To escape or to pursue: opposite decision making concerning a single moving object is influenced by starvation and sex.

Decision-making processes in the context of prey-predator interactions are studied from the side of the prey or the predator. Thus, prey capture and escape behaviours are researched separately, using different stimuli in different species. The crab Neohelice preys upon individuals of its own species; hence, it behaves as prey and as predator. These two innate opposite behaviours can be elicited by the same object moving on the ground. Here, we studied how the decision to perform avoidance, predatory or freezing responses to a moving dummy depends on sex and starvation level. In the first experiment, we assessed the probability of each response type in unfed crabs for 22 days. Males showed a higher predatory response probability than females. When starvation increased, the predatory response increased, while avoidance and freezing declined, but this only occurred in males. In the second experiment, we compared regularly fed and unfed males for 17 days. While fed crabs did not change their behaviour throughout the experiment, unfed crabs significantly intensified their predatory responses, displayed different exploratory activities and pursued earlier than fed crabs. Our results show the unusual situation of an animal that, to deal with a single stimulus, has to choose between opposite innate behaviours. This is a value-based decision as it is affected by factors other than the stimulus itself.

Visual determinants of prey chasing behavior in a mudflat crab.

The crab Neohelice granulata inhabits mudflats where it is preyed upon by gulls and, conversely, preys on smaller crabs. Therefore, on seeing moving stimuli, this crab can behave as prey or predator. The crab escape response to visual stimuli has been extensively investigated from the behavioral to the neuronal level. The predatory response (PR), however, has not yet been explored. Here, we show that this response can be reliably elicited and investigated in a laboratory arena. By using dummies of three different sizes moved on the ground at three different velocities over multiple trials, we identified important stimulation conditions that boost the occurrence of PR and its chances of ending in successful prey capture. PR probability was sustained during the first 10 trials of our experiments but then declined. PR was elicited with high probability by the medium size dummy, less effectively by the small dummy, and hardly brought about by the large dummy, which mostly elicited avoidance responses. A GLMM analysis indicated that the dummy size and the tracking line distance were two strong determinants for eliciting PR. The rate of successful captures, however, mainly depended on the dummy velocity. Our results suggest that crabs are capable of assessing the distance to the dummy and its absolute size. The PR characterized here, in connection with the substantial knowledge of the visual processing associated with the escape response, provides excellent opportunities for comparative analyses of the organization of two distinct visually guided behaviors in a single animal.