How male seahorses tap into their mothering side

If there were to be a “best dad” award in the animal kingdom, seahorses would be a shoe-in. That’s because males, not females, of these peculiar fish carry their young to term. They fertilize and nourish eggs deposited on their bodies in specialized “brood pouches” that function much like a mother’s womb. By studying how these pouches form, researchers now have discovered many similarities between male and female pregnancies — with one big exception.

In women and other pregnant animals, female hormones stimulate reproductive tissue to form a womb and placenta. In these seahorses, a male hormone underlies “motherhood,” researchers report November 11 in Nature Ecology and Evolution. 

“The evolution of the brooding pouch was not cut from whole cloth de novo but was built like a quilt with different patches of genes and cells that function the same way in different animals,” says Bill Cresko, an evolutionary geneticist at the University of Oregon in Eugene who was not involved in the study. “It’s way cool.”

The finding helps explain how pregnancy could have evolved more than 150 times in animals, albeit almost exclusively in females.

Most animals lay eggs, or else it’s the mothers that incubate their young. “Seahorses turn all of this on its head,” says Oliver Griffith, an evolutionary biologist at Macquarie University in Sydney. Thus, seahorses and their relatives — pipefish and sea dragons — have long fascinated biologists.

A family tree constructed more than 20 years ago revealed a progressive increase in male motherhood activities over evolutionary time in this group of fishes. More primitively, some species’ males just provide a sticky plate to keep eggs attached to their body as the eggs mature. Others provide an open-faced shelter on their tails or bellies. And a few, the seahorses, have evolved this closed brood pouch and supply the young inside with oxygen and nutrients.

“I don’t know of any other example where sex role reversal has gone so far,” says Axel Meyer, an evolutionary biologist at the University of Konstanz in Germany. 

The translation of the first genetic instruction book of a seahorse set the stage for Yali Liu, an evolutionary biologist at the Chinese Academy of Sciences South China Sea Institute of Oceanology in Guangzhou, to discover the genes that underlie male pregnancy. She removed belly cells from 10 lined seahorses (Hippocampus erectus) during different stages of the male’s development and measured how active the cells’ genes were at each stage. 

“To our amazement,” she says, a distinct set of skin cells were involved. These cells turn on some of the same genes that females of other animals rely on during their pregnancies, Liu, Meyer and their team found. “This reveals a deep commonality in the biology of pregnancy, regardless of sex,” she says. 

But instead of being activated by female hormones, the seahorse’s genes seemed to be turned on by a male hormone. It’s unclear whether that hormone was testosterone or another androgen. But when Liu and her colleagues exposed female seahorses to testosterone, which in humans typically causes beards to grow and voices to get deeper in young men, the female fish also developed brood pouches. This finding confirmed the role of a male hormone in seahorse pregnancies — even though males also have female hormones.

In the pregnant males, the hormone also stimulated the newly occupied pouch to thicken so it could supply the embryos with oxygen and nutrients, much like a placenta does. But in this case, this “placenta” was derived solely from the father’s skin not reproductive tissue as in all other female animals, Meyer says. 

The work “represents a quite phenomenal example of how genetic networks can be rewired to achieve the same end,” says Thomas Boehm, an immunologist at the Max Planck Institute for Biology Tübingen.

Camilla Whittington, an evolutionary biologist at the University of Sydney, saw similar rewiring in 2022, when her team compared gene activity in pregnant female mammals, reptiles and sharks. While some of the same genes get used in these species, other genes are unique, she says. “There are multiple evolutionary pathways that can produce the same result: pregnancy.”