Sea horses and their cousins in the syngnathid group are the only known animals in which the male gets pregnant and bears the offspring. In these unusual reproductive circumstances, however, the next generation often does not thrive—or even survive.
A new study of sea horse cousins called pipefish found that the males can be particular—and proactive—about how they treat their developing young. These fish's dedication to their unborn progeny appears to depend on how suitable they find the female mate to be, reports a pair of researchers from Texas A&M University.
"If the male likes the mom, the kids are treated better," Kimberly Paczolt, of the Department of Biology at A&M and lead researcher in the study, said in a prepared statement. Long presumed to be a dedicated place of unequivocal nurturing, the male brood pouch, where the female deposits eggs, "also may grant the male better control over reproduction," Paczolt and her coauthor Adam Jones, also of A&M's Department of Biology, wrote in their study, published online March 17 in Nature (Scientific American is part of Nature Publishing Group).
The researchers examined the behavior of male Gulf pipefish (Syngnathus scovelli) in controlled breeding experiments when they were presented with consecutive possible female mates. They found that when males were more reluctant to mate with a female, fewer healthy offspring resulted from the coupling.
Channeling the necessary nutrients to a brood (which can be five to 40 offspring carried for 12 to 14 days) is taxing for the males and can interfere with their ability to mate with other, possibly even more attractive females.
"It's almost as if he is saying, 'Are these babies worth my effort?' If he is not overly fond of the mother, the answer appears to be 'No,' and he invests fewer resources," Paczolt said.
Thus, "a father could benefit from spending more [biological resources] on offspring with good prospects…and less on low-quality eggs from low-quality mothers," Anders Berglund, of the Department of Ecology and Evolution at Uppsala University in Sweden, noted in a commentary that accompanied the new study.
The brood pouch can deliver nutrients to the developing embryos, but it can also take them away. If a male deems his mate to be unworthy of any of his efforts, noted Berglund, "he may even want to use the low-quality eggs as food for himself, to gain resources for future offspring with better prospects," actually consuming the developing young by absorbing them.
Although the researchers are not exactly sure how or why the male fish decides to care for broods differently, they did discover a strong preference for bigger females. "The one trait in the pipefish that seems to stand out is the size of the female," Paczolt said. "Males tend to seek out larger females to be their mates," she noted, and broods from these large females were bigger and more likely to be carried to term.
The new discovery adds another layer of intrigue to the rare reversal of sexual roles—and what implications it has for evolutionary advantages in sexual selection and conflict, the researchers noted. "The whole phenomenon of male pregnancy is full of conflict and far more complex than we had previously realized," Paczolt said.
Image of female Gulf pipefish courtesy of Nick Ratterman