Gas transfer by the neonate in the pouch of the tammar wallaby, Macropus eugenii

Abstract

The gas exchange characteristics of, and gaseous environment within, the wallaby pouch containing a single neonate (joey) (age 3–33 d) was determined in conscious animals from pouch gas samples obtained via an indwelling catheter. Pouch gas tensions were constant, with O₂ approximately 3 torr below, and CO₂ 4 torr above, ambient values. For a surgically closed and sealed pouch without a joey, the equilibrium values of O₂ and CO₂ were approximately 70 and 40 torr, respectively. For joeys 3–33 d old, O₂ uptake and CO₂ excretion rates were approx. 0.60 and 0.52 ml · g⁻¹ · h⁻¹, and the respiratory exchange ratio averaged 0.94. Of the O₂ from the atmosphere entering the pouch containing a 3-d joey, only 15% was consumed by the Joey, the remainder being removed by the pouch wall; similarly, 85% of CO₂ leaving the pouch was added to the pouch gas from the pouch wall. Histological studies confirmed that the maternal skin lining the pouch was well vascularized and the joey lung highly vascularized, but that the joey skin was poorly vascularized. Gas exchange in neonatal joeys must be almost totally pulmonary, given the markedly poorer skin vascularity and longer diffusion distances from skin capillaries across its epithelium, compared to that of the capillary network of the lung.