Not towing the allometric line: morphology and physiology of the respiratory system during postnatal development in marsupials

Abstract

On the whole, marsupials are born small; the fat‐tailed dunnart produces one of the smallest and most developmentally immature newborn mammals, weighing ~13 mg. Newborn marsupials, to varying extent, rely on cutaneous gas exchange and in the extreme, e.g. the dunnart, typically do not breathe until 3–4 days postpartum. Surface area and a functional lung volume in neonatal fat‐tailed dunnarts and tammar wallabies were determined with phase‐contrast imaging using a synchrotron radiation source. Values for surface area and lung volume were both less than that expected from allometry, and remained low until the animals reached a body mass of approximately 1 gram. Interestingly, this is also the age/size where we first observe the presence of secondary septa in the lung and cutaneous gas exchange ceases. The short gestation, low birth weight and less than allometrically predicted values for lung surface area and volume (and associated implications for pulmonary diffusing capacity) and higher than predicted respiratory system resistance may all play a part in reducing ventilation and hence placing a reliance of the newborn marsupial on cutaneous respiration in the first few days of life.