A morphological and morphometric study of the prosimian lung: the lesser bushbaby Galago senegalensis.
Among mammals, it is only in the order Primate that the extant members in fair measure present graded features approximating to the stages of the evolution from the early ancestral (prosimian) primates through to anthropoid apes and man (Huxley, 1863; Clark, 1962, 1970). The primates hence constitute a remarkably heterogeneous and variably specialised taxon exhibiting a notable range of body size and differing modes of life (Heglund, 1985). Classification of the primates attracts great attention due to its relevance to the evolution of man (Charles-Dominique, 1970). The prosimians which include the lemurs, bushbabies and tarsiers are regarded behaviourally, anatomically and physiologically to be the most primitive living primates (Szalay, 1968; Charles-Dominique, 1970, 1978; Bourliere, 1974) and exhibit many unique structural and functional features which, when viewed comparatively, illustrate the primate phylogeny (Napier & Walker, 1967). Studies of the prosimian biology, however, only received interest in the relatively recent past (Doyle & Martin, 1974; Martin, 1978; Stevens, Edgerton, Haines & Meyer, 1981). The present studies have illustrated that this group is in some respects functionally and structurally unique among the primates. The basal metabolic rate (BMR), for example, has been shown to be lower in the prosimians (Muller, 1979, 1985) than in the simian species, where the BMR is close to, or higher than, that predicted from body mass (Bruhn, 1934; Malinov & Wagner, 1966; Morrison & Middleton, 1967; Nakayama et al. 1971). Body temperature in the simiae is generally 10 to 2 °C above that of most prosimian species (Muller, 1985). Heat exposure was observed to cause great elevation of the respiratory frequency in the greater bushbaby (Galago crassicaudatus) than in the baboon (Hiley, 1976) and, in the same study, water loss through the skin in the bushbaby was found to be relatively very low. The response speed of the rhesus monkey was found to be by far greater than that of the prosimians (Ehrlich, 1968a,b). Morphologically the bushbabies have an atypically well-developed rete mirabile in the limbs for countercurrent heat exchange and a thick woolly cover (Whittow et al. 1977). The lesser bushbaby (Galago senegalensis), the subject of this study, is a small member of the family Lorisidae. It is the most populous primate in Africa (Haines, 1982) and adapts well to maintenance in captivity and to the rigours of experimental manipulation (Stevens et al. 1981). Despite these favourable features, the potential of the bushbaby as an experimental primate model for biomedical research has not been well exploited (Stevens et al. 1981; Haines, 1982). The main contributing factor may be due to the paucity of published detail on the morphology of these elusive, habitually arboreal and nocturnal animals compared to, for example, the more easily accessible New World and Old World monkeys and the anthropoid apes which are more frequently used as animal models for human studies. In spite of the assumed closeness between man and the other simians, it is becoming evident that the fact that two groups of animals are systematically placed in the same higher category is by itself no guarantee that a particular system under investigation is an appropriate model for a human system, as is apparent from accounts by Stevens et al. (1981) and Haines (1982). The prosimians, for example, in general have a well-developed appendix which is absent in the monkeys and present in man. These differences have apparently arisen from the differing extents in the evolution and adaptations that the various primate groups have undergone (Clark, 1970). The objectives of the present study were as follows. Firstly, to describe the morphology of the lung of the lesser bushbaby Galago senegalensis, which has hitherto not been investigated. Secondly, to establish the morphometric features of the structural components of the lung in an attempt to gain an insight into its anatomical gas exchange potential. Thirdly, to compare such data with those from other primates that have been investigated to a similar extent and then to correlate them with the mode of life that this bushbaby leads. Fourthly, to evaluate whether the lung of the lesser bushbaby could be, structurally, an appropriate organ model for biomedical pulmonary studies.