Male and female dusky pipefish look exactly the same in all but one aspect—males have a pouch for incubating eggs when they get pregnant. But it’s hard to spot, says Coley Tosto, author of a new study investigating what a pipefish thinks is sexy.
Tosto, a post-doctoral fellow at the University of Canterbury, caught a couple of hundred pipefish by dragging a makeshift comb attached to a net through seagrass beds. Dusky pipefish are maybe the length of your foot, and they look like a seahorse stretched out into a straight line.
“Pipefish are weird for many reasons, but one of them is that they don’t have a stomach,” says Tosto. “It’s just a tube from mouth to anus, and it means that in the wild they’re eating constantly.”
Feeding them constantly kept Tosto busy—especially because she wanted her matchmaking to succeed. She separated them into groups of 16, eight males and eight females, to see which fish would hook up.
“The official term is ‘experimental breeding populations’,” she says, “but the more fun term is ‘pipefish dating pools’.”
Many mating rituals involve males competing for the attention of females, but with pipefish, it goes both ways. Getting pregnant is a burden, so males are discerning about their mates. Females, meanwhile, are looking for the fittest male to take on the responsibility of fertilising and carrying their eggs.
Tosto’s analysis found that females generally favoured smaller males—which was so counter to expectations that she ran the results twice.
She theorises that smaller males may be younger—or look younger—and that females associate youth with good health. Another theory is that smaller males may perform better in the pipefish mating ritual, a display that begins with group twitching and wiggling and culminates with two pipefish twirling upwards through the water in synchronisation.
Or, perhaps, there is something invisible taking place: anything from genetic differences Tosto has yet to identify to a certain pipefish je ne sais quoi.