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Question: Why do male sea horses have babies?
Answer: First, one must remember that a sea horse is a fish. The idea of males being pregnant and carrying eggs until they hatch seems strange to us, but from a fish's point of view, it is much less surprising. Among fish that give parental care, the male cares for the eggs more often than does the female. To understand why fish are so different from mammals in parenting biology, you need to know some fish facts of life. In mammals and birds, fertilization of the egg and development of the embryo occur within the female's body. But most female fish release unfertilized eggs; the males then fertilize the released eggs and the embryo develops with the aid of food stored as yolk. The male of many fish species prepares a site to which he tries to attract females. If he is successful, a female will lay eggs at his site. Having done her part, she leaves. This process may be repeated with different females, so that the small carpet of fertilized eggs tended by the male may have been contributed by several females.
Most fish provide no parental care of the eggs, but among those that do; parental care can take many forms. In addition to preparing a place for the eggs to be laid, the males may use their fins to fan water across the eggs; the moving water removes sediment and improves gas exchange. The parent may pick up the eggs in his mouth to relocate or to clean them. However, all this effort is wasted if the parent cannot cope with the biggest threat to the eggs: predators. Eggs are an easy, nutritious meal in a sea full of hungry fish.
Seahorses and their relatives, pipefish and sea dragons, are slow moving and clumsy, but they have shapes and behaviors that help them blend into their surroundings. They would risk being eaten themselves if they tried to defend their eggs. Instead, they hide them. Fish have developed two remarkable egg-hiding behaviors: mouth brooding and external egg carrying. A mouth-brooding parent, usually the male, holds the eggs in his mouth for a week or two until they hatch. This behavior, which probably developed from tending eggs by mouth, has the disadvantage of interfering with the parent's feeding. A different way to protect the eggs is to take advantage of the adult's camouflage. Male pipefish and sea dragons have a special place on the lower surface of their bodies to which the females attach the eggs. Male seahorses are even more complex because they have evolved a complete pouch in which the female deposits the eggs. Protected by the pouch, the eggs are secure; in addition, oxygen and nutrients can pass through the male's skin to the developing embryos. Together these behavioral and structural features increase the likelihood that seahorse eggs will hatch into fry and that the fry will survive to reproduce. Producing offspring that survive to reproduce is the key to biological success.
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