Sex life of bizarrely beautiful seahorses

Peer at a seahorse, briefly hold one up to the light, and you will see a most unlikely creature; something you would hardly believe was real were it not lying there in the palm of your hand. Should we presume these odd-looking creatures were designed by a mischievous god who had some time on her hands? Rummaging through a box labelled ‘spare parts,’ she finds a horse’s head and, feeling a desire for experimentation, places it on top of the pouched torso of a kangaroo.

This playful god adds a pair of swivelling chameleon eyes and the prehensile tail of a tree-dwelling monkey for embellishment – then stands back to admire her work. Not bad, but how about a suit of magical colour-changing armour, and a crown shaped as intricately and uniquely as a human fingerprint? Shrink it all down to the size of a chess piece and the new creature is complete. No matter how tempting such a strange tale of creation may be, seahorses are real creatures, a product of natural selection and an endangered species. They inhabit a wide stretch of the oceans and are not, as we might suppose, restricted to warm azure waters that lap on equatorial shores.

Seductive dance

Suddenly, two tiny silhouettes come together like a pair of knights on a chessboard. The seahorses greet each other with a nose-to-nose caress and, wrapping their tails around a single blade of grass, they begin a seductive dance, spiralling round and round each other.

The first time a seahorse couple meet, this gentle courtship carries on for hours, days even, and it is a risky time. Driven by hormones that interfere with the instinct to hide, they abandon the camouflaged safety of their seagrass home. The female initiates sex by reaching up toward the surface, stretching her body as straight as it will go. This proves quite irresistible to the male, who immediately responds by pumping his tail vigorously up and down.

The couple halt in the open water column and hold their bodies close, forming a heart shape with their touching snouts and bellies. Their first attempt isn’t quite right, so they break apart and try again several times until their position is perfected, the female just above the male. Then an extraordinary thing happens. A short hollow tube emerges from the female, which she pushes into an opening in her partner's belly. The couple raise their heads and arch their backs as the female shoots an egg-laden liquid into the male.
Copulation is perfunctory, taking just six or seven seconds. When the male is full with the precious cargo, he wanders off, his bright mating costume already fading. He sways and wiggles his body, settling the eggs into position where they will remain for the next few weeks, growing in a protected internal pond.

Agonies of childbirth

The strangest thing about seahorses is that their males are the only ones in the world who experience – firsthand – the agonies of childbirth. Admittedly, there are many fathers who do a great job of helping out with the youngsters. In eastern Australia’s rainforests, tadpoles of the marsupial frog wriggle into special pouches slung on their fathers’ hind legs. Six weeks later, out hop the next generation of miniature frogs.
But only male seahorses become truly pregnant, nurturing their young inside their bodies, providing them with food and oxygen, whisking away waste products. This is all the more remarkable when we consider that pregnancy is a rare occurrence in fish, even among females.

When people first hear about seahorse males getting pregnant, the question that naturally follows is, “So what makes them male?” The simple answer is sperm. The distinction between scarce round eggs and prolific tadpole-like sperm is essentially all that separates woman from man, doe from buck, mare from stallion, and so on.Yet despite such a clear definition, it took marine biologists a long time to understand what was going on with seahorse sex. The ancient Greek philosopher Aristotle first wrote about the unusual reproductive habits of the Syngnathidae family, to which seahorses belong, in the third century BC. But it wasn’t until the 18th century that scientists finally realised something strange was going on, and began to study syngnathid sex in detail.

For four decades, arguments flared over which sex carried the eggs during a seahorse pregnancy. Everyone agreed that the females produced the eggs, but it wasn’t clear whether or not they handed them over to their male partners to look after. The academic tussle was played out on the pages of specialist journals, until the debate was finally laid to rest in the 1870s, when several scientists observed pairs of seahorses engaging in tight embraces within the confines of the laboratory. Those watching closely enough witnessed the transfer of eggs.

When evolutionary biologists discovered that male seahorses become truly pregnant, they rubbed their hands in anticipation. It gave them a perfect opportunity to test out their theories of how differences between the sexes evolve. They expected to find that the females, unshackled from the toils of pregnancy, had kicked up their heels and adopted a typically male habit, spreading their gametes as far and wide as possible.

But no, most female seahorses are loyal to one male throughout his pregnancy, and do not mate again until he is ready. In fact, many seahorses are monogamous throughout whole breeding seasons, returning to the same partner time and again. Some may even stay in devoted couplings for much of their lives.

Equally, if males are unlikely to find a profusion of other mates, it isn’t a huge sacrifice to settle down, be faithful, and become pregnant. And taking on the reins of pregnancy gives male seahorses one last added benefit: full reassurance that all the babies he is caring for are definitely his own – something other males, most notoriously human beings, can’t be absolutely sure of without a DNA test.

As fathers, their work is never finished. As soon as the arduous birth is over, the female returns and their courtship ritual resumes. The male may already be pregnant again by the next day – a tiresome life indeed, but one that maximises the output of offspring. Which is, ultimately, all that really counts.