A clown fish uses his fins to fan water across a glistening mass of eggs, keeping them aerated. A silver arowana scoops up his fertilized eggs with his mouth and holds them gently for two months, until a host of miniature adults swims free from his jaws. A seahorse drifts through coral, his belly pouch swollen with unborn young.
Most fish are uninvolved parents. They dump their eggs and sperm, then swim off and let nature take its course. But some species of fish take their parental duties more seriously — and among them, the majority of caring parents are dads. Care from mothers, or from both parents at once, is much less common. In a study published last fall in Evolution, researchers found evidence that paternal care, the system in which dads are the sole caretakers, has evolved dozens of times in fish.
These fish aren’t exactly helicopter dads. Their most common parenting style is simply guarding eggs after they’re fertilized. “Some people are surprised this is considered care,” said Frieda Benun Sutton, an evolutionary biologist at the City University of New York.
But it does count. To learn more about why this type of care in fish usually comes from dads, Dr. Benun Sutton and her co-author, Anthony Wilson, of Brooklyn College, took a deep dive into the family history of fish parents. They started with an evolutionary tree, built by other researchers in 2017 using genetic data, that shows how almost 2,000 fish species are related. Then they mapped onto the tree all the information they could find about parental care in those species: Were young cared for by fathers, mothers, both or nobody? They also added other factors including the size and number of each fish’s eggs and how they’re fertilized.
The completed tree showed that care by fathers is no evolutionary accident: It has arisen at least 30 separate times. Hundreds of the species in this sample have absent mothers and caring fathers. But why?
“We do see consistent patterns that occur across the entire evolutionary tree,” Dr. Benun Sutton said.
One important pattern is that every species with paternal care also uses external fertilization — sperm and egg meeting out in the open — and spawns in pairs, not groups. This makes sense, the authors say, because when fertilization happens in a group or inside a female’s body, a male can’t be sure of his paternity. Caring for those young could be a waste of his energy, evolutionarily speaking. But if he fertilizes a fresh batch of eggs and then stands guard over them, he believes he’s caring for fish that share his genes.
The link between external fertilization and doting dads is a hypothesis several decades old. But more recent research has shown that external fertilization doesn’t ensure fish dads end up caring for their own young, said Sigal Balshine, a behavioral ecologist at McMaster University in Ontario. Other males may sneak some of their own sperm onto the eggs.
In a 2012 study of plainfin midshipman fish, Dr. Balshine and her co-authors gave paternity tests to babies and found that only about half of the young in a nest belonged to the dad who was guarding it. “There was a lot more hanky-panky going on,” Dr. Balshine said.
The new study is the most comprehensive look yet at how parenting has evolved in fish, Dr. Balshine said. But it looked at only a small fraction of the world’s more than 30,000 fish species, and may not be a representative sample.
Dr. Benun Sutton agrees that the link between paternal care and external fertilization is an older, “perhaps dusty” hypothesis that her work revives. But if caring fish fathers were being cuckolded too often, she says, they would pass on their genes so sparsely that their behavior would die out altogether.
The puzzle of fish dads has long fascinated biologists, Dr. Balshine said.
“When we humans think about parental care, we think immediately of moms holding newborn babies,” she said. It’s true that females are the primary caregivers across much of the animal kingdom, from mammals to birds to reptiles. Fish provide an intriguing counterexample.
Dr. Benun Sutton said fish illustrate that, under the right circumstances, either parent’s care can be critical. There’s no evolutionary rule that moms have to be the main providers — a lesson we can apply to our own species, she said. “There is a very important role for dads.”