Odd as it may sound, in seahorses, it is the male that gets pregnant and gives birth to offspring. They incubate their embryos inside a special pouch found on their tail. This pouch is remarkably similar in features to the uterus of a female mammal, including the presence of a placenta that supports the growth and development of the baby seahorses. But the similarities between the two end here.

A new study is shedding more light on the elaborate and breathtaking process that unfolds when a seahorse dad is ready to give birth. Spoiler alert: it’s not at all like female mammals do it.

Not your typical dad

When a female mammal goes into labor, a flood of hormones including oxytocin causes the smooth muscles of the uterus to contract involuntarily. These spontaneous and uncontrollable movements allow the fetus to gradually exit the uterus.

Since previous research showed that reptiles’ uterine contractions also respond to oxytocin, Australian biologists at the University of Newcastle and the University of Sydney embarked on a new study in which they set out to see whether a similar process wasn’t involved in seahorses too.

The researchers reckoned that seahorse males likely use oxytocin-family hormones, such as the fish version of oxytocin called isotocin, in order to stimulate the brood pouch by contracting the smooth muscles inside.

Initially, the researchers started off with a very simple experiment in which they exposed pieces of tissue from a seahorse’s pouch and intestines to isotocin. The intestine tissue was supposed to act as a control for the study, meaning it would provide a baseline measurement because the assumption was that this tissue would have minimal contractions.

But the researchers were caught off guard by the reaction of the tissues, which was completely opposite from what they expected. The hormone produced no contractions in the brood pouch but caused them in the intestine tissue. The researchers eventually learned that was because the brood pouch contains very little if any bundles of smooth muscle – the kind that involuntarily contracts when stimulated. In fact, compared to your typical female mammal uterus, the brood pouch has basically no smooth muscle.

In the next phase of the study, the researchers used advanced imaging techniques and microscopy to compare in great detail the anatomical differences between male and female pot-bellied seahorses.

A conscious labor

They found that the males have three bones positioned right near their pouch’s opening, which support skeletal muscles. Unlike smooth muscles that contract involuntarily, skeletal muscles are controlled consciously. Think of flexing your biceps or extending your quads to pick up objects or move your legs. You actively and consciously perform these motions whereas the muscles inside your throat or around your heart contract automatically and completely involuntarily.

These bones and supporting muscles are much larger in male seahorses than in females, and are orientated in such a way as to control the opening of the brood pouch. This seems to allow the seahorse dads to consciously control the expulsion of their young at the end of their pregnancies.

The pregnancy starts when females deposit their eggs into the male’s brood pouch, where he fertilizes them. Just like human moms, the abdomens of the seahorse dads become swollen as the hundreds of embryos inside grow.

When they’re in labor, male seahorses bend their body towards their tail, alternating between pressing and relaxing, as if they’re doing some sort of underwater sit-ups. When they perform the pressing movement, their pouch opens up for a fleeting moment and their bodies jerk. The two movements combined allow seawater to flush through the pouch and tiny seahorse babies to exit. With time, the pouch opening gets increasingly larger, along with the number of seahorse babies ejected with each contraction. Within a few minutes, hundreds of new baby seahorses are expelled into the seawater.

These unexpected results show that male seahorses enter labor and deliver their babies in a radically different way from female mammals. While oxytocin-like hormones did not trigger the involuntary muscle movements they were expecting, the researchers believe that the hormones are still important, facilitating the kinds of behaviors that lead to birth. In many ways, seahorse dads have a unique style of giving birth, which only enriches the spectacle.

The findings appeared in the journal Placenta.