Hard-object feeding in sooty mangabeys (Cercocebus atys) and interpretation of early hominin feeding ecology

Abstract

Morphology of the dentofacial complex of early hominins has figured prominently in the inference of their dietary adaptations. Recent theoretical analysis of craniofacial morphology of Australopithecus africanus proposes that skull form in this taxon represents adaptation to feeding on large, hard objects. A modern analog for this specific dietary specialization is provided by the West African sooty mangabey, Cercocebus atys. This species habitually feeds on the large, exceptionally hard nuts of Sacoglottis gabonensis, stereotypically crushing the seed casings using their premolars and molars. This type of behavior has been inferred for A. africanus based on mathematical stress analysis and aspects of dental wear and morphology. While postcanine megadontia, premolar enlargement and thick molar enamel characterize both A. africanus and C. atys, these features are not universally associated with durophagy among living anthropoids. Occlusal microwear analysis reveals complex microwear textures in C. atys unlike those observed in A. africanus, but more closely resembling textures observed in Paranthropus robustus. Since sooty mangabeys process hard objects in a manner similar to that proposed for A. africanus, yet do so without the craniofacial buttressing characteristic of this hominin, it follows that derived features of the australopith skull are sufficient but not necessary for the consumption of large, hard objects. The adaptive significance of australopith craniofacial morphology may instead be related to the toughness, rather than the hardness, of ingested foods.

Figure 1. Fragmentation index of Sacoglottis gabonensis compared to familiar food materials.

E is elastic modulus (MPa) and R is fracture toughness (Jm⁻²). High values indicate stress-limited foods, i.e., those expected to require high occlusal forces to induce structural failure. Sacoglottis is materially hard (high stiffness) but its “honeycombed” interior also renders it structurally tough. Comparative data are from ref 15.

Figure 2. Seed casing of Sacoglottis gabonensis (top), and cross sections (bottom).

Both the outer casing and the material comprising the inner compartments are hard, whereas the seeds found within the compartments (not pictured) are oily and relatively soft.

Figure 3. Sooty mangabey female processing Sacoglottis gabonensis in the Taï Forest, Côte d'Ivoire.

Isometric biting with the postcanine teeth is an ingestive behavior associated almost exclusively (93.4% of all events) with Sacoglottis.

Figure 4. Enamel surface of the P⁴ of a Cercocebus atys specimen collected from Taï Forest, Côte d'Ivoire.

The topography of wear resembles that of other primate hard-object feeders (e.g., some Cebus apella and Lophocebus albigena).

Figure 5. Relationship of microwear complexity to state of wear on P₄ (P = 0.253 by Spearman's ρ) and M₁ (P = 0.0081) in Cercocebus atys.

Under an experimentwise α = 0.00625, none of the microwear texture indices covaries significantly with magnitude of attrition.

Figure 6. Relative tooth size in catarrhine primates (N = 33).

Cercocebus atys is represented by open circles. Male and female sooty mangabeys have relatively but not exceptionally large postcanine teeth relative to body size. Tooth area data from ref 19; C. atys was measured from the Taï sample (N = 8 females and N = 9 males). Body weight data from refs 53,54.