"Pregnant" Fish Fathers Abort Babies of Unsexy Females

When it comes to mating, pipefish males are always waiting for something better—and bigger—to come along, a new study shows.

That's because "pregnant" pipefish fathers will kill off embryos conceived by an undesirable female to make room for the offspring of a potentially more attractive female. (See a picture of a colorful Pacific pipefish.)

Scientists already knew that when paired up with a small—and thus unattractive—mate, a male pipefish will deliberately abandon some or all of its brood, absorbing nutrients from the doomed embryos.

It was previously thought that malnourished males needed the nutrients from the developing embryos. But new research on Gulf pipefish suggests the pregnant pipefish abandon embryos in hopes of finding sexier mates.

"Males are smart about where they devote their energies," said study co-author Kimberly Paczolt, an evolutionary biologist from Texas A&M University.

Pipefish Mystery: Why Is Bigger Better?

Like their seahorse cousins, pipefish exhibit the peculiar habit of male pregnancy. The female deposits the eggs into the male's brood pouch during mating. The male then nourishes the developing young. (See more pictures of doting animal dads.)

In their experiments, Paczolt's team mated males with several females and then observed how many embryos the males aborted. The team discovered that the males retained and nurtured a much higher percentage of embryos from larger females.

Scientists don't know why male pipefish are drawn to bigger females. But the species may follow a general trend among other fish, in which bigger females produce bigger, higher-quality eggs.

Whatever the reason, "the pipefish brood pouch is turning out to be more complicated than we thought," Paczolt added.

The research is detailed in this week's issue of the journal Nature.