Even in fish, one parent does more

Like bickering spouses in an underwater episode of “Dr. Phil,” parents in the fish world want their mates to take more responsibility for child-rearing – so they can do less, says CSUS biological sciences professor Ronald Coleman.

In convict cichlids, for example, Coleman finds that while parenting duties are shared, they’re not shared equally. “Both hope the other will do the work,” he says.
Convict cichlids are good parents who defend their children, Coleman says. But individual pairs vary in determining how much each parent should do for their offspring.

One of the factors influencing the decision is size. In his new lab Coleman matches varying sizes of partners, which affects the parenting choices. “If there is a small female and a large male, the female will do all the parental care. The male won’t do anything,” he says. “But if the female is larger, the male becomes intensely protective.”

Of course, in the fish world, the parenting decisions have potentially permanent consequences. “In parental situations, if they screw up, someone eats the kids,” Coleman says. “Natural selection doesn’t work on the species-it operates on the individuals.”

Coleman’s work has been published in several national journals. He pioneered the concept of “investment decision-making” in fish, which he continues to study.

“Fish make very careful decisions on costs and benefits,” he says.

A male fish, for instance, considers the value of what is in front of him-his children and a predator-relative to his expected future reproductive ability. He makes the decision based on what the children are worth in the grand scheme, not just the here and now.

“There’s no best move. It depends on what it’s worth to the partner,” he says. “If they value it more than you do, you can get them to do the work.

“The dark side is, maybe that’s why human parents do the things they do: why one parent may do more than the other, why one parent gets to go out and the other stays home, why one works and the other doesn’t.”

But, he is careful to point out, he doesn’t study human behavior.

Coleman’s parenting insight came into play in the lab’s first experiment, which confirmed a popular theory about fish coloration. “Many predators have a large spot on their tail,” Coleman says. “Most researchers thought it was an adaptation to make other fish avoid the eye. And that’s what we found.”

Coleman’s surprisingly simple method was to put a photo of a predator fish at the end of a transparent stick and make it “swim” in the tank of convict cichlid parents and their children. As expected, the parents try to protect the children. “When the ‘predator’ is in the tank, the guardian bites repeatedly at both ends, trying to get the eye,” he says.

In addition to his on-campus efforts, Coleman regularly goes to Costa Rica, Chiapas, Mexico and Nicaragua to study the fish in the wild and make sure he’s creating realistic scenarios in his lab experiments.