Better the devil you know: familiarity affects foraging activity of red-backed salamanders, Plethodon cinereus

Many animals can recognize previously encountered individuals and modify their behaviour towards them. Individuals also can have increased growth or other fitness components when interacting with familiar compared to unfamiliar individuals. One hypothesis explaining fitness benefits is that animals forage less in the presence of unfamiliar conspecifics because they spend time avoiding or interacting with these potentially more aggressive individuals. We tested this hypothesis in a territorial salamander, Plethodon cinereus, which is active on the forest floor at night. In our first experiment, we tested whether nocturnal activity was a suitable quantitative reflection of foraging. Level of nocturnal activity had a significant positive effect on mass gain and a significant negative effect on prey abundance in enclosures. Adult P. cinereus are known to be less aggressive towards familiar juveniles, and juveniles grow more when cohabitating with familiar adults, so we tested whether a lack of familiarity between juveniles and adults decreases nocturnal foraging activity of juveniles or adults. Juveniles were less active in enclosures with unfamiliar females than with familiar females with whom they had cohabitated in the laboratory. There were no differences in juvenile foraging activity based on familiarity with males. Surprisingly, adult salamanders were more active in enclosures with familiar juveniles than in enclosures with unfamiliar juveniles, even though juveniles are much smaller and do not pose physical danger towards adults. We conclude that adults and juveniles forage more when they interact with familiar conspecifics and we discuss hypotheses that may explain why familiarity affects interactions in these contexts.

► We tested effects of familiarity on foraging activity of Plethodon cinereus in enclosures.
► Level of nocturnal activity reflected foraging activity of salamanders.
► Foraging activity of juveniles was lower with unfamiliar adult females.
► Foraging activity of adults was lower with unfamiliar juveniles.