For most men, the thought of taking on the burden of pregnancy from their partners would seem like a nightmare, but it's all part and parcel of seahorse life. After mating, female seahorses and pipefish lay their eggs into a special pouch in the male's belly and he carries the developing babies to term. They may seem like a shoe-in for a Dad-of-the-year award but this apparent display of paternal perfection has several macabre twists. A recent study showed that pregnant pipefishes can also become vampiric cannibals, absorbing some of their brood for nutrition if their own food supplies are running low. Now, Kimberley Paczolt and Adam Jones from Texas A&M University have found that male pipefishes are also selective abortionists. They'll kill off some of the youngsters in their pouches if they've mated with an unattractive female, or if they've already raised a large group of young in an earlier pregnancy. The pouch isn't just an incubator for the next generation. It's a battleground where male and female pipefish fight a war of the sexes, and where foetal pipefish pay for this conflict with their lives. Paczolt and Jones studied Gulf pipefish, a species where females mate promiscuously with several males but where males mate with just one female at a time. When the duo acted as pipefish matchmakers, they found that for male pipefish, size matters. They were far more reluctant to mate with smaller females than larger ones.

The pouch of a male Gulf pipefish is transparent and with careful photographs, Paczolt and Jones managed to see each egg, ensconced in its own chamber. These photographs revealed that not only are liaisons with larger females more likely, they're also more successful. The females transfer more eggs to the male's pouch, and a greater proportion of these eggs survive. Throughout the entire sexual experience, from choice to pregnancy, it seems that male pipefish discriminate against smaller partners. Pipefish females even have to compete against their partners' exes. If the male's last partner was big and provided him with lots of youngsters, the current set of embryos had lower odds of making it out of the pouch alive. It seems that a big pregnancy is a draining experience and a difficult one to pull of twice in a row. Paczolt and Jones note that the pouch isn't just a sealed box - it's a way for daddy to channel oxygen and nutrients to his kids. If males aren't satisfied with the quality of their mate, they could simply restrict this flow of nutrients from their own body, forcing the siblings to compete for the limited resources and automatically starving out the weakest ones. Any youngsters that die could even be recycled. Earlier this year, another group of scientists showed that amino acids from pipefish eggs sometimes end up in the tissues of the male that supposedly carried them. Daddy, it seems, was cannibalising some of his kids. Another interesting possibility is that the females are influencing the pouch wars too. A larger female could produce eggs that are better at harvesting nutrients from their father, or they could lace the male with chemicals that increase his investment. But if these scenarios were true, you would expect that after a large and exhausting pregnancy, drained males would actually pursue smaller females. In fact, the opposite happens. That suggests that the males are the ones with the final say over the embryos' fates. These sorts of sexual conflicts are common in the animal kingdom. But this is the first time they've been documented in an animal where the traditional sex roles of pregnant females and promiscuous males have been swapped. These results cast the pouch of a male pipefish or seahorse in a new light. It's still a nurturing bag that shelters and provides for youngsters but it's also a way for males to control their investment in the next generation. The pouch is the male's secret weapon in the battle of the sexes. Reference: Nature http://dx.doi.org/10.1038/nature08861

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