...at least for guppies. Dr. Alexander Kotrschal and colleagues at Uppsala University (Sweden) either shrank or grew the brains of guppies over multiple generations to create animals with up to 8-10% variations in brain size. To test for "smartness" they had the fish count by training the animals to look for food where a card with two or four symbols was shown. According to the researchers, the animals with larger brains tended to learn the task, whereas those with smaller brains did not. The downside to being a bigger-brained guppy was fewer offspring and a smaller gut size. Dr. Kotrschal suggested that larger brains in primates may also be associated with fewer offspring compared to other mammals.
Other recent research from Dr. Maklakov from Uppsala University (Sweden) shows that urban songbirds tended to have larger brains (relative to body size) compared to their rural counterparts. The author of the study speculated that urban life requires bigger brains to handle its many challenges.