Inferring the mood of a fish has never been a precise science, but researchers claim at least one species responds the way humans do when separated from their other half. They say central American convict cichlids become glum when they split up.

Scientists at the University of Burgundy in Dijon believe they are the first to reveal the emotional attachments that fish form with their partners. Using a cognitive test, they found that female cichlids who lost their mates took a gloomier view of the world and were measurably more pessimistic.

“Emotional attachment to a partner is not a unique property of our species, since it has also evolved in at least one other species,” said François-Xavier Dechaume-Moncharmont, a senior author of the study. “It could indicate that these emotional biases are more than biases. Love is maybe not so irrational.”

Before assessing the emotional potential of the fish, the researchers investigated whether females were choosy about their sexual partners. The tests took place in a tank divided by clear partitions into three compartments. One by one, females were put in the middle compartment and single males were put in each of the side compartments.

Most of the females expressed a preference by cosying up, as much as the dividing walls allowed, to one or other of their male neighbours. The choice seemed to affect their fertility. When females were partnered with their chosen male, they spawned faster, spent more time attending to their eggs and had more fry than when they were partnered up with the rejected male.

For the next part of the study, female cichlids were trained to collect food from small containers placed in the fish tank. The containers either had black lids and were empty or had white lids and had food inside. The fish flipped the lids off by either sucking or pushing them and gradually learned which ones bore food.

Chloé Laubu, a behavioural ecologist and the first author of the study, then confused the fish by adding containers with grey lids to the tank. The scientists reasoned that, faced with ambiguity, optimistic fish would flip the grey lids off in the expectation of finding a treat, while pessimistic fish would hesitate or perhaps not even bother. “A good analogy would be the way you perceive the glass either half-full or half-empty according to your mood,” said Laubu.

Writing in Proceedings of the Royal Society B, the scientists describe how females flipped the grey lids off faster when their chosen mate was around than when the rejected male was on the scene. The findings suggest that being with the wrong male leads to a “pessimistic bias” in female members of the finny tribe. In other words, lovelorn fish have a gloomier outlook.

The scientists write: “Even if human relationships are particularly complex and refined, there is no reason to deny a priori the existence of emotional attachment to a partner in non-human species.”