Humans aren't the only species whose mental state is affected when they lose their lover. Female cichlids, a type of monogamous fish that primarily dwells in South America, become depressed and lovesick when their mate is removed and they're placed with a non-preferred male partner, a new study has found. Researchers came to this conclusion after the female fish took longer to investigate boxes that either contained food or were empty, demonstrating symptoms of apathy. Female cichlids, a type of fish that primarily dwells in South America, become depressed and lovesick when their mate is removed and they're placed with a non-preferred male partner In what's believed to be a first-of-its-kind study, researchers say they've determined fish can form attachments to sexual partners. Through a series of cognitive tests, they found that female fish were more likely to take on a'glass half-full' mental state when they remained with their chosen partners.

Despite what you might think, evolution rarely happens because something is good for a species. Instead, natural selection favours genetic variants that are good for the individuals that possess them. This leads to a much more complicated and messy world, with different selective forces pushing in many directions, even within a single species. A team of Swedish and Australian researchers led by Séverine Buechel from Stockholm Universitywondered if, like predator-prey conflict, sexual conflict might also affect the evolution of brain size. The team bred mosquitofish in a lab - both male and female.