Young mammalian herbivores are more vulnerable to harsh winter conditions than adults, especially among large circumpolar species like the muskox (Ovibos moschatus). We compared feeding responses of muskox calves (body mass 95 kg) with those of mature, non-reproductive females (body mass 227 kg) in mid-winter when air temperatures fell to −40 °C. Food intakes (1.8–2.2 kg digestible dry matter (DM) d−1), digesta fill (27–32 kg wet mass) and digestibility of hay (52–58% of DM; 49–55% of gross energy) were similar between age groups even though calves were much smaller than adults. Calves fed more frequently (12 vs. 8 feeding bouts per day) and thus spent more time feeding each day than adults (387 vs. 343 min per day). High mass-specific food intakes of calves indicate higher requirements for maintenance of body tissue than adults, which could be related to a larger intestinal tract in young muskoxen. Notably, cows and calves maintained a constant body mass throughout, indicating that they were feeding at maintenance levels and that the relatively higher intakes of calves were not related to growth. Together, these data suggest that limited food availability due to snow cover or high animal density may reduce the survival of muskoxen in their first winter.