Evaluating guppy, Poecilia reticulata, as a model for sex differences in cognition.

Whether cognitive differences exist between men and women is a question probably as old as experimental psychology though still highly controversial. Despite the large number investigations, sex differences in performance emerge only in a few cases, such as in visuo-spatial or verbal abilities. However, it is not clear if these differences are biological or due to cultural influences, possibly because we know very little about their underlying mechanisms and evolutionary causes. Sex differences in cognition have been investigated also in few other species, such as monkeys, rodents and chickens. Rodents, in particular, represent a powerful model in the field of spatial abilities, allowing, among the others, complex hormonal manipulations, drugs tests and between-species comparative studies. Recent years have seen a massive increase in the use of fish as models for cognitive research, at the point that fish are predicted to replace classical vertebrates models soon. The existence of cognitive sex differences in fish has not been investigated. Paradoxically, experimenters quite often use indiscriminately fish of both sexes in their experiments, introducing a confound if cognitive sex differences exist. In this thesis, I studied cognitive sex differences in the guppy, Poecilia reticulata, one of the most studied species in behavioural and cognitive research. Male and female guppies are characterized by considerable differences in ecology and behaviour. I compared the performance of males and females in a wide range of cognitive tasks, including visual discrimination learning, novel object recognition, rule learning, reversal learning, spatial navigation, spatial learning and quantity discrimination. Male and female guppies showed similar abilities in solving most of the tasks, with only two exceptions. In a reversal learning task, guppies were initially trained to obtain food by choosing a predetermined colour between two options, a task that the two sexes learned equally well; then, the reward contingency was reversed and subjects had to inhibit the learned preference to select the other stimulus. Females quickly started to choose the new rewarded colour while males persisted longer in selecting the previously rewarded colour. The second difference emerged in a quantity discrimination task. Guppies were required to select the larger between two groups of conspecifics, following a natural tendency of social fish to stay in larger shoals to reduce predation risks. At the beginning of the experiment, females were much better than males at solving the task; this difference vanished after some minutes. The two observed sex differences are possibly the result of differential action of natural selection on the two sexes. Indeed, male guppies are thought to undergo selection for high persistence that helps in courting females intensively; females are likely to undergo strong selection for predator avoidance skills as predators preferentially target females. The absence of sex differences in the other cognitive abilities can be due to the absence of sex differences in selective pressures, or, alternatively, to the existence of constraints that prevent differentiation. In many of the tasks in which males and females obtained similar scores, they were nonetheless observed to behave quite differently. In visual discrimination tasks and in a spatial learning task, males were faster than females in deciding which option to choose, suggesting greater impulsivity in males. In the novel object recognition task, males and females expressed equal ability in discriminating the two objects, yet males explored the novel object at the beginning of the test, females some time afterwards. In the spatial navigation task, males persisted longer than females in trying to reach a target behind a transparent barrier. The general lack of sex differences in cognitive abilities in guppies aligns with the scarce evidence of sex differences in cognitive abilities in mammals. However, I provided evidence that minor differences in performance, such as in persistence, impulsivity and exploratory tendency, typically found in mammals and birds are widespread in fish as well