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WHY FROGS HAVE EXTERNAL FERTILIZATION? AND HOW DOES IT OCCUR?

Animals that live in the water usually release their gametes directly into the water—plop, splash. The union takes place outside of the body, and is called external fertilization. Many fish, amphibians, and other sexual aquatic animals use external fertilization (the exceptions are aquatic mammals, sharks, and some other special types of fish).

External fertilization is beneficial because the mother does not have to physically carry the developing offspring. She can also lay a brood of many eggs at one time (we are talking hundreds to millions of eggs). On the other hand, it is also more dangerous because the developing offspring are at a risk to predators gobbling them up for breakfast or second breakfast. Humans aren’t the only ones who like caviar. Eggs are also not as protected from environmental changes (such as pH or temperature) as they would be inside their cozy mother.

It is important that both the eggs and sperm are released in synchronization. Courtship rituals are important to ensure that both parties are ready. Sometimes there is a single pair of fish, but since the eggs and sperm are just "out there," it is also common for multiple males or multiple females to spawn at the same time. There are even sometimes sneaky undesirable fish that hope to get in on the action. They release their gametes secretly near a better male's lady friend since they can't get one of their own. The reproductive process of frogs begins with fertilization, and like all other amphibians except caecilians, they fertilize their eggs outside of the female’s body.  The male and female frogs place themselves into what is known as the amplexus position.  They can remain in this position from a few hours to days at a time.   This position involves the mounting of the male frog onto the back of the female frog and the male then wraps his forelegs around his partner.  This position allows for the best chance at the sperm fertilizing the egg outside of the body.  The male and female who are copulating, release the egg and sperm to be fertilized at the same time, and many then leave their eggs to develop on their own.  Other frogs take care of their eggs in different fashions such as:  carrying their eggs in their vocal sacs/abdomens or burying their eggs on land (they keep them wet through periodic addition of water or urine to the nest).  A tadpole, the point in an amphibian’s life style that is a solely aquatic larval stage, emerges from the egg a couple of days to a couple of weeks after it is fertilized.  This tadpole then grows and changes in a liquid environment through a process known as metamorphosis (process of drastic changes from one life stage to the next in the development of an organism).  The body of the tadpole changes in the following order:  the back legs of the frog begin to form, followed by the front legs, the internal organs prepare themselves for land life, lungs develop, the digestive changes to be able to process a frog’s diet, the tail almost completely disappears.  Once the tadpole has completely made all of its changes through metamorphosis, the animal begins a life on land and in the water as an adult frog and the cycle begins once again.


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