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 favors survival. In this study, we analyzed how lifespan and other life-history traits of the lizard Psammodromus algirus vary along a 2,200 m elevational gradient in Sierra Nevada (SE Spain). Populations at intermediate altitudes (1,200–1,700 m), corresponding to the optimal habitat for this species, had the shortest lifespans, whereas 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 lifespan in 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 was maximum at both high (limited activity time) and low (limited food availability) altitudes, resulting in similar lifespans in areas with contrasting environmental conditions. Our findings also indicated that reproductive investment and body condition increase with elevation, suggesting that alpine populations are locally adapted.

The habitat of a particular species may be defined as a set of resources and conditions needed for survival and reproduction of individuals of that species. Accordingly, the central–marginal hypothesis states that zones with optimal or near-optimal conditions can be referred to as core habitats. Nevertheless, as one moves away from the core habitat areas, the environment usually becomes progressively less suitable for the species, implying lower survival probability and/or reproductive success, and hence decreased fitness. These habitats have border conditions that the species can tolerate for survival and reproduction and are, therefore, considered suboptimal or marginal.

Core and marginal habitats for a given species can be found along altitudinal gradients. Mountain environments harbour a high level of ecological heterogeneity because several abiotic factors change with altitude; mainly, temperature and the partial pressure of oxygen decrease with altitude, whereas solar radiation increases. These abiotic factors exert selective pressures on animals and plants, causing communities to vary along the elevation gradient. Hence, a species inhabiting an elevation gradient may occupy core habitats as well as marginal ones in a relatively small geographical area. As such, elevation gradients provide researchers with a natural experimental setting to study how life-history varies according to habitat quality.

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