Shelled shut-ins: a conditional escape task showing perceptual awareness in hermit crabs

Sentience is an increasingly popular topic in animal welfare and behavioural research. However, studying sentience can be difficult and contentious. Sentience has frequently been associated with ‘feelings’ and ‘subjective states’ not directly accessible to human observers; thus, investigating awareness in animals could be an alternative approach. Frameworks of awareness have been developed for the experimental examination of cognitive capacities that might underpin ‘feelings’ as affective states and subjective experiences. Using a framework of four hierarchically arranged levels of awareness (that is, perceptual, cognitive, assessment and executive), the awareness of an intertidal crustacean, the common hermit crab, Pagurus bernhardus , was assessed. Inspired by recent work on cognitive abilities and task solving in terrestrial hermit crabs, a series of four experiments was designed to test the ability of P. bernhardus to solve a simple conditional task: if the crab changes its shell, then it can escape confinement. Crabs were exposed to different levels of confinement, including complete confinement, confinement requiring shell changes for escape, confinement requiring no shell changes for escape and no confinement. Apart from testing crabs in the absence of an external motivation (experiment 1), crab responses were tested in the presence of food (experiment 2), an additional shell (experiment 3) and under hypoxic conditions (experiment 4) to ensure that individuals were motivated to escape. Crabs were more likely to escape confinement if they did not have to change shells. Sequence analysis of the behavioural patterns of crabs revealed that they are not tactically assessing different components of their confinement to aid in task solving. Collectively, these experiments indicated that P. bernhardus displays perceptual awareness without evidence of insight or forward planning.