In Paracerceis sculpta, a Gulf of California isopod, α-males usually defend aggregations of breeding females within the spongocoels of intertidal sponges. Breeding isopods are also found in habitats other than sponges, but interactions among individuals in these locations are poorly known. To investigate plasticity in male-male interactions associated with mate acquisition in breeding habitats other than sponges, we examined the behavior of pairs of α-males in two distinct environments; those in which breeding sites were absent, but females were present, and those in which both breeding sites and females were absent. We established males as residents in the presence and in the absence of females by allowing them to occupy arenas for 1 h before intruders were introduced. Residents and intruders engaged in significantly more total acts and grappled more in the presence of females than when females were absent. In the presence of females, residents became active first in most cases, and gripped females more than intruders. Latency to activity was significantly longer when receptive females were present than when they were absent. However, residents seldom interacted with intruders when females were lacking. Our results suggest that in locations other than sponges, α-males compete for females and can recognize their status as residents and as intruders. These results also suggest that, as in many vertebrate and invertebrate species, P. sculpta α-males attempt to mate opportunistically, and that contests for mates in atypical breeding habitats may be common in nature. We discuss the importance of distinguishing rare and highly contingent behaviors from those likely to represent adaptive behavioral plasticity, and we propose a simple criterion of commonality for their identification in behavioral analyses.