Which Animals Carry Their Young in a Pocket? More Than You Would Guess

Carrying developing young in an enclosed space on the body solves several problems at once. It protects eggs and embryos from predators, which take an enormous share of eggs released freely. It keeps them at a controlled temperature and, in aquatic animals, at a controlled salinity and oxygen level. It lets the parent move while brooding rather than being tied to a nest site. And in the more elaborate versions the parent supplies oxygen and nutrients directly, which extends how far development can proceed before the young must fend for themselves.

The arrangement has arisen independently many times:

• •Marsupials, where young complete development attached to a teat inside
• •Seahorses and pipefish, where the male carries and gives birth
• •Many crustaceans, which hold eggs beneath the abdomen
• •Several frogs, including species that brood in the vocal sac or the stomach
• •Some fish that brood in the mouth rather than a pouch
• •Certain insects and spiders that carry an egg mass attached to the body

Male pregnancy in seahorses is not simply carrying eggs and is the most complete reversal known. The female transfers eggs into a sealed pouch on the male's front, where they are fertilised internally and then embedded in tissue that develops a rich blood supply, supplies oxygen, removes waste, regulates the salinity of the fluid around them and provides nutrients. The male carries them for weeks and expels the young by muscular contractions. The tissue involved expresses many of the same genes that operate in pregnancy elsewhere in the vertebrates, which is a striking case of the same genetic toolkit being recruited for the same job in an entirely different sex and lineage.

Male care is rare across animals generally and is the rule in the family containing seahorses, which makes the reasons worth examining. Where fertilisation happens inside the male's pouch, he can be certain the young are his, which removes the uncertainty that otherwise discourages males from investing. The female meanwhile is freed to produce the next batch of eggs while the male carries the current one, so the pair can cycle faster than either could alone. That reversal also inverts the usual pattern of competition, and in several pipefish species it is the females that compete for males and the females that carry the brighter markings.

The arrangement is expensive and the costs are measurable, which is why not every species does it. A brooding parent carries weight that impairs swimming and increases the energy cost of moving, and in fish that brood in the mouth the parent frequently cannot feed at all for the whole period and loses substantial body mass. Space limits how many young can be carried, so brooding species produce far fewer offspring per attempt than broadcast spawners and invest far more in each. And the parent is a single point of failure, since losing it loses the entire brood, which is the trade being made against the protection it provides.

An enclosed space for developing young protects them from predators, controls their conditions and lets the parent keep moving, and it has evolved separately in marsupials, seahorses, crustaceans, frogs and others. The seahorse version is a genuine pregnancy in the male, with tissue supplying oxygen and nutrients using many of the same genes as pregnancy elsewhere. Brooding parents carry fewer young and frequently cannot feed.