Pregnancy in seahorses is a male affair: it is not the mothers, but the fathers who carry the offspring. The babies develop in a brood pouch on the father’s stomach. A study now shows how this male pregnancy works on a cellular level. Accordingly, the male sex hormones ensure that the abdominal pouch takes on similar functions to the uterus and placenta in female animals. They also regulate the immune system so that it does not reject the offspring.
Roles reversed: In seahorses, the males carry the offspring. When mating, the female lays her eggs in a special brood pouch on the male’s stomach, where they are fertilized by his sperm. The embryos develop inside the brood pouch and are supplied with nutrients and oxygen by the male’s body until the young seahorses are finally born via live birth – almost like marsupials.
© Jinggong Zhang
Pregnant males
But how does male pregnancy work on a cellular level? A team led by Yali Liu from the South China Institute of Oceanography in Guangzhou has now addressed this question. To do this, the researchers analyzed how the cells in the brood pouch of male seahorses (Hippocampus erectus) develop over the course of the gestation period and which genes are active. They compared their results with the development of the female uterus and placenta during female pregnancy in mammals.
The result: The seahorse fathers’ brood pouch develops in a very similar way to the female uterus during pregnancy. As with pregnant female mammals, hormones play a key role in pregnant male seahorses. But while female hormones such as estrogen are primarily involved in female pregnancy, male hormones control pregnancy in seahorses.
“Our studies confirmed that androgens – male sex hormones – play a central role in the development of embryos in the brood pouch instead of classic female hormones,” reports Liu’s colleague Axel Meyer. “The androgens induce the thickening and vascularization of the skin layer in the abdominal area to form a structure that is similar to the placenta of mammals. This shows an interesting difference to the development of the female uterus of mammals including humans, which is typically guided by female hormones.”
Changes in the immune system
The immune system also adapts during seahorse pregnancy. To ensure that the embryos are not recognized as foreign bodies and rejected, the immune response of the parental body must be suppressed. In many viviparous species, a gene called foxp3 is responsible for this. “Surprisingly, seahorses lack the foxp3 gene, which raises questions about their unique immune tolerance strategies during male pregnancy,” said the researchers. Although the study could not clearly clarify which mechanisms exactly regulate the immune system of the seahorse fathers, they suspect that male sex hormones also play a role. “Androgens often have an immunosuppressive effect, i.e. suppressing the immune system. This could contribute to this unique immune tolerance,” explains Meyer.
It is possible that this very property could even have led to male pregnancy becoming commonplace in seahorses over the course of evolution. “The different evolutionary stages within the seahorse family Syngnathidae make them an excellent model for understanding the development from egg-laying reproduction of their ancestors to live birth,” says Meyer. A first step could have been for the females to attach sticky eggs to the males’ bodies. Only later did the brood pouch develop, offering the offspring more protection and thus increasing survival rates.
“Thanks to our studies, we now better understand the genetic, molecular and cellular mechanisms underlying this remarkable evolutionary development – how pregnancy developed repeatedly in female mammals and male seahorses, but through different genetic and hormonal pathways,” says Meyer.
Source: Yali Liu (South China Sea Institute of Oceanology, Guangzhou, China) et al., Nature Ecology & Evolution, doi: 10.1038/s41559-025-02883-5