A place in the sun: interspecific interference affects thermoregulation in coexisting lizards

Solar radiation is in theory an unlimited resource on Earth, but can be locally limited. Heliothermic organisms use solar radiation to elevate their body temperatures, leading them to compete for sun-exposed areas, where interference for limited resource can occur. In coexisting lizard species, interference for basking sites could promote and direct interspecific interactions, which can be related to broader geographic species distribution patterns. We compared two competing lizard species (Podarcis muralis and Iberolacerta horvathi) that occupy sun-exposed gaps in a forested landscape in the Northern Dinaric Mountains (southern Europe). Both species exhibit an altitudinal segregation pattern with a high zone of overlap at middle altitudes. Using experimental thermal gradients, we tested for existence of interference competition. We used adult male lizards in three social contexts, alone as a measure of preferred body temperature (T (p)) and in conspecific or heterospecific pairs as a measure of realized body temperature (T (r)). I. horvathi achieved lower T (r) compared to its T (p) (alone), while T (r) of P. muralis did not vary between social contexts. The evidence for interference found in I. horvathi could not be explained by a change in their general movement pattern in a thermal gradient. Our results suggest that interference was a result of direct agonistic interactions, assessed by the frequency of approaches and touches. I. horvathi showed higher interference-susceptibility than P. muralis indicating an asymmetric interaction between the two species. Sunlight (as a resource) is an important factor influencing distribution patterns in assemblages of heliothermic ectotherms where sun-exposed basking sites are limited.