In almost every species that engages in sexual reproduction, the female provides the prenatal care for the next generation. Only in family Syngnathidae, which includes seahorses, is this pregnancy role reversed. Consequently, much research has focused on the unique behavior of these species and their reproductive behaviors which include monogamous mating, parental birth pouches, and male seahorse contractions.
In most animal species, males compete for female partners. According to Texas A&M researcher Adam Jones, seahorses are different. Females compete; males are more choosy. This difference reflects a reversal of common sex roles in animal mating behaviors. Male seahorses choose females who are close in size to themselves - a system known as assortative mating. Once a male seahorse chooses a mate, however, the pair remains monogamous for that breeding season unlike their relatives the pipefish. Female pipefish can mate with multiple males during a single season.
When mating occurs, the female deposits her unfertilized eggs into the male’s brood pouch. As many as 200 eggs can pass to the male, who then releases sperm into the pouch to fertilize the eggs. His body responds to the eggs by providing a protective environment for their growth. Like human mothers, male seahorses control the nutrition received by their embryos. They also maintain appropriate oxygen and salinity levels in the pouch to ensure the babies have everything they need. During the male’s pregnancy, the female drops by daily to check on her mate and her offspring.
Depending on the seahorse species, the male’s pregnancy may last from 10 days to more than 40. When the time comes to give birth, the male begins to experience a form of contractions that essentially push the hatched babies out of his brood pouch and into the water. During the birth process, the male seems to be making a forward-then-backward motion over and over again. Within only a few hours of releasing his offspring, the male’s brood pouch shrinks back to normal size and he can receive more eggs from his mate.
Speculation & Seahorse Reproduction
Because of the uniqueness of seahorse mating and reproduction behavior, researchers have tried to understand the advantage of the reversed sex roles for the species. According to "National Geographic" magazine, one proposed theory is that the reversal allowed a shorter reproductive cycle. In most species, the female must hold the fertilized eggs, release them or give birth, then ready a new batch of eggs for fertilization. With seahorses, the female can begin growing new eggs immediately after inserting hers into the male's brood pouch so that, when he is ready for a new pregnancy, so is she. "Scientific American" magazine's website reported research suggesting seahorse reproduction methods could lead to the development of new seahorse species because they choose to mate with similar-size partners.
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