When a Pregnant Pipefish Dad Spots an Alluring Female, Things Get Weird

A robust ghost pipefish in waters off Indonesia. With some species of pipefish, fathers are susceptible to what scientists call the Bruce effect.

Pipefish, along with their cousins sea horses and sea dragons, defy convention in love and fertility. In a striking role reversal, fathers give birth instead of mothers.

During courtship, females pursue males with flashy ornaments or elaborate dances, and males tend to be choosy about which females’ eggs they’ll accept. Once pregnant, these gender-bending fathers invest heavily in their young, supplying embryos with nutrients and oxygen through a setup similar to the mammalian placenta.

But this investment may also be cruelly conditional, according to a new study in Proceedings of the Royal Society B. Studying pipefish, scientists found evidence that pregnant fathers spontaneously abort or divert fewer resources to their embryos when faced with the prospects of a superior mate — in this case, an exceptionally large female.

The researchers named their finding the “woman in red” effect, after the eponymous 1984 Gene Wilder film about a married man’s obsession with a woman in a red dress that becomes damaging to his family life.

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The reported effect is an interesting instance of sexual conflict, which is ubiquitous among animals, said Sarah Flanagan, a pipefish expert at the University of Canterbury in New Zealand.

If you’re a romantic, you might think of mating as harmonious. But in nature, reproduction is more often a vicious power struggle between mothers and fathers with competing interests.

A maternal analogue to the “woman in red” effect occurs among mice. Males are willing to kill a female’s offspring, if they are unrelated to him, before mating with her.

In anticipation, a pregnant mother may terminate her pregnancy when exposed to a new male, rather than spending resources on doomed offspring.

This behavior, named the Bruce effect (for Hilda Bruce, the British zoologist), has been documented in lions, horses and monkeys, too, but a similar strategy has never been reported apart from mammals until now, said Nuno Monteiro, a researcher at the CIBIO Research Center in Biodiversity and Genetic Resources in Portugal and an author of the new study.

Though pipefish don’t commit infanticide, Dr. Monteiro wondered whether they experienced something similar to the Bruce effect because pregnancies are so costly for males. Previous research had found that pipefish fathers invest less in pregnancies from small females if they have bred with larger females before.

At the risk of anthropomorphizing, Dr. Flanagan said, it’s as if male pipefish are thinking, “I’ve done better in the past.”

In their study, Dr. Monteiro and his collaborators bred male black-striped pipefish with large females, then kept each father-to-be in a tank partitioned from a new female of equal size, a new female much larger in size (the woman in red), or its original mate.

Males in the “woman in red” group — exposed to the new, “sexier” female — had the highest rates of abortion and shortest pregnancies. They also birthed smaller offspring, some of which had abnormalities.

It seems that these fathers stopped investing in existing offspring to save up for a better brood, said Jacinta Beehner, an associate professor of psychology and anthropology at the University of Michigan who studies reproductive strategies in primates.

In fact, because pipefish fathers are known to absorb nutrients from aborted embryos (essentially cannibalizing their young), they should be able to reallocate energy from abandoned pregnancies toward future ones, Dr. Monteiro said.

This fish-eat-fish explanation is a temptingly Machiavellian one, he added, but exactly how the “woman in red” effect occurs remains unknown.

For instance, are male pipefish the ones deciding to hold out for a more appealing mate? Or are dominant females short-circuiting male pregnancies through some type of cue?

For now, Dr. Beehner said, “This study offers insight into the dark side of paternal care in pipefish and sea horses that probably won’t find its way into many children’s books.”

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