Sex-switching fish mates for life

For tiny hermaphrodite fish found in coral reefs off Panama, a lifelong monogamous relationship comes with a bit of give and take.

The pair switch reproductive roles at least 20 times a day.

The strategy allows individual fish to fertilize about as many eggs as it produces, giving the fish a reproductive edge.

“Our study indicates that animals in long-term partnerships are paying attention to whether their partner is contributing to the relationship fairly—something many humans may identify with from their own long-term relationships,” says Mary Hart, adjunct professor of biology at the University of Florida.

The duo motivate one another to contribute eggs to the relationship. If one partner lacks eggs, the other will simply match whatever it produces. The only way for a partner to convince its mate to produce more eggs, is to pick up the slack and generate more itself, she says.

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Scientists observed the short-lived chalk bass, Serranus tortugarum, for six months—and were surprised that every couple stayed together for the duration.

With only 3 to 5 percent of animals known to live monogamously, this is a rare find—and one of the first for a fish living in a high-density social group, says coauthor Andrew Kratter, an ornithologist with the Florida Museum of Natural History.

“I found it fascinating that fish with a rather unconventional reproductive strategy would end up being the ones who have these long-lasting relationships,” he says. “They live in large social groups with plenty of opportunities to change partners, so you wouldn’t necessarily expect this level of partner fidelity.”

Published in the journal of Behavioral Ecology, the new research lays the groundwork for studies that investigate mechanisms that govern partnerships in the wild.

An occasional fling

Scientists have long studied cooperative behavior in animals, like primates that groom each other or vampire bats that regurgitate food for relatives in need of a blood meal. But it has remained a point of debate among scientists whether or not these animals are paying attention to the amount of resources being exchanged. For the chalk bass, matching reproductive chores helps partners succeed, even when there are opportunities to mate with other fish, Hart says.

“We initially expected individuals with partners that were producing less eggs would be more likely to switch partners over time—trading up, so to speak. Instead we found that partners matched egg production and remained in primary partnerships for the long term.”

For their entire adult lives, the fish mating partners come together for two hours each day before dusk in their refuge area, or spawning territory. They chase away other fish and begin with a half-hour foreplay ritual of nipping and hovering around each other, an activity that helps strengthen the partners’ bond. Eventually it becomes apparent which fish is going to take on the female role for the first of many spawning rounds.

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Finding a new mate every evening is time-consuming and risky for a fish that only lives for about a year. Having a safe partner may help ensure that individuals get to fertilize a similar number of eggs as they produce, rather than risk ending up with a partner with fewer eggs.

But all of this doesn’t mean the chalk bass is completely opposed to an occasional fling.

If one partner has more eggs than the other, it may share the extra with other couples, an option that, while infrequent,can add stability to the system of simultaneous hermaphroditism paired with monogamy.

Scientists are only beginning to understand how mutually beneficial relationships among animals are maintained, much as humans in general still strive to determine what makes long-term relationships last.

“Not even one of the original pairs that I observed switched mates while its partner was still alive,” Hart says. “That strong matching between partners and the investment into the partnership was surprising.”

Source: University of Florida

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