(CNN) Throughout history and across cultures, people have written songs and poems lamenting lost loves. Now, researchers at a French university say they have discovered that fish also pine for their partners, challenging the view that only humans form emotional attachments.
Researchers at the University of Burgundy performed a series of tests on convict cichlids -- a species of small, monogamous fish native to Central America -- to measure their mood and emotional attachment.
They allowed 33 female fish to choose a male mate and then monitored the behavior of the females when they were not paired with their preferred partner. They found that the females who were denied their first choice adopted a more "pessimistic" outlook on life.
The study, published Wednesday in The Proceedings of the Royal Society B journal, found that females separated from a preferred partner showed a "pessimistic bias" -- in other words, a bleaker outlook on life -- when presented with a challenge.
In a series of tests, researchers trained the fish to recognize and open boxes, distinguishable by their color and position in the tank: a "positive" box filled with a reward of food and an empty "negative" box. Biologists then presented the fish with a third "ambiguous" box and measured the reactions of the female fish before and after separation from their preferred partner.
Scientists found that the females who were paired with their preferred partner were quicker to investigate the ambiguous box, but if a female had been separated from her preferred mate, it affected her responsiveness to the task.
"It is a measure of their emotional state, their optimism or pessimism," said François-Xavier Dechaume-Moncharmont, a behavioral ecologist who was one of the authors of the study.
"If you are in an optimistic mood, you will see the glass half-full. If you are in a pessimistic mood, you will see it as half-empty. It is exactly the same for the fish. We offered them an ambiguous box, and their reaction to this corresponds to their emotional state."
Scientists also found that females paired with their preferred male "invested more in reproduction," spawned earlier and spent more time attending their eggs.
The study, researchers believe, may help us to understand love and emotional attachment better.
"Many psychologists decided that only humans can feel some emotion to their partner, but with this study, we have shown that is not correct. This will question very deeply the adaptive value of such behavior," Dechaume-Moncharmont said.
"Many people consider love as a pollution to our decisions; when you are in love, you may behave a little strangely. But maybe this emotional response has some adaptive value. If it has evolved independently in many species, that could prove that maybe it has some adaptive value."