The function of the reproductive system is to produce sex cells, called gametes, to produce offspring that will continue the species. All organisms have some mode of reproducing. Vertebrates, such as mammals and birds, can have complex reproductive systems. This is because these organisms have a high degree of parental care.
Some organisms, like many fishes, do not have much in the way of parental care. In many species of fishes, the female lays her eggs and the male sprays his sperm over them. The fertilized eggs are then left to develop without any parental guidance or protection. Because of this low level of parental care, most of the eggs will never hatch and very few of the offspring will reach adulthood to reproduce.
Birds and mammals, however, have a high degree of parental care before and after the offspring are born. In the case of mammals, the developing fetus is usually retained inside the mother and nourished through the placenta. In this way, the young are given a ‘head start’ in development before they are exposed to the environment. After birth, one or both of the parents will care for the young and may even teach them skills necessary for survival. In some species, such as wolves, even individuals that are not directly related to the young will spend much energy caring for the young. Of course, the name for mammals comes from the mammary glands, which produce milk to feed the young, another sign of parental care.
Birds follow a similar pattern of parental care except rather than retain the young inside the mother the offspring are inside a hard-shelled egg for protection. After the chick hatches from the egg, then typically both of the parents care for it by keeping warm and bringing it food.
The high degree of parental care exhibited by birds and mammals means that their offspring are more likely to survive to adulthood. Because of this, birds and mammals may only produce a few dozen offspring in their lives. This is a small number compared to the thousands of eggs laid by fish. Though parental care is very time and energy consuming for the parents, the trade off of having offspring that are very likely to live is worth the effort.