Lurking among the underwater plants in Australia’s ponds and streams is a fish called the mouth almighty. The species is named for its impressive jaws, which grip transient prey. But the males also use their almighty mouths to gently carry hundreds of babies.
The fathers do this oral care, called mouth brooding, for two or three weeks at a time. Like other mouthbrooding fish, they do this at great personal cost. But according to a study published Wednesday in the journal Biology Letters, empowered fathers sometimes carry babies that aren’t theirs.
“If it’s true, it’s actually pretty neat,” said Tony Wilson, an evolutionary biologist at Brooklyn College who studies reproduction in fish and was not involved in the study.
The study’s lead author, Janine Abecia, is a Ph.D. candidate at Charles Darwin University in Australia’s Northern Territory, where she has studied the mouth almighty or Glossamia aprion, as well as the blue catfish Neoarius graeffei. Both live in Australia’s freshwater environments. Fathers of both species scoop fertilized eggs into their mouths and carry them until after the young have hatched.
Her research has suggested that these two species don’t eat at all when on father duty: “I opened the stomachs of mouthbrooders and they were empty,” said Ms. Abecia.
Research on other types of mouthbrooders — who can be fathers or mothers, depending on the species — have shown that they don’t eat either. Having a mouth full of offspring can also make it difficult to breathe. And it seems to slow down the parent, potentially making it harder to escape predators, Ms Abecia said.
Given the cost, it makes evolutionary sense that fish parents should only care for babies they know are their own. Yet scientists don’t know how often this is true. “It’s actually a question I’ve been interested in for a long time,” said Dr. Wilson.
Mrs. Abecia collected mouthbrooding fathers of both almighty and blue catfish from rivers in the Northern Territory. She collected additional adult fish, with no fry in their mouths, for genetic comparison. Then she selected about 10 eggs or babies from each father’s mouth and analyzed their DNA to find out where they came from.
With the blue catfish everything was as expected. All nine fathers seemed to be carrying their own young, and those baby fish all had the same mother.
Within the powerful jaws of the almighty mouth, however, things were a little weird. The all-powerful species apparently mate in the lab, Ms Abecia said. But of the 15 groups of youngsters she studied from the wild, four didn’t quite fit this story.
Two groups of pups had multiple mothers, suggesting that the male courted a female while he already had eggs in his mouth. One batch had multiple fathers, perhaps because another male had secretly fertilized some eggs before the brooding father fertilized them and gulped them down. And in one batch, the fry were totally unrelated to the fish she was carrying.
“It’s a very small study,” said Dr. Wilson, so it would be “premature” to draw conclusions about how common these duped fathers are. He noted that while the blue catfish in this study appeared monogamous, there may have been a handkerchief that the researchers’ sample didn’t catch. “Personally, I’d like to see more data,” he said.
But, he added, the genetic techniques used in this study make it easier for scientists to ask these kinds of questions about the private lives of seemingly monogamous animals. “Stories like this are probably just the beginning of understanding the complexities that exist in nature,” he said.
Scientists have already discovered other mouth-brooding fish carrying the wrong babies. In one type of cardinal fish, about 8 percent of the broods included the young of a second father. A study of fish called silver arowanas found that out of 14 breeding fathers, two had mouths full of offspring that were completely unrelated.
For their efforts, these fathers will not pass on any of their genes. Why hasn’t evolution made them more cautious?
One possibility is that a mouth full of baby fish makes them look sexy.
“Some female fish of other species are attracted to males who are already caring for their young,” Ms Abecia said. Men who get stuck with the wrong babies now can make up for it later; perhaps more females will happily fill those males’ scaly jaws with eggs.
“It goes to show that it’s not just the females who go to great lengths to care for their offspring,” Ms Abecia said. “In a way it’s inspiring.”