Pregnant Male Pipefish Are the Sea’s Swaggery Swingers

Everyone loves seahorses. With their trumpet-tipped snouts, gender-bending pregnant papas and tendency to dabble in monogamy, the long-necked equines of the sea have easily coiled their boxy prehensile tails around human hearts like so much sea grass.

But every family has a black sheep. Pipefish, the lesser-known, straight-bodied cousins of the common seahorse, aren't interested in your outdated notions of monogamy. As with seahorses, pipefish males are the ones who carry eggs to term—but scientists now report that these fickle fellows can compromise their current brood of eggs if they so much as glance at a more attractive passing female.

“This is an extremely clever and insightful study… which [features] the dark side of paternal care in pipefish,” says University of Michigan biopsychologist Jacinta Beehner, who did not participate in the work. “[It] probably won't find its way into many children’s books featuring cartoon ocean animals.”

The idea of curbing the viability of an ongoing pregnancy sounds macabre, but it’s actually not unheard of in the animal kingdom. In the late 1950s, a scientist named Hilda Bruce found that pregnant female mice that encounter unfamiliar males can undergo spontaneous abortions. Initially baffled, Bruce eventually realized these females’ behavior isn’t borne out of cruelty; rather, it’s a form of self-preservation. Had the fetuses been carried to term, the new, dominant male would have likely slaughtered his competitor’s pups to make room for his own. Rather than waste precious resources on a doomed litter, female mice resign themselves to a bleak fate.

And these morbid little mice are not alone. In recent years, gelada monkeys, horses, lions and several other rodents have joined the grisly cult of the so-called Bruce effect. No observations of compromised pregnancies had been made outside of mammals: It seems having an intimate connection between parent and fetus—like that provided by the placenta—is critical for the level of control needed to terminate a pregnancy. But if any species were to disprove the placenta postulate, it might be the rule-breaking pipefish.

Pipefish and their relatives don’t have placentas—but they do have male brood pouches. When females deposit their eggs into one of these pouches, a dialogue is immediately initiated between father and offspring. The pouch siphons off some of the male’s resources, including nutrients, oxygen and immune factors, and provides them to the developing eggs. Pregnancy lasts only a couple weeks; after the eggs hatch, the folds of the male’s pouch slide open like the doors of an elevator, unleashing a teeny cavalry into the open seas. The tearful goodbye is final: After birth, pipefish don’t care for their young. Because of this, the entirety of a father’s influence over his progeny is exerted during mating and pregnancy—meaning that every choice made in this window of time is critical.

If you’re a male pipefish on the prowl, there is nothing hotter than a “huge, sexy female,” says senior author Nuno Monteiro, an animal behaviorist at the University of Porto in Portugal. The bigger the pipefish, the more fertile—and males take note. They’ll invest their time and energy in courting the broadest of broads in the hopes that their kids sport the best possible genes. And dads don’t stop at mate choice: Even after females have passed their eggs to males, fathers can have favorite children. Some males even dote on eggs from highly alluring females, passing them extra resources during pregnancy, and nutritionally neglect those from more petite mates. With these clear preferences, Monteiro saw an opportunity to test if a male’s caprice might have other effects on his developing offspring.

“If a male [pipefish] gets pregnant, he should have some degree of control over his pregnancy, just like a female [mammal] would,” he explains. “It can’t just be a burden they’re unable to modulate.”

Together with his student Mário Cunha and their colleagues, Monteiro devised a series of experiments modeled on Bruce’s original findings. First, they allowed male black-striped pipefish to be impregnated by healthy, average-sized females. Then, some of these moms were replaced by a weighty lady the males might consider a comparatively better catch.

As the pregnancies progressed, the researchers found that males who had glimpsed fuller-bodied females were much more likely to abort some of their eggs than those who hadn’t. And while all males gave birth to at least some living offspring—a much milder display of the spontaneous abortions previously induced in mammals—the pipefish dads exposed to a fresh set of womanly wiles experienced slightly shorter pregnancies and hatched smaller newborns.

Notably, none of the pipefish males actually had the chance to couple up with these new arrivals: In each case, the researchers waited until the males had sealed their pouches to further implantation before introducing a second female, then separated the pairs with a physical barrier to guard against any inadvertent funny business. But the mere sight of a new lady looming large was enough to steal these pipefish hearts—and perhaps the lives of their children.

Clearly, male pipefish have a thing or two to learn about fidelity. The explanations behind such behavior remain tough to disentangle, however: Despite their fluted snouts, these lanky lotharios aren’t big talkers. “I have difficulty understanding by own motivations sometimes,” says Monteiro. “Other animals’ motivations are even harder.”

Cunha and Monteiro theorize that these males might be banking on more auspicious future pregnancies by way of these husky heavyweights—leading them to divert resources away from their current, crappier brood. In fact, previous work has shown that males can actually reabsorb nutrients from their eggs—meaning there’s a good chance that these dads aren’t just withholding resources, but actively reclaiming them. The placenta-like pouch operates as a two-way street: A father can giveth, but also taketh away, with quite a high degree of control.

“This is really exciting and well done,” says University of Tampa biologist Emily Rose, who studies pipefish but was not affiliated with this study. “[The authors] took a really thorough approach.”

For now, many questions remain. And although these pipefish seem a little sleazy, their abortive attempts still pale in comparison to the true Bruce effect observed in some female mammals. But Cunha and Monteiro were never expecting such a dramatic result in their pipefish. For one, female pipefish don’t seem to go out of their way to kill other females’ offspring, which appears to motivate many of the males in mammalian species in which the Bruce effect is observed. (Notably, both male and female pipefish will occasionally slurp up juveniles of their own species through their straw-like mouthparts, but this is probably more run-of-the-mill cannibalism than sexual competition.) Additionally, because pipefish dads hit the road almost immediately after giving birth, they don’t stand to lose as much in terms of childcare as other animals that spend a great deal of time rearing their young.

In their next experiments, Cunha and his colleagues hope to uncover the chemical bases of these behaviors, including what signals giga-gals might be sending males, and the hormones that mediate male abortion. Additionally, it’s still unclear if males hoarding resources from less-than-desirable young are actually able to reallocate them to future broods. Even if two-timing pipefish save up their energy in the short term, this doesn’t guarantee that they’ll even get the chance to tango with the daunting dame in question.

“The authors have discovered a fascinating fact,” says behavioral neurobiologist Justin Rhodes of the University of Illinois at Urbana-Champaign. Rhodes, who was not involved in the research, cautions against attributing too much intention to these flighty little fish. “Many details are still unclear,” he adds. “It’s worth keeping in mind that there’s not always a function for everything we see in nature.”

For now, scientists have only “hints,” says Cunha. We may never know what’s truly going on in the heads of the sea’s sveltest swingers—but in the meantime, what’s clear is this: In the presence of the most buxom babes, even these boys can’t seem to keep it in their pouch.

Katherine J. Wu | | READ MORE

Katherine J. Wu is a Boston-based science journalist and Story Collider senior producer whose work has appeared in National Geographic, Undark magazine, Popular Science and more. She holds a Ph.D. in Microbiology and Immunobiology from Harvard University, and was Smithsonian magazine's 2018 AAAS Mass Media Fellow.