Male Pregnancy - in Animals

In Animals

The Syngnathidae family of fish has the unique characteristic of a highly derived form of male brood care referred to as "male pregnancy." The family is highly diverse, containing around 300 different species of fish. Included in Syngnathidae are seahorses, the pipefish, and the weedy and leafy sea dragons. The males of some of these species possess a brood pouch on the trunk or tail; in other species, the eggs are merely attached to the male's trunk or tail when the female lays them. Although biologists' definitions of pregnancy differ somewhat, all members of the family are considered by ichthyologists to display male pregnancy, even those without an external brood pouch.

Fertilization may take place in the pouch or in the water before implantation, but in either case, syngnathids' male pregnancy ensures them complete confidence of paternity. After implantation in or on the brood pouch or brood patch, the male incubates the eggs. Many species osmoregulate the brood pouch fluid to maintain proper pH for the developing embryos. In at least some species, the male also provisions his offspring with nutrients such as glucose and amino acids through the highly vascularized attachment sites in or on his body.

This period of incubation can take much longer than the production of another clutch of eggs by the female, especially in temperate regions where pregnancies last longer, leading to a reproductive environment in which sexual selection can be stronger on females than on males due to increased male parental investment. Interestingly, this reversal of traditional sex roles has only been found in pipefishes, whereas seahorses have largely been accepted as monogamous. Some pipefish species display classical polyandry because of this unique situation. Male syngnathids usually prefer females with large body size and prominent ornaments such as blue skin pigmentation or skin folds. Intriguingly, syngnathid males in some species are apparently capable of absorbing eggs or embryos while in the brood pouch. In these cases, embryos with the highest survival rate are those whose mothers display the preferred phenotype.

Syngnathidae is the only family in the animal kingdom to which the term "male pregnancy" has been applied.