Seahorses and a few related fish are the only species where the males can get pregnant. These males take eggs from females and nurture them in their brood pouches. Researchers have discovered that male pipefish, one of those related species, may selectively abort young resulting from mating with less attractive females. The above video, which is preceded by a commercial, describes the results of this study. From Scientific American:
"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.[...]
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.