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Guanaco populations face different feeding constraints along their altitudinal migrations in Andean mountains. The guanaco’s diet and food availability were analyzed using microhistological analysis and point-quadrat transects at four sampling sites from the summer range, and four sites from the winter range of a High Andean migratory population. Significant differences were detected with Kruskal-Wallis ANOVA, feeding selection by the chi(2) test, and dietary preferences by Bailey’s confidence interval. Summer range was characterized by having higher plant diversity with more availability of grass-likes (Ciperaceae and Juncaceae) and forbs, compared to winter range. Sites with vegas (moist areas associated to streams and ponds, with dense hydrophytic vegetation) showed higher plant cover and diversity, also diet diversity was higher. Diet was dominated by grass-likes and grasses at sites with wetlands, only by grasses at the other sites, especially in winter. The highest diet-availability similarity occurred in the vega microhabitat, but species from slopes were also eaten at sites with wetlands. Grasses were preferred and shrubs were avoided in both seasonal ranges. The narrower guanaco’s diet in winter, and the higher dietary diversity where plant cover and diversity are higher, both agree with the hypothesis of selective quality. A summer opportunistic feeding behavior shifts to a more selective behavior during winter. Winter browsing barely occurred, and grasses prevailed in the diet of both seasons. Altitudinal migration, forced by the deep snow, could favor guanacos to maintain a grazer strategy year round. Vegas and grasslands, in the summer and winter ranges, have a high feeding relevance for this guanaco population, and connectivity between both ranges is essential for their survival. (C) 2011 Deutsche Gesellschaft fur Saugetierkunde. Published by Elsevier GmbH. All rights reserved.