A shrew-sized origin for primates

Abstract

The origin of primates has had a long history of discussion and debate, with few ever considering the impact of the original body weight on subsequent primate adaptive radiations. Here, I attempt to reconstruct early primate evolution by considering the initial size of primates as well as the critical functional-adaptive events that had to occur prior to the early Eocene. Microcebus is often viewed as a living model, and thus 40 -65 g might represent a practical ancestral weight for the origin of primates. I consider a smaller original body weight, likely 10 -15 g in actual size, and I address the biological implications for shrew-sized primates by comparing the behavioral ecology of mouse lemurs, our smallest living primates, to another tiny-sized mammalian group, the shrews (Family Soricidae). Several behavioral and ecological characteristics are shared by shrews and mouse lemurs, and several mammalian trends are evident with decreased size. I suggest that a shrew-sized ancestral primate would have had high metabolic, reproductive, and predation rates, relatively low population densities, and a dispersed and solitary existence with a promiscuous mating system. Although small mammals like shrews provide insights concerning the ancestral size of primates, primate origins have always been tied to arboreality. I assess other potential arboreal models such as Ptilocercus and Caluromys. By combining all of this information, I try to sequence the events in a functional-adaptive series that had to occur before the early Eocene primate radiations. I suggest that all of these important adaptive events had to occur at a small body size below 50 g. Yrbk Phys Anthropol 47: 40 -62, 2004. © 2004 Our attempts to understand the origin of primates have had a long history, with central themes of arboreality, small branches, and the visually based predation of insects (

Internal carotid arterial canal size and scaling in Euarchonta: Re-assessing implications for arterial patency and phylogenetic relationships in early fossil primates

“…The absolute interspecific variation in size of the hearing organs may therefore be constrained by demands on the auditory system, unlike brain size or body size. It is worth noting that even if this hypothesis about constraints on the stapedial artery is correct, it seems that additional, unrelated factors (perhaps still related to hearing specialization) must be responsible for the initial loss of the stapedial artery in stem haplorhines, which are thought to have been smallbodied (Gebo, 2004).…”

Primate origins, human origins, and the end of higher taxa

“…If they were smaller than any living primates, as has been argued, we would expect them to have had a correspondingly more insect-centered diet. 101 As Bloch and his coworkers acknowledge, 64 the visual peculiarities of euprimates -enlarged, frontated, and approximated eyes and orbits, reduced retinal summation, enlargement of the visual parts of the brain, and a complete postorbital bar -seem to have evolved later than at least the initial stages of specialization for pedal grasping (Fig. 5).…”

“…Although their affinities are widely debated, most of the fragmentary early (Paleocene to mid-Eocene) fossils that have been proposed recently as candidate basal anthropoids (Eosimias, Algeripithecus, Biretia, Altiatlasius, Anthrasimias) exhibit some combination of diminutive size, moderately trenchant cheek teeth, and enlarged orbits, suggesting derivation from a small, visually predatory ancestor. 101,[108][109][110][111][112][113] Can we then still attribute the origin of the distinctive euprimate traits to a basal adaptive shift involving visually directed predation? I am now inclined to think that the whole notion of a defining adaptive shift that accounts for ''primate origins'' is another hallucination born of ignorance.…”

Foraging Cognition in Nonhuman Primates

“…Many primate species have opposable thumbs and toes, allowing them to grasp and reach fruit at the terminal tree branches, which are inaccessible to many other animals. Hindlimb dominanceand grasping ability enable many primates to leap between trees in an energetically efficient way, in contrast other arboreal mammals such as most tree squirrels (Gebo, 2004;Sussman, 1991;Taylor et al, 1972). Other adaptations concern forwardfacing eyes and stereotypic vision, which facilitates hand-eye coordination and foraging at high speed (Cartmill, 1972;Gebo, 2004).…”

“…Hindlimb dominanceand grasping ability enable many primates to leap between trees in an energetically efficient way, in contrast other arboreal mammals such as most tree squirrels (Gebo, 2004;Sussman, 1991;Taylor et al, 1972). Other adaptations concern forwardfacing eyes and stereotypic vision, which facilitates hand-eye coordination and foraging at high speed (Cartmill, 1972;Gebo, 2004). Similarly, diurnal activity, high visual acuity, and color vision enable spotting of fruit and their nutritional value from large distances (Barton, 2000;Polyak, 1957;Riba-Hernández et al, 2005;Sumner & Mollon, 2000).…”