Young arctic ungulates must grow quickly to use forages available during brief summers. We measured growth and organ maturation in Rangifer tarandus (reindeer, caribou) and Ovibos moschatus (muskoxen) at 3 stages: the neonatal period (1–30 days), during transition from milk to plants (30–60 days), and at maturity. Although reindeer provided greater maternal investment than muskoxen in utero (6.9% compared with 4.8% maternal mass), both species grew at similar rates (0.002 compared with 0.005 day−1). High thyroid secretion and large hepatic lipid reserves in neonates indicated high rates of energy metabolism for thermogenesis and use of nutrients sequestered in utero. Mass gains during the neonatal period were associated with large gastric abomasa that would allow young ungulates to digest milk soon after birth. Body growth continued during transition to forage (60 days of age), which was accompanied by expansion of rumen, small intestine, and colon. Growth and organ maturation of arctic ungulates, therefore, emphasizes maternal investment in utero and the duration of growth, with little variation in growth rate during both neonatal and transitional phases.
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