Fertilization can be externalwhen it occurs outside the body, in the environment, or internalwhen it occurs in the body of the individual who produces the eggs.
Some amphibian species have external fertilization. The male stimulates the female to release the eggs in the environment and the male releases the sperm on them.
The energy cost of gamete production is especially high in species whose fertilization is external. In these cases many male and female gametes are formed, which guarantees the chance of a chance encounter between them, resulting in the largest number of zygotes. Of these numerous zygotes, however, not all survive the adversity of the environment. Only a small number form adult individuals, continuing the species.
In animals in which fertilization is internal, the number of gametes produced is lower, thus, the energy cost of their production is also lower. The cost of developing the embryo also depends on the animal being oviparous, oviviviparous or viviparous.
Animals oviparous lay eggs and their embryonic development occurs mainly outside the mother's body. Embryos depend on nutritive material present in eggs. As an example of an oviparous animal we can mention the birds, insects, reptiles and monotremes mammals.
Animals ovoviviparous They retain eggs within the body until hatching, and embryos also feed on the nutritional reserves in eggs. As an example of an ovoviviparous animal we have lebistes, which are common freshwater fish, scorpions, sharks and poisonous snakes.
Lebiste and viper
We viviparous The embryo depends directly on the mother for its nutrition, which occurs through physiological exchanges between mother and fetus. There is no shell isolating the egg. As a rule, embryonic development is completed within the maternal body and individuals are already born formed. The energy cost is especially high, as females invest energy in the nutrition and development of the embryo within their bodies. In the case of these species, fewer embryos are formed, but they have a higher chance of survival. It is viviparous, for example, placental mammals, as is the case of the human species.