What Develops First, the Back or Front Legs on a Frog?

Before a tadpole can transform into an adult frog, he has to grow larger. Once he reaches the ideal size, his thyroid begins producing a hormone that triggers metamorphosis. If life in his watery world becomes too dangerous, the thyroid may begin producing the hormone early to help him transition to land and escape the water. This can occur if too many tadpoles congregate in one spot, more predators than usual are lurking around, food is harder to find or the water is drying up.

A tadpole’s back legs begin to emerge after he reaches the ideal size. The back legs appear first as tiny limb buds at the base of his tail and eventually grow into very large legs. He keeps them folded against his body while they're growing. With his legs tucked up under his round belly, he's able to swim without being slowed down too much.

The front legs develop inside the gill pocket of the tadpole’s body. They don't emerge until he's almost ready to leave the water because they slow him down even more than the back legs. Once all four of his legs have popped out, the tadpole is called a froglet. He must now grow into an adult frog quickly to shorten the amount of time he's in the water with reduced swimming ability.

After the froglet develops all four legs, he begins to lose his tail through a process called apoptosis. As the tail is slowly reabsorbed into the froglet’s body, he won't eat -- reabsorbing his tail is a bit like eating a big meal, providing nourishment. Once he loses his tail, the frog is ready for life on land. He has powerful back legs for jumping, and his front legs help pad his landing.