16 - The physiology of phyletic dwarfi sm in Cheirogaleidae

Summary

Introduction

Hill's (1953, p. 28) multivolume monograph, Primates: Comparative Anatomy and Taxonomy, presented a phylogenetic tree of living primates in which cheirogaleids occupied a position at the base of the Malagasy “lemuroid” radiation, 40–45 Ma if the time scale is read literally. Some years later, Charles-Dominique and Martin (1970) sought to provide evidence to support this view of cheirogaleid archaism. They pooled their expert knowledge of dwarf galagos and mouse lemurs to derive an extensive list of characters shared by the smallest representatives of the Afro-Asian and Malagasy strepsirrhine lineages, which they then interpreted as plesiomorphic retentions from the common strepsirrhine ancestor. The list included small size, nocturnal habit, occupation of a “fine branch and creeper” microhabitat, omnivorous but partly insectivorous diet, use of woven, globular nests and oral transport for the protection of altricial infants, and a “noyau” social system mediated largely by urine washing and other scent marking behaviours. Cartmill (1974) elaborated his model of the adaptive origins of the primate clade and its diagnostic characters (forward-facing and convergent orbits; grasping hands and feet; claws transformed into nails) from a similar scenario. In Cartmill's view, the primate ancestor was primarily a predator of flying insects, anchoring itself to a branch with its grasping feet, and launching its body with grasping hands outstretched to capture passing prey, its manual dexterity guided by its forward-facing eyes and consequent stereoscopic vision. The scenario found ready acceptance among many primate evolutionary biologists, and other evidence in its support was rapidly adduced. Mouse lemurs, with their large numbers of small acrocentric chromosomes (2n = 66), were perfect ancestors for a model of karyotypic evolution that favoured Robertsonian fusions (Dutrillaux, 1979). Small fossil teeth from Eocene and Oligocene sediments were declared to resemble those of cheirogaleids (Marivaux et al., 2001; Godinot, 2006). Models of the evolution of primate sociality were grounded in cheirogaleid (especially Microcebus) social systems (Kappeler, 1998).

Cheirogaleids have an additional feature, unique among primates, to recommend them as model ancestors: they use energy-saving strategies similar to those of other small mammals like rodents and insectivores. Cheirogaleids accumulate fat stores which they draw on in periods of food scarcity. This energetic facility offered an appealing resolution to the conundrum of early primate biogeography. Primates are currently distributed on various landmasses which are separated by substantial tracts of ocean.