The lung of the emu, Dromaius novaehollandiae: a microscopic and morphometric study

Abstract

The emu (Dromaius novaehollandiae) is the only extant member of the family Dromaiidae and is the most widespread of Australian flightless birds (Cameron & Harrison, 1978). After the African ostrich (Struthio camelus), it is the world's second largest living bird. The respiratory and cardiovascular physiology of large flightless birds has been investigated by Crawford & Schmidt-Nielsen (1967), Crawford & Lasiewski (1968), Schmidt-Nielsen et al. (1969), Calder & Dawson (1978), Jones, Grubb & Schmidt-Nielsen (1983) and Grubb, Jorgensen & Conner (1983). The ratites are generally considered to be phylogenetically among the most primitive of the extant groups of birds (Storer, 1971). They have a lower body temperature (about 38 °C) than carinate birds (Calder & Dawson, 1978; Jones et al. 1983). Furthermore the ostrich (Schmidt-Nielsen et al. 1969) and the emu (Jones et al. 1983), when heat stressed, are exceptionally unsusceptible to the respiratory alkalosis that overtakes most other birds after prolonged panting; this has been attributed to unknown structural and functional pulmonary adaptations (Jones, 1982a, b). Gross investigations of the lungs of the emu and the kiwi have categorised them as primitive because they lack the neopulmonic system of parabronchi that characterises species which are supposed to be phylogenetically advanced (Duncker,- 1971). Morphometric studies indicating the potential of the lung for gas exchange are evidently lacking for ratite birds. An apparently mature and healthy adult emu, surplus stock from an Australian zoo, became available to us. In view of the rarity of such material, especially under conditions allowing fixation for electron microscopy, we decided to investigate the general microscopic and morphometric characteristics of its lungs. It is hoped that this would further elucidate the emerging picture (Maina, 1988) that, as in the terrestrial mammals, the pulmonary design of a bird reflects the oxygen demand upon it and th