Ecology

The lifeways (not to mention the identities) of the earliest primates have been much debated. But the fossil record has long shown that the basic package of prosimian/strepsirhine adaptations was in place by the early Eocene, about 55 myr ago; and if the earliest primate colonizers of Madagascar significantly predated this time, the strepsirhine bauplan would have been present substantially before. Early theories of euprimate origins (e.g., Jones, 1916; Smith, 1924) held that it was adoption of arboreality itself that was the key to the fixation of such primate features as grasping hands, binocular vision, and brain enlargement. However, since many other arboreal mammals do very well without such characteristics, this explanation is at the very least incomplete. Cartmill (e.g., 1972) added visual predation to the mix, with the implication that early primates had been at least mainly insectivorous. In contrast, Sussman and Raven (1978) noted that euprimate diversification followed closely on the heels of the radiation of flowering plants, and proposed that it was the "windfall" resources of abundant fruits and flowers that had provided early primates with the opportunity to radiate. Based on a field study of the "prosimian-like" neotropical marsupial Caluromys, Rasmussen (1990) integrated these two notions by concluding that the primate ancestor had been a visual predator that foraged in the fine terminal branches of the angiosperm canopy for a "combined windfall" of fruits and flowers and the insects attracted by them. Most authorities would currently accept this hypothesis.

Modern strepsirhine body sizes vary enormously, and within this large range Bugtilemur and the Fayum lorisoids vary from tiny to small. The majority of Eocene adapiform primates were larger than this, most of them also exceeding their omomyoid contemporaries in body size. Interestingly, this places the Fayum strepsirhines in the general size range of the modern lorisoids, while the adapi-forms are more comparable to the midrange of Malagasy lemurs. As to locomotion, the Fayum lorisoids are known only from cranial material, but the adapiforms show a variety of locomotor types that apparently ranged from rather loris-like slow arboreal quadrupedalism in Adapis (Dagosto, 1983), to more active quadrupedalism in forms like Pronyticebus (Szalay and Dagosto, 1988) and thigh-powered leaping in Smilodectes (Covert, 1986). Smallish orbit sizes in almost all adapiforms point to diurnal activity, and molar form and wear suggest a preponderance of frugivory among many members of this group although some adapiforms, among them Adapis and Leptadapis, possess sharply crested molars that are suggestive of folivory (Covert, 1986).

It is, then, possible to view the radiation of adapiforms in the Eocene as a sort of early euprimate parallel of today's radiation of diurnal lemuriforms in Madagascar. If the modern lorisoid families had indeed diverged by the late Eocene, the Fayum genera (which derive from deposits that were laid down in moist lowland tropical forest conditions) were presumably fairly close ecological equivalents of their living counterparts. Interestingly, heterothermy, a potential facilitating factor in any rafting scenario, is absent in the few lorisoids so far studied (Mzilikazi et al., 2004), so that its presence in certain cheirogaleids seems most likely to be a specialization acquired in Madagascar rather than a dispersal advantage possessed by the original colonizers. The many adapiform genera are simply too diverse to suggest any ecological thread more precise than a preference for the canopies of tropical or semitropical forests. If the ancestral strepsirhine was a very early adapiform, it is thus difficult to surmise its exact ecological preferences; adapids were probably mostly frugivorous, but Adapis, the adapiform most frequently compared to lemurs, had the molar morphology of a folivore. On a comparative basis there is thus little to suggest the precise ecological niche or niches of Madagascar's founding primate. The problem is, of course, only exacerbated by the fact that Daubentonia, the probable outgroup of the diverse remaining Malagasy primate fauna, is so highly autapomorphic. And, in a fauna with such conspicuous diversity at low taxonomic levels, what may also be surprising is the typically rather eurytopic signal that emerges from field studies of living lemur species. Without doubt, this generalist tendency has been with euprimates from the very start, and we would do well to emphasize the role of geography/normal population variation above that of adaptation as determinant of the current genus- and species-level diversity of lemurs.

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