Greater pipefish hiding in japweed. Grevelingen, the Netherlands.

I will never tire of observing pipefish. The one in this underwater photo is a greater pipefish Syngnathus acus, blending into the Japweed Sargassum muticum. It seemed quite confident that I didn’t notice it—until it discovered its own reflection in my camera’s dome port, that is. Then the little fish came out of hiding and swam right up against it.

Cute as that may be, the really cool thing about pipefish is that some of them show what’s known in biology as sex role reversal.

You see, in nature it’s generally the males who romance the females, and the females who get to accept or reject their advances. Likewise, females tend to spend more energy and time on care for offspring than males do. Not so in pipefish, however.

In pipefish and the closely related seahorses and seadragons (collectively the syngnathid family), a female dances with a male to seduce him. What’s more, it’s the male who becomes pregnant, not the female. If she succeeds, their dance culminates in the transfer of eggs to a special brood pouch on the male’s belly.^[1] Males can then fertilize the eggs, and will care for them until they hatch, while females don’t provide any parental care at all.

This enchanting Life footage, narrated by the ever inspiring David Attenborough, shows the dance of one of the most graceful syngnathids, the weedy seadragon Phyllopteryx taeniolatus:

In addition to keeping the eggs safe, the male’s pouch acts like a placenta in the sense that it provides aeration, osmoregulation and nutrition to the developing offspring during a pregnancy that can last several weeks. Like any pregnancy, this is energetically demanding, but it turns out it’s not all fatherly dedication.

A pregnant male is able not only to provide nutrition to its broods, but also to take it from them.^[2] What’s happening is the result of a conflict of interest between individuals of both sexes.

Males like larger females over smaller ones, but caring for the large broods of large females is a big hit on the energy reserves of the male. To compensate, males appear to absorb broods from smaller females as an energy boost.^[3] In other words, pregnant fathers cannibalize their offspring at the expense of the reproductive interests of those females. Males should benefit from this, because it allows them to raise more offspring from larger mothers in the future.

Male pipefish may have lost the battle over parental care, but they haven’t lost the war.

1. In some species males don’t have a pouch; the eggs are simply glued to the belly. This might explain how syngnathid males evolved to become pregnant, but that’s a story for another article.
2. Reference: Sagebakken G, Ahnesjo I, Mobley K, Goncalves I, Kvarnemo C (2009). Brooding fathers, not siblings, take up nutrients from embryos. Proc R Soc Lond B 277:971–977. DOI: 10.1098/rspb.2009.1767.
3. Reference: Paczolt K, Jones A (2010). Post-copulatory sexual selection and sexual conflict in the evolution of male pregnancy. Nature 464:401-404. DOI: 10.1038/nature08861.