1. Who Lays the Eggs and Who Carries Them?
During mating, which follows a courtship dance, the female transfers her eggs into the male's brood pouch. Inside this pouch, the embryos are supported by a tissue called the "pseudo-placenta." [1] Therefore, saying "the male lays the eggs" is not accurate. The male gives birth, but the female is the one who lays the eggs. [1]
Male pregnancy is not unique to seahorses. It is also found in the Syngnathidae family, which includes pipefish and sea dragons. [6] This trait is rare among vertebrates, though it is also seen in Darwin's frog, an amphibian. [6] Thus, having a brood pouch is not exclusive to seahorses. [6]
2. How the Brood Pouch Protects the Embryos
According to a laboratory at Sophia University, the brood pouch is only present in mature males and is not seen in immature ones. The pouch forms only after the male reaches maturity. [5]
University of Konstanz reports that embryos receive nutrients and oxygen from the male's body, and the male gives birth to live young. During pregnancy, the tissue in the brood pouch changes into a structure similar to the placenta in mammals. [2] However, it is not the same organ as a mammalian placenta, but a different tissue that performs a similar function. [2]
Research on the giant seahorse shows that during pregnancy, thin blood vessels develop in the brood pouch, significantly reducing the distance between the father's blood and the embryos. [3] In this species, a male can carry up to about 1,000 young, and the pregnancy lasts about 30 days. [3] However, the number of young and the duration vary depending on the species and conditions. [3]
3. How Muscles Help the Male Give Birth
A study published by Dudley et al. in 2022, introduced in an article by the Japan Science and Technology Agency (JST), explains that the male's brood pouch has less smooth muscle compared to a mammalian uterus. Instead, skeletal muscles and bones involved in childbirth are larger near the opening of the pouch. [4]
The JST article states that it is believed the male uses these muscles to push the young out of the pouch. [4] As the phrase "it is believed" suggests, this is an explanation that includes speculation, rather than a conclusion confirmed by direct observation. [4]
4. Lost Immune Genes and Changes in the Male's Spleen
It is reported that fish in the Syngnathidae family have lost three immune genes (TLR1, TLR2, and TLR9 of the Toll-like receptors) that many vertebrates possess. [7] These genes are also involved in the mother's body accepting the fetus in the uterus and placenta. It has also been reported that species with more developed brood pouches tend to have fewer types of immune genes. [7] However, this report does not show that losing these genes caused male pregnancy to become possible; it is cautious to read this as a change that happened at the same time. [7]
In a study comparing the genomes of four seahorse species, a mutation specific to the lineage including seahorses was found in the tlx1 gene. Zebrafish with this same mutation lost their spleens in all individuals examined. [1] The seahorse's spleen is said to be non-functional. It has not been shown that the absence of a spleen is the cause of male pregnancy. [1]
In 2025, the University of Konstanz introduced a study in Nature Ecology & Evolution reporting that pregnancy progresses due to male hormones (androgens). It also noted that the foxp3 gene, important for immune tolerance, is absent, suggesting the immune system might be suppressed by a different mechanism. [2]
5. Why Do Males Get Pregnant? Scientists Still Don't Know
When males become pregnant, the number of eggs they can raise (limited by the pouch size) becomes the upper limit of the male's reproductive success. A 2024 mathematical model showed that as the pouch becomes smaller, the "Bateman gradient," which usually favors males, becomes equal for males and females, and can eventually reverse. [6] This means the female side could be the one that benefits from having multiple partners. [6] However, this is the result of a model, not an explanation of why male pregnancy originated. [6]
In the Syngnathidae family, research suggests that "role reversal," where females compete or court conspicuously, is not predicted by male pregnancy alone but tends to be determined by mating patterns. [8] Monogamous species tend to have conventional roles, while species with reversed roles tend to have multiple partners. [8] There are also differences between species. [8]
6. Where to Learn More About Seahorse Pregnancy
You can compare the abdominal shapes of males and females in encyclopedias or photos to find the location of the brood pouch. JST introduction articles and university laboratory pages available in Japanese can be accessed via the source links. [4] [5]
When visiting an aquarium, observe the movements of males and females alongside the exhibit descriptions. Please do not tap the tanks or use flash photography. [4]