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A free-ranging population of adders (Vipera berus) was studied from 1987 to 1993 in a prealpine area of western Switzerland situated at about 1400-1600 m a.s.l. Adders of two chromatic morphs (melanistic and zig-zag) are found in this locality. Body sizes of melanistic specimens (including both males and females) significantly exceeded those of zig-zag (cryptic) ones, and fecundity was higher in black than in cryptically coloured females. The existence of a dwarf mountainous "ecotype" in this alpine region is suggested. The frequency of melanism was higher in females.
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Monney et al. (1995) studied this question.
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