Male Octopus Avoid Using Hectocotylized Arm Under Situations With Unpredictable Risks

Anti‐predator behaviours are generally considered as those that prevent death from predation. However, even if prey animals survive, they might suffer sub‐lethal injuries. Although it is predicted that prey would minimise injury by protecting vital body parts, such defensive strategies have not been widely recognised in previous studies. While cephalopods frequently lose arms, males rarely lose their hectocotylus, an essential arm for reproduction. This is considered to be a result of the male's protection of the hectocotylus, but it has not been verified. This study investigated arm loss in Octopus parvus in the wild and tested whether males protect their hectocotylus (right third arm, R3) by avoiding its use during tasks with unpredictable truncation risk. Arm loss was observed in 78% of wild‐caught individuals, and males lost R3 arm significantly less often than females. In Experiment 1, males did not use their hectocotylus at all to physically explore a novel object (fishing sinker), while females used R3 arm just as frequent as her other arms. In Experiment 2, the number of individuals explored prey in a hole using R3 arm did not differed significantly between sexes, but males explored the hole with arms other than R3 arm longer than females before inserting R3 arm. These results suggest that O. parvus males avoid risk more for the hectocotylus than for the other arms, i.e., hectocotylus arm protection. Males did not always protect their hectocotylus, and they sometimes inserted their hectocotylus after exploring the hole with other arms in Experiment 2. Males might suffer behavioural disadvantages by persistently guarding their hectocotylus.