The tiny Central American convict cichlid may only be a few centimeters long but it's a fish with a big heart.  

Researchers in France found that the tropical fish, known to be a loyal and monogamous partner, suffers from heartache when separated from its lover.

Even more heart-rending perhaps is that female cichlids, when paired with a male who is not their preferred partner, were shown in experiments to exhibit "pessimistic" behavior.

"Positive" box with fish food and "negative" empty box had either black or white lids for the fishes to distinguish and lift. A gray "ambiguous" was then placed in the center of tanks. 

Researchers reasoned that an optimistic fish would flip open the gray lid, whereas a pessimistic one might hesitate or leave the box alone altogether.  

Studies show female fish spend much more time trying to lift off the gray lids when their favored males were in the tanks with them. 

AFP