Changes in nutritional yield (<i>y</i>-axis) as a result of increasing food volume with body mass 28–59 kg (<i>z</i>-axis) at 10-minute increments of feeding on C₄ sources (<i>x</i>-axis) for yearling baboons (a) and yearling baboon with increased manipulatory capabilities m = 2, i.e. a doubling the efficiency (<i>B_j</i>) with which they process corms (b).

<p>Incremental steps are highlighted by shaded bands.</p

Baboon Feeding Ecology Informs the Dietary Niche of <i>Paranthropus boisei</i>

<div><p>Hominins are generally considered eclectic omnivores like baboons, but recent isotope studies call into question the generalist status of some hominins. <i>Paranthropus boisei</i> and <i>Australopithecus bahrelghazali</i> derived 75%–80% of their tissues’ δ¹³C from C₄ sources, i.e. mainly low-quality foods like grasses and sedges. Here I consider the energetics of <i>P. boisei</i> and the nutritional value of C₄ foods, taking into account scaling issues between the volume of food consumed and body mass, and <i>P. boisei</i>’s food preference as inferred from dento-cranial morphology. Underlying the models are empirical data for <i>Papio cynocephalus</i> dietary ecology. <i>Paranthropus boisei</i> only needed to spend some 37%–42% of its daily feeding time (conservative estimate) on C₄ sources to meet 80% of its daily requirements of calories, and all its requirements for protein. The energetic requirements of 2–4 times the basal metabolic rate (BMR) common to mammals could therefore have been met within a 6-hour feeding/foraging day. The findings highlight the high nutritional yield of many C₄ foods eaten by baboons (and presumably hominins), explain the evolutionary success of <i>P. boisei,</i> and indicate that <i>P. boisei</i> was probably a generalist like other hominins. The diet proposed is consistent with the species’ derived morphology and unique microwear textures. Finally, the results highlight the importance of baboon/hominin hand in food acquisition and preparation.</p></div

Summary diagram of the composition of diet eaten by a 34–49 kg hominin.

<p>In (a) the empirical data for yearling <i>Papio cynocephalus</i> are shown. In (b) the basic model shown in (a) is scaled up to account for larger body masses and feeding on all C₄ sources is increased until the target of approximately 9700 kJ is reached (i). Then, once the model has been scaled to larger body masses, only the time feeding for stolons, leaves, meristem and seeds is increased (ii.), or on leaves (iii) or corms (iv); feeding time on fruits and invertebrates was kept constant to the level of yearling baboons (ii–iv). In (c) the models outlined in (b) are repeated with improved manipulations skills for the processing of corms (m = 2). In (d) only C₄ food sources that are well-suited to be broken down by <i>P. boisei</i> dento-cranial morphology, i.e. hard, brittle or soft, are selected. The effects of manipulatory capabilities (m) were tested. The models shown in (e) are considered most appropriate for inferences about the feeding ecology of <i>P. boisei.</i> These are achieved when all C₄ sources are selected, but only feeding time on corms is increased beyond the time observed in yearling baboons. The total time available for feeding, including foraging, is assumed to be 50% of the day in all models, i.e. 360 minutes.</p

Illustration of the tensile stresses (σ) and resulting breakages in <i>P. boisei</i> teeth.

<p>Tensile stresses (σ) would occur when lateral loads are applied to a straight-walled tooth and the force vector is directed outside the dental tissue. Without decussating enamel, i.e. bundels of enamel prisms crossing over, transverse cracks initiated on the unloaded side will propagate through the tissue and will lead to catastrophic failure of the tooth. Cracks tend to travel along the protein-rich prism sheaths and are stopped by differently-oriented prisms. Such oblique/transverse breakages are frequently found in <i>P. boisei</i> teeth and are illustrated here in a sample of SEM pictures. Although these breaks may have occurred post mortem, they illustrate the plane of least resistance and thus allow an assessment of the loading conditions to which the tooth should not have been subjected <i>in vivo</i>. Images are not to scale and are for illustration only.</p

Stable isotope data for the subsample of <i>Gorilla</i> (○) and <i>Pan</i> (□) hair (see grey box in Figure 1B) that also preserve skulls from the area used for analyses of morphological/developmental differences.

<p>Separation in isotope space between infants (light grey), i.e. individuals prior to M1 being in functional occlusion, juveniles (middle grey) and adults (dark grey), i.e. after M2 in functional occlusion (dark grey) are also shown. Individuals with M1 in the process of eruption are indicated in red.</p

Boxplot of brain sizes by developmental stages for (A) gorillas and (B) chimpanzees.

<p>Females are shown in light grey and males in dark grey. Group 1: individuals with deciduous dentition only, Group 2: individuals with alveolar emergence of M1 up to M1 being in functional occlusion, Group 3: individuals with M1 in functional occlusion up to M2 being in functional occlusion, Group 4: M2 in occlusion to M3 reaching the occlusal plane, Group 5: adult. Sample sizes for each boxplot are indicated at each lower end, whilst descriptive statistics are given in Table S4 in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0102794#pone.0102794.s001" target="_blank">File S1</a>.</p