Oecologia (2003) 136:317–323 DOI 10.1007/s00442-003-1257-9 GLOBAL CHANGE ECOLOGY Robert B. Weladji · Øystein Holand Global climate change and reindeer: effects of winter weather on the autumn weight and growth of calves Received: 21 October 2002 / Accepted: 14 March 2003 / Published online: 18 April 2003  Springer-Verlag 2003 Abstract Reindeer/caribou (Rangifer tarandus), which constitute a biological resource of vital importance for the physical and cultural survival of Arctic residents, and inhabit extremely seasonal environments, have received little attention in the global change debate. We investi- gated how body weight and growth rate of reindeer calves were affected by large-scale climatic variability [mea- sured by the North Atlantic Oscillation (NAO) winter index] and density in one population in central Norway. Body weights of calves in summer and early winter, as well as their growth rate (summer to early winter), were significantly influenced by density and the NAO index when cohorts were in utero. Males were heavier and had higher absolute growth than females, but there was no evidence that preweaning condition of male and female calves were influenced differently by the NAO winter index. Increasing NAO index had a negative effect on calves’ body weight and growth rate. Increasing density significantly reduced body weight and growth rate of calves, and accentuated the effect of the NAO winter index. Winters with a higher NAO index are thus severe for reindeer calves in this area and their effects are associated with nutritional stress experienced by the dams during pregnancy or immediately after calving. Moreover, increased density may enhance intra-specific competition and limits food available at the individual level within cohorts. We conclude that if the current pattern of global warming continues, with greater change occurring in northern latitudes and during winter as is predicted, reduced body weight of reindeer calves may be a consequence in areas where winters with a high NAO index are severe. This will likely have an effect on the livelihood of many northern indigenous peoples, both economically and culturally. Keywords Individual performance · Global warming · North Atlantic Oscillation · Rangifer tarandus · Reindeer industry Introduction Climatic variation (local and global) has been reported to affect several life-history and population parameters of northern ungulates (see reviews by Putman et al. 1996; Saether 1997; Post and Stenseth 1999; Post et al. 1999a; Gaillard et al. 2000; Ottersen et al. 2001; Weladji et al. 2002b), including body weight. Climatic effect on ungulates may be direct. For example, the effect of severe cold may lead to increased costs of thermoregu- lation (Parker and Robbins 1985; Putman et al. 1996), and increasing snow depth to increased costs of locomotion (Parker et al. 1984) and cratering (Fancy and White 1985), thereby affecting ungulates’ body condition. Climate may also act indirectly on ungulates through its effect on quality and biomass production of forage plants in summer (Bø and Hjeljord 1991; Berendse and Jonasson 1992; Lenart et al. 2002). In winter, extensive and deep snow cover may limit forage availability. Moreover, cold weather following wet snow or rain may lead to formation of ice layers in the snow or “icing” on the ground surface limiting access of herbivores to vegetation (Miller et al. 1982; Forchhammer and Boertmann 1993). There is increasing evidence that the globe is currently warming (Mann et al. 1999), the changes being more pronounced in northern latitudes and during winter (Dickinson 1986; Maxwell 1997). Recently, there has been increasing evidence that the large-scale atmospheric phenomenon, the North Atlantic Oscillation (NAO), through its effects on vegetation and regional weather conditions, influences several aspects of life histories (e.g. body mass, early growth, fecundity, survival, etc.), as well as demographic and population dynamic processes (e.g. cohort variation, sex ratios, abundance, etc.) of several terrestrial large mammalian herbivores (Post et al. 1997, 1999b; Forchhammer et al. 1998, 2001; Post and Stenseth R. B. Weladji ( ) ) · Ø. Holand Department of Animal Science, Agricultural University of Norway, P.O. Box 5025, 1432 s, Norway e-mail: robert.weladji@ihf.nlh.no Tel.: +47-64948056 Fax: +47-64947960