Child rearing is commonly thought of as "women's work,"but there are plenty of exceptions to this notion throughout the animal kingdom. In fact, outside the class of mammals, males share, and in some cases assume sole responsibility for, the care of their young. Males of many species of birds, for example, incubate, brood,and feed their young. And among species of fish, males more often are responsible for guarding the developing eggs than females are. Few males in any animal class, however, rival the reproductive commitment of male seahorses, which protect and nourish their developing offspring inside their bodies.
In the animal kingdom, pregnancy is perhaps the greatest commitment a parent can make to its offspring, and it is typically a commitment left to the female. There is probably a simple physiological reason why reproductive roles have evolved this way. Given that eggs are much larger than sperm, and females produce the eggs, it is more efficient for the female to keep the eggs in her body and take on sperm from the male than vice versa. But "vice versa" is exactly the way seahorses breed.
Female seahorses deposit their eggs inside of males, and the males subsequently undergo a pregnancy that is not unlike a typical pregnancy for females. Sperm from the male fertilizes the eggs shortly after the female deposits them into the male's brood pouch. The eggs then embed in the wall of the pouch, where they will develop as embryos until the birth of the young seahorses anywhere from10 days to 6 weeks later, depending on the species. During development, the male provides oxygen and nutrients to the embryos through a capillary network in the pouch. Later, to prepare the young for life after birth, he changes the pouch environment to be more like seawater.
Scientists know very little about why seahorses evolved this reproductive strategy. One seahorse scientist, however, suggests that it may have something to do with a simpler strategy that is common to most fish. It is typical for males of a wide variety of fish species to provide all of the parental care. Most often this care comes in the form of guarding a nest. But just as in mammals, protecting developing embryos inside the body, as opposed to casting them off into the world to fend for themselves, dramatically improves the chances that any one offspring will survive. It follows, then, that if pregnancy and live birth were going to evolve in a group of animals, it would likely evolve along previously established gender roles. Thus, the primary caretakers --males in the case of most fish groups -- would be the ones to evolve the capacity for pregnancy.