Title, basically. Females are the ones that give birth, or are there more exceptions like the seahorse?
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The Midwife Toad does something similar. The female releases a string of eggs which the male fertilizes and wraps round his legs, carrying them about until they are ready to hatch.
Evolution does what works, there is no set plan or “right” way of doing something. In the case of seahorses and some amphibians, evolving a way of protecting eggs rather than just dumping them in the water, hoping that some survive, has led to their enhanced survival.
Female animals produce eggs (the larger sex cell, via a process called oogenesis). Males produce sperm (the much smaller sex cell, via spermatogenesis). Generally the evolutionary game plan is to have the little sperm to travel to the larger less mobile egg, hence female incubation being the norm. But it’s not a requirement.
In many aquatic creatures, for example salmon or jellyfish, the females and males both just release eggs/sperm into the environment together. There’s no proper internal incubation or even sex involved.
To say that male seahorses give birth is very misleading. Seahorses reproduce in more or less the same way as most fish: the female lays eggs, the male fertilizes them with sperm, and after some length of time the eggs hatch.
Seahorses are unique because instead of just releasing their eggs into the open water or sticking them to a rock or something, the male seahorse holds them in a pouch on its belly. To be clear, the eggs are still external to the male’s body, so it is not like a mammal’s uterus. Keeping the eggs in a pouch protects them from predators and helps to regulate their temperature, salinity, and oxygenation; the male seahorse also secretes nutrients that help the embryos grow. As for why the male does it and not the female, that is unclear. One compelling theory is that by handing off the eggs to the male, the female seahorse can focus its energy on producing the next brood of eggs, which ultimately results in more offspring.
While this behavior is unique it is not completely without comparison. There are several species of fish, including arowanas and some types of cichlid in which the male carries fertilized eggs in its mouth. In some cases the young continue to live in their parents mouth even after they hatch, unlike in seahorses which do not nurture their young at all after they hatch.
They basically house the fertilised eggs transferred from the female and protect them while they develop and later give birth to a more developed form of seahorse.
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