×may change their phenotype along the altitudinal gradient being adapted to the local conditions. Consequently, elevational gradients constitute a good model to examine geographic variation offering a natural experiment for carrying out research in evolutionary ecology. In this thesis, I carry out different approaches to study two lizard species along two elevational gradients in the Mediterranean region.On the one hand,I studythe Atlas day gecko Quedenfeldtia trachyblepharusin the High Atlas of Morocco. On the otherhand, I studythe large Psammodromus Psammodromus algirusin Sierra Nevadaof Spain. In the first chapter I study the Atlas day gecko trophic ecology.It is hypothesized that increased competition should reduce niche breadth. However, there are scarce field tests on this hypothesis. TheAtlas day geckofaces fewer competitors as altitude increases, and thereby, we predict that this species should increase niche breadth and relevant fitness parameters with altitude.We tested this prediction by analysingthe isotopic signature of carbon (δ13C) and nitrogen (δ15N). Our results reveal that specimens from higher altitudes showed higher values for both carbon (δ13C) and nitrogen (δ15N) isotopes, had better body condition and a greater isotopic breadth when compared to specimens from lower altitudes. Altitudinal variation in carbon values was not explained by variation in isotopic concentration in the baseline of the trophic chain. Therefore, resultssupport the prediction that relaxed interspecific competition favours increased trophic niche breadth. These results also suggest that global warming may represent an important threat for this species, as it may provoke the ascent in altitude of competitors, with negative consequences for the conservation of this endemism. In the second chapter, I study the variation in probability and intensity of infestationof blood-sucking mites parasitizingthe Atlas day gecko depending on host body condition, sex, and elevation.Parasitism is one of the main selective forces in nature,strongly affecting host fitness. Still, knowledge is incomplete concerning how variation in probability and intensity of infestationdepends on body condition, sex or geographic variables. I found that mite prevalence was58.75% and probability of infestation decreased with hostbody condition.However, parasitism intensity tended to increase with body condition. The parasite load ranged from 0 to 16 mites per individual, with a mean intensity of 3.0 ± 0.37 (SE) in infested geckos.Prevalence was higher in males (2/3 parasitized) than in females (1/2 parasitized), but intensitydid not significantly differ with sex. Neither prevalence nor intensity varied with elevation. In conclusion, geckos in better body condition harboured heavier parasite loads, but animals with the highest body condition were not infested. These findings suggest that animals with goodbody condition may tolerate heavier mite infestations, but only animals with the highest body condition may resist infestation.In the third chapterIanalysehow lifespan and other life-history traits of the lizard large Psammodromus Psammodromus algirusvary along a 2,200 m elevational gradient in Sierra Nevada.Lifespan is one of the main components of life history. Shorter lifespans can be expected in marginal habitats. However, in the case of ectotherms, lifespan typically increases with altitude, even though temperature –one of the main factors to determine ectotherms’ life history–declines with elevation. This pattern can be explained by the fact that a shorter activity time favours survival. Populations at intermediate altitudes, corresponding to the optimal habitat for this species, had the shortest lifespans, while populations inhabiting marginal habitats (at both low and at high altitudes) lived longest. Therefore, this lizard did not follow the typical pattern of ectotherms, as it also lived longer at the lower limit of its distribution, nor did it show a longer lifespanin areas with optimal habitats. These results might be explained by a complex combination of different gradients along the mountain, namely that activity time decreases with altitude whereas food availability increases. This could explain why lifespan wasmaximum at both high (limited activity time) and low (limited food availability) altitudes, resulting in similar lifespans in areas with contrasting environmental conditions. This studyalso indicated that reproductive investment and body condition increase with elevation, suggesting that alpine populations are locally adapted. In the fourth chapterIinvestigate the role of life-history traits such as age, sex, body size, body condition, and tail autotomy (i.e self-amputation) in shaping telomere length of six populations of the large Psammodromus in Sierra Nevada. The timing of organisms’ senescence is developmentally programmed but also shaped by the interaction between environmental inputs and life-history traits. In ectotherms, ageing dynamics are still poorly understood despite their particularities concerning thermoregulation, regeneration capacity, or growth trajectory. Our cross-sectional study shows that older lizards have longer telomeres, which might be linked to a higher expression of telomerase across their life or caused by the selective disappearance of individuals with shorter telomeres. Variation in telomere across elevation was explained by age structure of lizards, thus altitude had no effect on the telomere length of lizards. In contrastto our predictions, telomere length was unaffected by tail autotomy, and was sex-independent, but positively correlated with body condition. Regarding the lack of impacts of elevation on telomeres of lizards, our results suggest that life-history traits such as age and body condition can be major drivers of telomere dynamics for this lizard, whereas environmental conditions apparently had scarce or no effects on their telomeres. Our findings emphasize the relevance of understanding species’ life histories and habitat characteristics for fully disentangling the causes and consequences of lifespan trajectory in ectotherms.