Male Pregnancy in Seahorses

Seahorses have a unique mode of reproduction: male pregnancy. Male seahorses provide all post-fertilization parental care, yet despite the high levels of paternal investment, they have long been thought to have conventional sex roles, with females choosing mating partners and males competing for their attention. However, clutch, egg and offspring size all increase with female body size in seahorses, suggesting that males may obtain fecundity benefits by mating with large-bodied females.

Seadragons and pipefish, which are related to the seahorse, also have the male being the one to become pregnant.  In the case of the pipefish, though, the male may decide to digest the embryos, rather than bring them to term, if food is scarce or if he simply found his mate to be less than desirable.