tadpole's thyroid gland, and almost every organ is subject to modification. The more obvious changes are the legs and tail. However, the cartilaginous skull of the tadpole is also replaced by the mostly bony skull of the adult frog. The teeth the tadpole uses to tear up aquatic plants disappear as the mouth and jaw take a new shape, and the insect-catching tongue of the frog develops. The large intestine characteristic of herbivores shortens to suit the more carnivorous diet of the adult frog. The gills regress, and the lungs enlarge. 

At the end of the transformation, a fully developed frog with lungs, legs, and no tail emerges from the water. This frog will live mostly on land, with occasional swims. This frog then proceeds to find a mate to reproduce with, and the cycle begins anew.

Sources:

https://www.pnas.org/content/97/6/2615

https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_Introductory_Biology_(CK-12)/12%3A_Vertebrates/12.13%3A_Amphibian_Reproduction_and_Development

https://natureinstitute.org/pub/ic/ic33/frog.pdf

Photo credit: Encyclopædia Britannica, Inc.