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.
More on the war of the sex:
- Ballistic penises and corkscrew vaginas - the sexual battles of ducks
- Study reveals sexual tactics of male flies by shaving their genitals with a laser
- Female antbirds jam their partners' songs when other females approach
- Chimps call during sex to confuse fathers, recruit defenders and avoid competitors
Random picks from the archive
- Delay not deviance: brains of children with ADHD mature later than others
- Fake cleaner fish dons multiple disguises
- Deformed skull of prehistoric child suggests that early humans cared for disabled children
Maybe I hang out over at PZ's place to much but as soon as I saw that the father will both abort and eat babies I started to think that he was the perfect atheist, at least in the minds of some (not me of course, I am a long time abortionist and baby eater).
Ha, I had just watched this vid on Nature youtube channel and was lamenting the fact that I don't have a subscription to read the original research. But popping over the NERS I find all the hard work done for me and nice summary of the paper presented, thanks Ed!
Does this imply that the pipefishes remember something about who they mated with? That is a little bit hard to believe. Or are they responding to a discernable quality of the eggs they're carrying?
Nathan, do you mean the fact that the last pregnancy affects the current one? No memory required - it could just be that after a pregnancy with high brood survivorship, males simply don't have enough energy to carry another full brood to term. Your sensory hypothesis is possible too.
I'm keying off of "If males aren't satisfied with the quality of their mate, they could simply restrict this flow of nutrients from their own body..." That depends on the male either remembering something about who the eggs came from, or noticing something about the eggs themselves. We know that one such thing is that there are more of them. Maybe they're bigger, too. Whatever it is, if his daughters are small, both they and their eggs will be discriminated against. So, maybe with fewer eggs to care for, he cares for them less well, to have more resources left for the eggs of the next mate, who might be bigger?
Cook stuff! I don't think that the presence of aminoacids from the offspring proves anything though. The males presumably have to process the excreta of the growing embryos and this will include proteins too. Even cells from your kids - if you are a female, obviously - stay with you after pregnancies, but that does not imply human females are cannibalistic. Embryos also could die from natural causes (malformations, inbreeding, etc) and efficient dealing with such dead embryos, such as reabsorption, would be advantageous both for the male (recycling of resources) and for the growing young (decreasing the chances of infection). I am not sure how the pouch works though!