With the exception of birds, the majority of egg-laying animals play no part in helping their young to survive. A large proportion of their young die, and to offset this, they often produce a huge number of eggs. A housefly, for example, can lay over a thousand eggs in the course of its life, while a female cod can lay 3 million.
Showing the characteristic contrasting white patches above the eyes and under the jaws, a male and female killer whale, Orcinus orca, swim protectively on either side of their baby. Killer whales maintain close ties to the social structure of their natal pods, or groups, for life. To prevent inbreeding, however, the whales typically seek mates outside of their original pod.
Most amphibians and reptiles lay smaller clutches of eggs, and some of them remain with their eggs and guard them until they hatch. Birds lay smaller clutches still, and the parents incubate the eggs, or keep them warm until they hatch, and continue to care for their young once they have hatched. Most ground-nesting species protect their young and lead them to food, but typical tree-nesting birds provide their young with both food and shelter until they are able to fend for themselves. Without this parental care, the young birds would have no hope of survival.
A female American redstart turns toward her offspring, which begs for food with its gaping mouth.
Parental care is equally important in mammals, which provide food for their young in the form of milk. Raising a family in this way creates a close link between the mother and her young. This method also allows the young to learn important patterns of behavior by watching their mother at work. In small rodents, this learning period lasts for just a few days, but in larger mammals, it can last for more than a year.
Kangaroos are born hairless and immature about a month after conception, before ears, eyes, or hind legs have developed. The newborn joey follows a path of fur licked smooth by its mother to her pouch, where it develops for the next five to nine months. The joey continues to nurse by placing its head in her pouch, and from that point the young maleís development proceeds much more quickly than does the young femaleís. Once a joey has left the motherís pouch, she may give birth again immediately.