Daddy fish: the male Gulf pipefish (Image: Touterse/Flickr) A baby Gulf pipefish, in the brood pouch (Image: Charlyn Partridge) Inside a pregnant male’s brood pouch, 10 days into gestation. The embryos are visible through the pouch folds, curled into little balls around the remnants of their yolk. The large black circles are their developing eyes (Image: Charlyn Partridge)


Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world.

Species: Syngnathus scovelli

Habitat: shallow, near-shore waters from Florida to Brazil. Seaside maternity units.

Who’s the daddy? If we’re talking about Gulf pipefishes, it’s easy to spot: he’s the pregnant one.

Males and females across the animal world have always attempted to foist responsibility for caring for their offspring onto each other. In pipefishes, seahorses and sea dragons, the females seem to have won: the males get pregnant and carry the offspring as they develop.

But the males are no evolutionary pushovers. They may be left holding the babies, but they assert themselves by selectively aborting offspring from less attractive females.

Sex back to front

Pipefish reverse every stereotype we have about the roles of the sexes.

In species where the females get pregnant or otherwise care for the offspring, the males tend to be more promiscuous and the females more choosy over who they mate with. Pipefish do things the other way round: in Gulf pipefish, males mate once per pregnancy while the females mate with several males.

As a result, females compete more for the males’ attention. The females are also larger than the males, and have conspicuous V-shaped silvery stripes that may make them more attractive to the opposite sex.

It is also the female that initiates courtship: she erects her dorsal fin, darkens the skin around the silvery stripes, and holds her body in an S-shape. Just to make the point, she sticks out her ovipositor, the organ with which she lays eggs. Then both the male and the female swim with their long thin bodies held vertically.

If the fussy male is willing, they intertwine their bodies and the female “ejaculates” her eggs into a pouch on his chest. Within this protective cocoon, the male fertilises the eggs then nurtures the young as they develop. The pouch has a structure rather like the placenta found in female mammals, which allows the father to transfer nutrients to his offspring.

You always loved him more

But it seems these parents are guilty of favouritism. Kimberly Paczolt and Adam Jones of Texas A&M University in College Station have shown that the males are selective at all stages of the breeding process, including after they have become pregnant.

For starters, they are picky about which females they mate with, preferring to go for big ones. Paczolt found that males would generally mate with the largest females within a day of encountering them, but would um and ah for weeks before bestowing their favours on a smaller suitor.

That’s not just sizeism: larger females provided more eggs. A higher percentage of those eggs made it to birth, too, though that could be in part because the fathers took better care of their large mates’ offspring: when they had just had a brood with a large female, fewer offspring would survive from the next one.

In other words, the males seem to be selecting which broods they lavish the most care on. The unlucky offspring that the males aren’t bothered about are more likely to die before birth.

Paczolt does not know how the males help their favoured offspring to thrive. But another recent study raises an intriguing possibility.

Pregnant males of the closely related broad-nosed pipefish can drain nutrients from their offspring through the brood pouch – nutrients that were originally included in the eggs. The “placenta” that the males use to nourish their young can also be used to take food off them – talk about stealing candy from a baby.

Journal reference: Nature, DOI: 10.1038/nature08861 (in press)