Male seahorses take chivalry to the extreme, bearing and nurturing babies themselves instead of burdening their female counterparts. But this egalitarian behavior masks a dark side. A new study reveals that the males favor broods from attractive females and abort those from less desirable mates-a tactic that raises questions about sexual conflict in the seahorse world.
Pipefish, seahorses, and sea dragons—a fish family known as syngnathids—are best known for the unique sex role reversal in which males incubate the eggs in womb-like pouches on their bellies while females vie for their attention. Larger ladies tend to win out; males prefer to mate with them and only reluctantly couple with smaller females. During their showy courtship dance, prospective mates pirouette around each other, changing colors and intertwining tails before the female deposits her eggs in the male's pouch. He then fertilizes them and carries them, providing nutrients and oxygen until they hatch. Monogamous seahorse females stick around to help, but other syngnathid females strut their stuff to the next would-be father and start the process over.
Researchers understood the particulars about how syngnathids choose and woo their mates, but they knew little about what comes after. To find out, biologist Adam Jones and graduate student Kimberly Paczolt of Texas A&M University in College Station studied the Gulf pipefish, a syngnathid that looks like a stretched-out seahorse. They paired 24 males with females of varying sizes. The researchers monitored each male's pregnancy, then presented him with a new mate and watched a second pregnancy unfold.
The team found that fathers could control how much they invested in each brood. If males mated with larger, more attractive females first, their second broods tended to do poorly, implying that the males had channeled so many of their resources into the eggs from more desirable mates that they had little left over for the second pregnancy. Fathers matched with small females aborted their broods or drained resources from them through tissues in their brood pouches, but their subsequent broods with large females survived better. The pattern of egg survival suggests the crafty dads sabotaged their less desirable offspring to save resources in hopes of landing a better mate and investing more in her brood later on. The results, published in tomorrow's issue of Nature, offer the first evidence that pipefish exert an additional, post-mating control over their offspring and that their pouches play a role in this sneaky selection.
"Males only have so many resources to invest, so they invest preferentially," explains Paczolt. "They're more 'clever' than we thought."
"This is a very nice, very convincing study that provides another reason for males to have this pouch," says Anders Berglund, a biologist at UppsalaUniversity in Sweden. He notes there are alternative explanations for the outcomes Paczolt observed—higher-quality eggs may somehow force their fathers to devote extra resources to them, for example. Either way, these reproductive machinations point to sexual conflict in syngnathid society, long regarded as an egalitarian ideal. "Males do a lot for their babies," says Berglund, "but it's not so cute as one would have imagined."