Male pregnancy in seahorses is remarkable in more ways than one.
A pair of seahorses mating [Photo by George Grall, National Geographic Creative]
The fact that male seahorses are the ones that carry the young of their species is quite well-known. However, many of us probably don't know much about seahorse pregnancies beyond the fact that it's the males that undergo it. Interestingly, the roles for males and females in almost every part of the seahorse reproductive process are reversed. The entire reproductive process of the seahorse is actually quite remarkable, if by “remarkable” we mean “strange but compelling”.
Watch a seahorse give birth
Unlike most other species that engage in sexual reproduction, the male seahorses are the ones that carry young and subsequently give birth. However, this isn't the only thing that sets seahorses apart.
There's a misconception that seahorses are monogamous and mate for life. Evidence shows that seahorses actually stay monogamous for at least an entire breeding season. It's possible that the longer a mating pair stays together, the more likely they are to reproduce successfully. However, there's genetic evidence that specific seahorse species are monogamous.
Another thing is that in most other species, it's the males that compete with each other for a female's favors. The reverse is actually true in seahorses. The males of the species are the ones that select their mates, and they tend to select females that are similar to them in size. Researchers say that mate selection in seahorses may spur the creation of new seahorse species.
Males have a brood pouch on the front of their bodies, also known as the ventral side. A female deposits her eggs into the male's brood pouch, where the male facilitates fertilization. The female visits her male partner daily to check on him and their brood.
The gestation period can last 10 to over 40 days, depending on the species. When the time is up, the male experiences contractions of a sort, ejecting his young into the water like he's blowing bubbles. The male can also receive another set of eggs from the female on the same day he gives birth.
Photo by Rudie Kuiter, Aquatic Photographics
Unfortunately, the attentiveness of the parents ends at birth. Once the babies are born, they are on their own. As a result, they fall prey to predators or get swept away by the current from sources of nutrition. Fewer than five in 1000 seahorse babies survive to adulthood.
This may be why male pregnancy is the biological norm for seahorses. Scientists say that this way of doing things may lead to shorter cycles of reproduction. While the male is carrying the fertilized eggs, the female can begin preparing new eggs. This is how the males can get pregnant again on the same day they give birth.
Male pregnancy is rare, but seahorses aren't the only animals to exhibit it. Seahorses belong to the family Syngnathidae, which also includes pipefish and the leafy seadragon. The males of both of these species also have a larger role in parenting than the males of other species.
Scientists aren't completely sure yet what the benefits are in having males carry their young and birth them, but there's definitely an evolutionary advantage there.
Get weekly science updates in your inbox!