Mechanical Properties of Plant Underground Storage Organs and Implications for Dietary Models of Early Hominins

Abstract

Constantino P, and Lucas PW. The mechanical properties of plant underground storage organs and implications for the adaptive radiation and resource partitioning of early hominins. Evolutionary Biology 35(3): 159-175. Abstract The diet of early human ancestors has received renewed theoretical interest since the discovery of elevated d RESEARCH ARTICLE13 C values in the enamel of Australopithecus africanus and Paranthropus robustus. As a result, the hominin diet is hypothesized to have included C 4 grass or the tissues of animals which themselves consumed C 4 grass. On mechanical grounds, such a diet is incompatible with the dental morphology and dental microwear of early hominins. Most inferences, particularly for Paranthropus, favor a diet of hard or mechanically resistant foods. This discrepancy has invigorated the longstanding hypothesis that hominins consumed plant underground storage organs (USOs). Plant USOs are attractive candidate foods because many bulbous grasses and cormous sedges use C 4 photosynthesis. Yet mechanical data for USOs-or any putative hominin food-are scarcely known. To fill this empirical void we measured the mechanical properties of USOs from 98 plant species from across sub-Saharan Africa. We found that rhizomes were the most resistant to deformation and fracture, followed by tubers, corms, and bulbs. An important result of this study is that corms exhibited low toughness values (mean = 265.0 J m -2 ) and relatively high Young's modulus values (mean = 4.9 MPa). This combination of properties fits many descriptions of the hominin diet as consisting of hard-brittle objects. When compared to corms, bulbs are tougher (mean = 325.0 J m -2 ) and less stiff (mean = 2.5 MPa). Again, this combination of traits resembles dietary inferences, especially for Australopithecus, which is predicted to have consumed soft-tough foods. Lastly, we observed the roasting behavior of Hadza hunter-gatherers and measured the effects of roasting on the toughness on undomesticated tubers. Our results support assumptions that roasting lessens the work of mastication, and, by inference, the cost of digestion. Together these findings provide the first mechanical basis for discussing the adaptive advantages of roasting tubers and the plausibility of USOs in the diet of early hominins.

A new species of Miocene wombat (Marsupialia, Vombatiformes) from Riversleigh, Queensland, Australia, and implications for the evolutionary history of the Vombatidae

“…Brewer et al (2008) suggested that R. crowcrofti could have been rhizophagous. Mechanical processing of Underground Storage Organs (USOs) would seem like a potential match for the tooth morphology and wear pattern evident in this species (and to a lesser extent N. boodjamullensis; Dominy et al, 2008). USOs would have been available in both a rainforest environment and in more open/drier/seasonal environments, as they are today.…”

Dental microwear texture analysis of late Pliocene Procynocephalus subhimalayanus (Primates: Cercopithecidae) of the Upper Siwaliks, India

“…This suggests hardobject feeding and extractive foraging, characterized at least a portion of the diet of Paranthropus robustus and to a lesser extent, Australopithecus africanus. A strong C 4 signal is also exhibited by Paranthropus and Australopithecus [33], possibly from corm and bulb consumption [34]. Differences in surface texture complexity may correspond to changes in habitat from a more closed environment of late Pliocene Australopithecus africanus compared to the relatively open savannas and grasslands associated with early Pleistocene Paranthropus robustus.…”

“…Underground storage organs contain particles with relatively resistant fracture properties [34]; grit adhering to USOs would cause additional microwear [35,36]. The enamel textural complexity of Procynocephalus may stem from the consumption of USOs, hard-object feeding, or both, given its enamel texture complexity exceeds that characterizing Lophocebus and Cebus.…”

“…In xeric habitats, leaves, fruit and herbs can be covered by a thin layer of grit, particularly those proximate to ground level [42]. Underground storage organs may be consumed without leaving a large degree of enamel texture complexity as some bulbs and corms have outer tunics that are easy to skin or handwash [34]. In addition, semi-terrestriality does not always indicate a reliance on USOs.…”

“…Objects in nature present differences in fracture properties. Fracture in this context can be defined as a perforation in plant tissues as a function of the force applied [34]. The degree to which tissues respond to displacement from the teeth reflect the mechanical properties of the fibers.…”