To examine environmental effects on age structure, growth rate and body size, we studied three epigean populations (490–2350 m a.s.l.) and one subterranean (550 m a.s.l.) population of the Pyrenean newt Euproctus asper. The age structures were measured by skeletochronology of phalanges. In the three surface populations, where winter dormancy on land causes an annual growth stop, lines of arrested growth (LAGs) were observed. LAGs in the subterranean population, where individuals stay in the water throughout the year, indicate yearly (at least cyclical) periods of arrested growth even though animals occupy a relatively constant temperature/light environment. The age structures of adults did not differ between surface populations (5–28 yr) and the subterranean population (7–16 yr). Body lengths were highest for the surface population from the lowest altitude. The age, body size and growth traits of the subterranean population came closer to the values obtained for the surface population located at 490 m a.s.l. than for those living at higher altitudes. We hypothesize that LAGs in subterranean newts result from reduced feeding in winter and that the similarity of age and size structures of subterranean and low-altitude populations argues for a recent colonization of the hypogean environment, where populations can find remaining optimal environmental conditions favorable to these stenothermic and rheophilous animals.
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