Effects of methylphenidate on responses to novelty in a teleost fish (Poecilia reticulata).

Novelty seeking, the willingness to explore novel stimuli, can have important fitness consequences. The neurotransmitter dopamine has been linked to this behavior in studies on lab animals including rodents and fish; however, few studies have investigated this association in individuals from natural populations. Trinidadian guppies (Poecilia reticulata) show variation in willingness to explore novel objects and environments, and females tend to show a preference for novel males. In this study, we asked whether we could enhance interest in several types of novel stimuli in lab-reared, female Trinidadian guppies by administering methylphenidate hydrochloride, a stimulant known to increase dopamine levels. We scored their responses to three different types of novelty: novel environments, objects, and males. Treated females showed enhanced exploratory behavior: they traversed relatively more inner squares of the novel environment (open-field test); they spent more time inspecting a novel object; and they showed greater interest in the second male guppy to which they were exposed than control fish. We also found a positive association between our metrics of exploration in the open field and novel object tests. Our other assays suggest that these differences were not the result of increased activity or reduced levels of stress. Therefore, our results suggest that dopamine plays a role in the responsiveness of guppies to novelty; this opens the door to studies of behavioral mechanisms in natural populations.