Dear enemies and nasty neighbors in crayfish: Effects of social status and sex on responses to familiar and unfamiliar conspecifics

• Pairs of crayfish (Procambarus clarkii) fought to establish a dominance hierarchy.
• Crayfish later distinguished familiar from unfamiliar opponents of the same status.
• Subordinate crayfish preferred to be near unfamiliar conspecifics.
• Dominant females preferred to be near and fight familiar conspecifics.
• The behavior of dominant females is consistent with the nasty neighbor effect.

Our experiment examined the ability of crayfish (Procambarus clarkii) to discriminate between familiar and unfamiliar conspecifics of equivalent social status, and investigated whether this species displays dear enemy or nasty neighbor effects. Pairs of size and sex matched crayfish fought to establish social status and the resulting dominant and subordinate crayfish then participated in a choice phase in which they interacted with two conspecifics tethered in an arena. Both choice conspecifics had the same social status and sex, but one was familiar (the focal animal's previous opponent) and the other was novel. We found that subordinate focal animals of both sexes spent significantly more time in proximity to the unfamiliar choice animal, behavior inconsistent with the dear enemy and nasty neighbor hypotheses. In contrast, male and female dominant focals differed significantly: females spent more time close to and fighting with the familiar choice animal while male dominants responded equivalently to the two choice animals. Thus the response of crayfish toward familiar and unfamiliar conspecifics was complex and not explained by a single hypothesis. We suggest that, in addition to familiarity and unfamiliarity, the perceived threat-level of opponents influences the behavior of crayfish toward conspecifics.