In nature, the single-parent family is the overwhelming rule—and that parent is typically female. From building a nest and gathering food, to giving birth and protecting their young, in many species the females do it all. However, throughout the animal kingdom, there are exceptions to this rule. 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 are females. However, few males in any animal class rival the reproductive commitment shown by one improbable fish: the seahorse.

Male seahorses protect and nourish their developing offspring inside their bodies. In essence, they undergo a pregnancy that is similar to a typical mammalian pregnancy. When seahorses mate, the female deposits her eggs inside the male's brood pouch, where they are fertilized with his sperm. The fertilized eggs then embed in the wall of the pouch, and develop there until birth, anywhere from 10 days to six weeks later, depending on the species. During development, the male provides oxygen and nutrients to the embryos through a capillary network in the pouch. The pouch's internal conditions are also transformed near the end of embryo development, becoming more like the ocean environment, an adaptation that helps the developing seahorses acclimatize to the outside world before they are born.

Once on their own, young seahorses face many challenges. On average, only two of the thousands of young that a pair produces will reach maturity. And adult seahorses don't have it much easier. Storms are probably the most common natural cause of death of adult seahorses. Although seahorses can use their tails to cling to aquatic vegetation, heavy surf can still pull them free and cast them ashore, or cause them to die of exhaustion in open water. Predators such as crabs, tuna, skates, and rays also take a steady toll on seahorse numbers.

In addition to these natural hazards, however, a new threat has emerged. Sought after by human collectors for a variety of reasons, including Chinese medicines, souvenirs, and the aquarium trade, seahorse numbers have fallen dramatically in recent decades. As slow-swimming fish, seahorses rely heavily on camouflage to avoid being caught. Yet, destruction of sea grass beds, mangroves, and coral reefs continues to compromise their ability to avoid detection. As with so many other vulnerable organisms, the survival of seahorses may rely on human intervention to undo the environmental disturbances that challenge their existence.

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