Studies on the slow worm (Anguis fragilis) in the Iberian Peninsula have primarily focused on northern and north-western populations, with limited data, based mainly on distribution short notes, on the north-eastern populations in Spain. This study examines the ecology, demography, and phenology of a slow worm population located at a suboptimal site near the southern edge of its distribution range on the Iberian Peninsula, characterized by a Mediterranean climate, in southwestern Europe. The study area encompasses 2.9 hectares in Tarragona Province, Spain, at an altitude of 990 m. Sixteen sampling stations were established to cover all available habitats. In this population, adults comprised 73.73% (n = 87) of individuals, with a sex ratio (♂♂/♂♂+♀♀) of 0.44. Of the captured individuals, 46.61% (n = 55) exhibited non-intact tails. The estimated apparent survival probabilities during the study period (1.5 years) were 0.76 (0.54, 0.94) for immatures, 0.79 (0.29, 0.95) for males, and 0.83 (0.68, 0.96) for females. Population density was estimated at 16.11 ind./ha (7.78, 45.42) for immatures, 44.39 ind./ha (30.38, 72.57) for males, and 64.08 ind./ha (35.35, 129.93) for females. The activity period extends from March to October, with males emerging from hibernation earlier than females, peaking in May. Female emergence occurs one month later, peaking in June. A total of 41.18% of captured males had recent scars and wounds, indicative of fighting during the mating season from mid-April to mid-June. Pregnant females were observed from mid-May to early September, with a peak incidence in June, which is an extended period compared to other European populations at similar altitudes and latitudes.
