Organisms face numerous environmental stressors, which can affect developmental precision. Developmental instability can be used as an efficient and reliable bioindicator of environmental stressors and individual health. However, little information is available about effects of interspecific interactions on occurrence of developmental instability. We analyzed fluctuating asymmetry in two sympatric lizard species (Iberolacerta horvathi and Podarcis muralis), which exhibit a competitive interaction, to determine potential effects of altitude, interspecific competition and urbanization on a set of physical characteristics. We sampled 16 syntopic and allotopic populations and used methods of geometric morphometrics to analyze head morphology. We detected significant effect of altitude, but the trend was not linear: lizards had most asymmetrical heads at mid altitudes, which could be related to lower availability of favorable lizard habitats (open areas) at this altitude and therefore higher competition. We observed that lizards from allotopic populations attained larger head sizes compared to lizards in syntopy that experienced interspecific competition. This could suggest that in sites where competing species is absent, individuals can invest more energy in the development of the body. Moreover, head shape of Iberolacerta horvathi was more asymmetric compared to Podarcis muralis, which could indicate difference between species in the vulnerability to stress effects. We did not detect any statistically significant effect of urbanization, which could relate to relatively low degree of pollution and habitat degradation in our study area. Our results highlight the possibility of using lizards and developmental instability for bioindication of environmental stressors.

The tps files contain 2-D digitized landmarks of the dorsal view of the head. 28 landmarks were taken on each specimen. Landmark recording was repeated twice for each animal.

"Specimen" label in classifier file corresponds to ID label in the landmark-data file, "species" indicates which species it is (IHOR=Iberolacerta horvathi, PMUR=Podarcis muralisI, "sex" (M or F) indicates animal gender, "svl" indicates snout-vent length, "pop" indicates the population type (allotopic/syntopic). Landmark recording was repeated twice for each animal and is indicated by "image" (1 or 2). "Individual" is the consequent number of the specimen, "type" represents type of environment (urban/natural), under "altitude" are altitude meters, under "kategorija" are altitude categories (nizka=low, srednje_nizka=medium-low, srednje_visoka=medium-high, visoka=high.

The excel file contains supracilliar granules counts, which were counted twice, each time in a different order with several days of rest between the first and second counting, by the same person.