A significant proportion of the environmental stochasticity was explained by variation in climate during autumn and late winter. These climate effects remained even after accounting for the effects of known killings by humans. In correspondence with our results, survival of muskox calves in the Arctic Wildlife Refuge in north-eastern Alaska was related negatively to the snow depth in May and June (Reynolds 1998). Thus, the long-term growth rates of muskox populations are affected by stochastic climate effects operating during the nonbreeding season, as has been found for many other ungulate populations (Sæther 1997; Coulson, Milner-Gulland & Clutton-Brock 2000).