Doors to the Homes: Signal Potential of Red Coloration of Claws in Social Hermit Crabs

Abstract

Red colouration on a signaller's body may be an informative signal in many animals. For species that inhabit architecture (e.g., burrows, nests, or other structures), certain parts of the body are more exposed than others, potentially serving as superior platforms for signalling via colouration. Yet if animals differentially advertise red colouration on body parts that are more versus less exposed from their architecture remains to be tested. Here we systematically quantified red coloration in social hermit crabs (Coenobita compressus). These crabs inhabit architecturally remodelled shells and have claws that visibly block the shell entrance, like doors to their home. We hypothesised that red colouration of claws may be a signal of resource-holding potential (RHP). Consistent with this RHP signalling hypothesis, we found that within the same individuals’ bodies, exposed claws showed significantly greater red colouration than unexposed carapaces. Furthermore, larger body size predicted greater red colouration of claws. Competing hypotheses (e.g., interspecific signalling, camouflage, and UV protection), while not explicitly tested, nevertheless appear unlikely based on natural history. Red claw coloration may therefore function as a signal to conspecifics, and experiments are now needed to test recipient responses. Broadly, relative to surrounding architecture, exposed body surfaces offer rich potential as signalling platforms for colouration.