Date: 08 Aug 2010

Adaptive Profile Versus Adaptive Specialization: Fossils and Gummivory in Early Primate Evolution

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Abstract

Gummivory, a rare dietary habit among modern primates, has figured prominently in interpretations of the earliest primates (plesiadapiforms) largely on the basis of a morphological analogy with sugar gliders, and it has also been proposed as a key adaptation pertaining to early strepsirrhines (Adapis, Leptadapis) and the origins of the toothcomb, partly by analogy with marmosets. In reexamining these hypotheses, it is important to consider the following: distinguish gum-gouging from gum-gleaning; assess adaptive compromise and preadaptation; examine system-wide linkages between gum harvesting and collateral behaviors relating to diet; reevaluate the morphological correspondence between purported analogs; empirically evaluate tooth wear, perhaps the most direct morphological signal of gouging behavior. A distinction is drawn between facultative gummivory as part of a species’ Adaptive Profile and obligate gummivory as an Adaptive Specialization, and the testability of both notions. Using marmosets as the most stringent morphological and behavioral model of an obligate modern primate gummivore – exhibiting a distinctive functional suite of features and evidence of heavy upper and lower anterior tooth wear – none of the test cases present cogent examples of gum-gouging adaptation. The sugar glider Petaurus breviceps also differs from marmosets in tooth morphology and wear; they appear to be gum-gleaners. The derived “gracilization” of upper incisors in all strepsirrhines living and extinct, possibly indicative of an obligatorily folivorous ancestry, may have been ­preadaptive to a shift from harvesting to socio-sexual biological roles in anterior tooth use, presaging the toothcomb’s evolution as a grooming adaptation. The procumbent, often styliform, large lower incisors of many plesiadapiforms may have served as precision probes and pincers for these primitive face-feeding primates that likely lacked the advanced hand–eye coordination of euprimates. Working with the uppers in beak-like fashion, they would probably have been well suited to harvesting and ingesting small seeds, a prelude to the full-blown coevolutionary relationship established between euprimates and angiosperms as the latter evolved a diverse array of larger and more fleshy fruits. Then non-tarsiiform primates, with more discriminating eyes, touch-sensitive prehensile hands, mobile athletic bodies, and more versatile front teeth, could reinvent themselves as a unique mammalian guild of obligate arboreal frugivores and folivores.