Resource assessment in hermit crabs: the worth of their own shell.

Animals gather information about the quality of a resource through its assessment and behave accordingly as a result of adaptive motivational changes. In the hermit crab Pagurus longicarpus, we investigated whether an individual was affected in its motivation to acquire a new shell by the quality of both the domicile shell (own resource value, ORV) and the offered shell (external resource value, ERV), and asked whether its motivation was altered by the information gathered during shell investigation. We analyzed the behavior of hermit crabs inhabiting shells of differing qualities and compared their willingness to acquire an offered shell – either optimal, or smaller than optimal, or larger than optimal – by measuring the latency to approach it, the number of bouts of shell investigation, and its total duration. Crabs occupying small shells were more quick to approach the offered shell, while crabs in larger than optimal shells were found to investigate the offered shell more thoroughly. The readiness of crabs to approach the offered shell and the extent of its investigation were independent of the quality of the external resource but were exclusively affected by the ORV, while the number and duration of shell investigation did not change with time as investigation proceeded, except for crabs in poor quality shells. These results suggest that P. longicarpus’ motivation to acquire a new shell is exclusively influenced by the value of the shell it inhabits rather than by the quality of the shell it is offered with, and that this species does not gather – or does not use – information about ERV during investigation.