Cooperate or compete? Influence of sex and body size on sheltering behaviour in the wall lizard, Podarcis muralis

Reptile sheltering behaviour, despite profound life history ramifications, remains poorly investigated. Whether or not individuals share a suitable shelter or, conversely, exclude conspecifics may depend on associated costs (resource partitioning, sexual harassment, disease or parasite contamination) and benefits (predation risk dilution, thermal resilience, information sharing). We performed two experiments on field caught wall lizards (Podarcis muralis), a highly territorial species, to investigate the relative roles of sex and body size in night sheltering. In the first experiment, random pairs of lizards were offered two identical shelters. Lizards either shared a shelter, or sheltered separately. In the second experiment, different random pairs of lizards were offered only one shelter so as to elicit a share or compete response. Body size and sex both appeared as significant drivers for sheltering patterns. Unexpectedly, wall lizards often chose to share shelters. When only one shelter (too small to accommodate two adult lizards) was available, many lizards rejected the sheltering option in preference for aggregation. Such aggregative behaviour was not sex dependant, and may reflect thermoregulatory or anti-predatory benefits. Our results nevertheless suggest that cooperative behaviour may exist in wall lizards.