Reproduction II

In sexual reproduction, two parents donate genes to the young, resulting in offspring with a mix of inherited genes. These genes are donated through a process called fertilization. There are two mechanisms by which fertilization can take place. The first is external (the eggs are fertilized outside of the body), and the second is internal (the eggs are fertilized within the female reproductive tract).

External Fertilization

External fertilization occurs mostly in wet environments and requires both the male and the female to release their gametes into their surroundings (usually water). An advantage of external fertilization is that it results in the production of a large number of offspring. One disadvantage is that environmental hazards such as predators greatly reduce the chance of surviving into adulthood. Amphibians and fish are examples of animals that reproduce this way.

Internal Fertilization

Animals that use internal fertilization specialize in the protection of the developing egg. For example, reptiles and birds secrete eggs that are covered by a protective shell that is resistant to water loss and damage. Mammals, with the exception of monotremes, take this idea of protection a step further by allowing the embryo to develop within the mother. This extra protection increases the chances of survival because mom supplies everything that the embryo needs. In fact, most mammalian mothers continue to care for their young for several years after birth.

Male or Female?

It is important to note that not all animals are strictly male or female. Animals such as sea anemones may have both male and female reproductive parts-- hermaphrodites. It is possible for some hermaphrodites to self-fertilize, but most must find a mate to reproduce. Since both parties involved become fertilized, this process doubles the number of young that are produced. Hermaphroditism is a good solution to the scarcity of potential mates. Another solution is the ability to change gender from a male to a female (protandry) or from a female to a male (protogyny). Certain fish, like wrasses, may change from female to male as they mature into adulthood.

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For additional information on reproduction see:

Animal Sexual Reproduction

Asexual Reproduction