If the male seahorse gives birth – why do we call it the male? What is it that the female can do that is more female than giving birth? It turns out to be the case who provides the eggs.
The male seahorse is equipped with a brood pouch on the ventral, or front-facing, side. When mating, the female seahorse deposits up to 1,500 eggs in the male’s pouch. The male carries the eggs for anywhere from 9 to 45 days until they emerge, expelling fully-developed, miniature seahorses in the water. Once the seahorse babies are released into the water, the male’s role is done and he offers no further care.
Before breeding, seahorses court for several days. Scientists believe the courtship behaviour synchronizes the animals’ movements so that the male can receive the eggs when the female is ready to deposit them. During this time they may change colour, swim side by side holding tails or grip the same strand of sea grass with their tails and wheel around in unison in what is known as a ‘pre-dawn dance’. They eventually engage in a ‘true courtship dance’ lasting about 8 hours, during which the male pumps water through the egg pouch on his trunk which expands and opens to display its emptiness. When the female’s eggs reach maturity, she and her mate let go of any anchors and snout-to-snout, drift upward out of the seagrass, often spiralling as they rise. The female inserts her ovipositor into the male’s brood pouch and deposits dozens to thousands of eggs. As the female releases her eggs, her body slims while his swells. Both animals then sink back into the sea-grass and finally she swims away.
The male releases his sperm directly into seawater where it fertilizes the eggs, which are then embedded in the pouch wall and become surrounded by a spongy tissue. The male supplies the eggs with prolactin, the same hormone responsible for milk production in pregnant mammals. The pouch provides oxygen as well as a controlled environment incubator. The eggs then hatch in the pouch where the salinity of the water is regulated; this prepares the newborns for life in the sea. Throughout gestation, which in most species requires two to four weeks, his mate visits him daily for ‘morning greetings’. They interact for about 6 minutes, apparently reminiscent of courtship. The female then swims away until the next morning, and the male returns to vacuuming up food through his snout.
Research indicates the male releases sperm into the surrounding sea water during fertilization, and not directly into the pouch as previously thought.
Why the male seahorse carries the offspring through gestation is unknown, though some researchers believe it allows for shorter birthing intervals, in turn resulting in more offspring. Given an unlimited number of ready and willing partners, males have the potential to produce 17 percent more offspring than females in a breeding season.
Also, females have ‘time-outs’ from the reproductive cycle that are 1.2 times longer than those of males. This seems to be based on mate choice, rather than physiology. When the female’s eggs are ready, she must lay them in a few hours or eject them into the water column. Making eggs is a huge cost to her physically, since they amount to about a third of her body weight. To protect against losing a clutch, the female demands a long courtship. The daily greetings help to cement the bond between the pair.
One common misconception about seahorses is that they mate for life. Many species of seahorses form pair bonds that last through at least the breeding season. Some species show a higher level of mate fidelity than others.