How do water skinks avoid shelters already occupied by other lizards?

Animals rely on a wide range of sensory modalities to obtain information about their environment, and to determine the most appropriate behavioural response for a give situation. Scincid lizards use both chemosensory and visual cues in tasks such as mate location, shelter-site selection, and predator evasion. Published literature has focused primarily on the use of chemosensory information by these lizards, but our experiments reveal an alternative scenario. Water skinks (Eulamprus heatwolei) are sympatric with larger black rock skinks (Egernia saxatilis) in southeastern Australia, and use similar long-term retreat-sites. However, Eulamprus avoid shelter-sites occupied by Egernia because of the risk of attack and injury. We conducted laboratory trials to clarify the kinds of information used by Eulamprus to avoid already-occupied shelters. In the field, shelter-sites might provide three types of information about prior occupancy: either scent deposited by a previously present Egernia; evidence of the current presence of an Egernia, perhaps involving visual, chemical, and/or auditory information; or a direct behavioural interaction with a resident lizard (potentially involving an attack). In our trials, Eulamprus based their shelter-site avoidance only on direct interactions. Thus, Eulamprus reacted strongly to Egernia with which they were able to interact physically, but not to either Egernia scent deposits, or a (caged) Egernia with which they could not physically interact. The behaviour of the resident was critical, in that Eulamprus fled from the crevice in response to bites, movements, or open-mouth displays by the resident Egernia. We suggest that Eulamprus base their shelter-site choices on the presence of an actual resident animal rather than more indirect cues (such as scent), because only a direct encounter provides reliable information about the magnitude of threat associated with crevice occupancy.