Seahorses are the most famous example, but they’re far from the only one. Several animal species have evolved systems where males carry developing offspring, either inside their bodies or attached externally. These range from fish with fully enclosed brood pouches to frogs that incubate tadpoles in their throats and insects that haul eggs on their backs.
Seahorses and Their Relatives
The seahorse family, known as Syngnathidae, includes over 200 living species of seahorses, pipefish, and seadragons. In every single one, the male is the pregnant parent. After a female deposits her eggs into the male’s body, he fertilizes them internally and carries the developing embryos in a ventral brood pouch on his abdomen or tail.
This isn’t just egg storage. The male seahorse’s pouch functions remarkably like a mammalian uterus. Developing embryos settle into individual compartments lined with vascularized tissue that acts as a pseudo-placenta, allowing the exchange of gases, nutrients, and waste. Males actively supply proteins and lipids to the growing embryos. Research on the Australian pot-bellied seahorse found that newborns contained significantly more protein mass than the freshly fertilized eggs they started as, confirming that fathers transfer nutrients directly to their young during pregnancy.
The hormone prolactin, best known for its role in milk production in mammals, plays a key part here too. Receptor activity for prolactin ramps up in the brood pouch during the second half of pregnancy, helping regulate the salt and water balance inside the pouch. This is strikingly similar to how the same ion-exchange systems work across the placenta in mammalian pregnancies.
Brood sizes vary enormously by species. Smaller seahorses may carry as few as five offspring at a time, while the pot-bellied seahorse, the largest species at up to 35 cm, can release around 2,000 hatchlings in a single brood. Most species fall somewhere in the range of 100 to 300.
Within the family, the level of male involvement varies on a spectrum. Some pipefish species simply glue eggs to the outside of the male’s body. Others have partially enclosed pouches. Seahorses and certain pipefish represent the most advanced end: a completely sealed pouch where embryos are fully integrated into paternal tissue and nourished through the pseudo-placenta. This evolutionary range, all within one family of fish, shows a clear progression from basic external egg carrying to true internal pregnancy.
Darwin’s Frog
Darwin’s frog (Rhinoderma darwinii), a small species native to the forests of Chile and Argentina, has one of the strangest parenting strategies in the animal kingdom. The male carries his offspring inside his vocal sac, the same structure most frogs use to croak.
After the female lays eggs on the forest floor, the male guards them for about 20 days. Once the embryos are developed enough to wriggle inside their eggs, he scoops them up into his mouth and they slide into his vocal sac. The embryos hatch into tadpoles roughly three days later, and then the entire process of development and metamorphosis happens inside the father’s throat. This post-hatching period lasts 50 to 70 days. During that time, the male nourishes the tadpoles with viscous secretions produced inside the vocal sac. When metamorphosis is complete, the tiny froglets crawl from the vocal sac into the father’s mouth and hop out into the world.
Brooding males are easy to identify because their vocal sacs are visibly swollen with developing young. This species is currently endangered, making its unusual reproductive biology both fascinating and increasingly rare to observe in the wild.
Giant Water Bugs
Male giant water bugs in the genus Appasus carry eggs glued directly onto their backs. After mating, the female cements her eggs to the male’s wings, and he swims around for weeks tending the clutch. He aerates the eggs by doing pushups at the water’s surface, exposing them to air to prevent fungal growth and ensure oxygen supply.
This arrangement comes with an unexpected perk. Research on Appasus major published in Royal Society Open Science found that males already carrying eggs were more attractive to additional females. The visible egg burden essentially signals that a male is a proven, committed parent, turning childcare into a mating advantage. This suggests that sexual selection, not just offspring survival, may be a major force keeping this dads-only care system in place.
Nurseryfish
The nurseryfish (Kurtus gulliveri), found in the brackish rivers of northern Australia and New Guinea, has a feature unique among all fish: a bony hook protruding from the top of the male’s head. After fertilization, the male carries the entire egg cluster suspended from this forehead hook as he swims. The eggs dangle between his eyes in a gelatinous mass until they’re ready to hatch. No other fish species carries eggs this way.
Why Males Carry Offspring at All
In most of the animal kingdom, females bear the greater burden of parenting. Male pregnancy and egg carrying tend to evolve in environments where offspring face high predation risk and where one parent investing heavily in protection and nutrition leads to better survival rates. In aquatic environments especially, keeping eggs attached to a mobile parent rather than leaving them in a nest means the young can be moved away from threats and kept oxygenated.
For seahorses, male pregnancy may also free up the female to produce the next batch of eggs while the male is still gestating. This allows pairs to breed more frequently than if the female had to carry and protect offspring herself. In giant water bugs, the dynamic is reversed in an interesting way: males that invest in carrying eggs gain a reproductive advantage by attracting more mates, creating a feedback loop that reinforces paternal care.
These species are scattered across unrelated branches of the animal tree of life, from fish to amphibians to insects. Male parental carrying didn’t evolve once and spread. It arose independently multiple times, each time solving the same basic problem: keeping vulnerable offspring alive long enough to fend for themselves.