Apneic uptake of atmospheric O2 by deeply hypothermic nestlings of the white-footed mouse (Peromyscus leucopus): circulation and lungs

Nestling white-footed mice (Peromyscus leucopus) are born in the earliest days of spring in cold climates. If the nestlings are by accident exposed to ambient temperatures near freezing (0-7 °C) at early ages (2-10 days old), they may experience body temperatures (T_bs) equally low. During such hypothermia, although their heart keeps beating, they become apneic (cease inhaling and exhaling). However, they have an exceptional ability (e.g., compared to Mus musculus) to tolerate these conditions for at least several hours, after which they revive if rewarmed by parents. This paper addresses the physiology of the apneic period. We show that apneic, hypothermic nestlings undergo physiologically important exchanges of gases with the atmosphere. These gas exchanges do not occur across the skin. Instead they occur via the trachea and lungs even though the animals are apneic. Most significantly, when hypothermic neonates are in apnea in ordinary air, they take up O₂ steadily from the atmosphere throughout the apneic period, and the evidence available indicates that this O₂ uptake is essential for the nestlings' survival. At T_bs of 2-7 °C, the nestlings' rate of O₂ consumption varies quasi-exponentially with T_b and averages 0.04 mL O₂ g^{- 1} h^{- 1}, closely similar to the rate expressed by adult mammalian hibernators in hibernation at similar T_bs. Morphometric analysis indicates that, at all focal ages, O₂ transport along the full length of the trachea can take place by diffusion at rates adequate to meet the measured rates of metabolic O₂ consumption.