Individual recognition in guppies does not require large brains

Abstract It is often assumed that group-living animals require larger brains in order to deal with the various social challenges they encounter. One such key challenge is the need to recognize and discriminate between specific group members. Individual recognition is often deemed the most cognitively demanding form of recognition. Hence, one could expect this ability to be facilitated by the evolution of larger brains. So far, this hypothesis remains largely untested. In this study, we investigated the link between relative brain size and individual recognition, using Trinidadian guppies ( Poecilia reticulata ) from artificial selection lines for either increased or decreased relative brain size. In a first experiment, we compared the selection lines in their ability to spontaneously discriminate between a familiar and unfamiliar conspecific. In a second experiment, we actively trained guppies from the selection lines to associate a particular individual with the presence of food. Overall, we found evidence for individual recognition, confirming earlier research on this species. However, individual recognition was independent of brain size selection regime in both experiments. Guppies spontaneously recognized and preferably associated with a familiar individual. In the trained association experiment, however, fish showed no preference for either stimulus fish. Our study suggests that although small fishes like the guppy are capable of individual recognition, larger brains do not necessarily facilitate this ability. Our study demonstrates that to fully understand the link between sociality and cognition, one needs to verify which exact social challenges require the evolution of larger brains. Significance statement Living in a group is a complex challenge, and is thus said to require relatively large brains. Despite this assumption, there is very little known about which particular aspects of group-living are actually cognitively demanding to a degree that they require a higher investment in brain tissue. Here, we tested the specific hypothesis that the ability to recognize and remember specific individuals, i.e. individual recognition, a keystone of sociality, is cognitively challenging by comparing guppies with known differences in relative brain size in their ability to recognize a familiar shoal-member and learn the difference between two new individuals. Although guppies demonstrated individual recognition, relative brain size did not affect their performance. Our results provide valuable insights in the evolution of sociality and its link with relative brain size.