Hunt 1994
Classification: Feeding Models
Mnemonic: Hand to Mouth
Specific Model: Postural Feeding Hypothesis
Original Proponent(s): Hunt (1994)    
Assessment: Popularity: Feeding Models were ranked 2nd (out of 9) most popular in the texts reviewed. 36% of texts referred to Hunt's Postural Feeding Hypothesis.
Simple: #2 (=2)  / 42 (84%)
Detailed: #5  / 42 (76%)
Basic Summary: Feeding on relatively short bushes provided sufficient selection for traits in favour of upgright posture leading to bipedalism.    
Discussion: This model arguably has the best evidence from extant apes. It was first proposed by Hunt (1994), but revised in subsequent papers (e.g. Hunt 1996, 1998.) Based on over 700 hours of observation of wild chimpanzees at Gombe in Tanzania, it was found that the behavioural context most likely (almost 80% of recorded instances) to result in bipedalism was postural feeding. It should be noted, however, that these bipedal incidents were themselves relatively rare, comprising only about 3% of all observations recorded in the chimpanzees.

A major premise of this model is that “contexts that elicit bipedalism in extant apes may provide evidence of the selective pressures that led to hominid bipedalism” (Hunt 1994:p183). Hunt begins by briefly reviewing differing views of the “positional repertoire of the earliest known hominids” and the “hypothesized selective pressure(s) that led to the evolution of bipedalism”, suggesting that a majority of authors see them as “compromised terrestrial bipeds with substantial adaptations to arboreality” (Hunt 1994:183) and that of all the speculated selective pressures, feeding models are arguably the most supportable because feeding is such a major aspect of the daily lives of extant primates.

His argument acknowledges the contradictory signals from australopithecine anatomy: some indicating arboreality whilst others terrestrial bipedalism: “Arboreal hypotheses cannot completely account for the extent to which bipedalism has been perfected, since refinements such as the shock-absorbing nature of the foot would seem to be unnecessary for low-stress branch walking. Terrestrial hypotheses cannot convincingly explain why arboreal traits persist for over a million years after the adoption of terrestrial bipedalism” (Hunt 1994:184.)

In a later paper, Hunt (1998) reported his findings even more fully and also placed greater stress on other feeding models as a basis for his, notably the original views of Jolly (1970), the revised form of this in Jolly & Plog (1987), Tuttle (e.g. 1981), Rose (e.g. 1991) and Wrangham (1980). Hunt pointed out that as primate time budgets are dominated by feeding behaviours it is likely that such behaviours would have been important in whatever factors led to a change in posture and mode of locomotion, in addition to other early anatomical changes signalling ape-human divergence, such as dental reduction and encephalisation.

His study is one of the most detailed of wild extant ape locomotion to date and produced a number of interesting findings:

a. Chimpanzees are rarely bipedal. Only 97 instances of bipedalism among 21 individuals was recorded in over 700 hours of observation (Hunt 1998:399.)

b. Of that rare bipedality most of it (almost 80%) was found to have been in the context of postural feeding (Hunt 1998:399.)

c. Almost half of the recorded arboreal bipedal feeding activity was in relatively small, forest edge, small fruit-bearing trees (Hunt 1998:404.)

These findings led him to promote the hypothesis that increased postural feeding in some groups of hominin could have begun the evolutionary trajectory towards obligate bipedalism. Specifically, Hunt suggests that this kind of behaviour would be most favoured in habitats where tree size and density was reduced, perhaps in ecotones bordering on savannah grassland and woodland and the types of trees being foraged were ones bearing small-diameter fruit.
 Strengths: The strengths of the postural feeding model are two-fold:

Firstly, like the others discussed so far, it links a change in locomotor repertoire to a behaviour that would clearly have benefited the selective fitness of those performing it;

Secondly, it is perhaps the best model discussed so far in terms of being backed up by evidence in extant apes. It also offers some good evidence against other models. “Responding to threat” was a rarely recorded context at 4.1% and “Carrying objects” was not recorded as a context that was elicited bipedalism at all. The model is made all the more compelling by Hunt as he draws upon paleontological evidence to back up his model. He suggests that contradictory interpretations of that anatomy between those who find the evidence suggesting a very human-like terrestrial mode of life (e.g. Indications of lumbar lordosis, valgus knee and arched feet, Lovejoy 1981) and those who take the evidence to suggest a more ape-like arboreal life style (e.g. long and curved pedal phalanges, abductable toe, Stern 1983) may be resolved if one considers that the earliest forms of their undoubted bipedality were in some kind of arboreal, postural feeding, context.

Furthermore Hunt suggests “although australopithecine habitats are not as tropical and closed as those in which chimpanzees are typically found they are better described as open-forest or woodlands than savannah or bush land. Habitats in which chimpanzees are bipedal, therefore, are probably quite similar to those which australopithecines were found. An open-forest, small fruit-eating adaptation for early hominids also accords well with micro wear evidence that australopithecines were frugivores, and made little use of grazing or subterranean resources” Hunt (1994:191.)
Weaknesses: However, like many of the models, the postural feeding idea shares a weakness in explaining why, if this factor led to the evolution of bipedalism in hominids, it failed to do so in the other Hominoids. Hunt’s view on this is that australopithecines may have inhabited those forest edge habitats where the density of relatively small trees, bearing relatively small fruits was sufficiently height to mitigate a general shift in locomotion towards more bipedalism. However, chimpanzees that do inhabit similar habitats today, such as those at Mt. Assirik, Senegal (McGrew et al 1981) have not been demonstrated to be any more bipedal than chimpanzees or bonobos living in much denser woodland. Indeed the most bipedal of the apes, the gibbons, are also the most arboreal.

Also it is not so clear why a relatively passive feeding behaviour might transform into a general shift in locomotion. Here, Hunt concurs with the view of Wrangham (1980) and Rose (e.g. 1981) in postulating that it would be more efficient to maintain an upright posture whilst shuffling from tree to tree, than to repeatedly shift from bipedalism to quadrupedalism and back again.

Perhaps the strongest criticism is against the evidential basis of the model itself. The study defined bipedalism “as locomotor or postural modes in which the body weight was supported principally by the hind limbs.” Hunt (1994:184.) This definition is broad enough to include any instance of upright posture, whether on land or in trees, supported or unsupported, whilst moving or whilst standing still and, as such, could be seen as rather too inclusive. Indeed it is only to be expected that such a definition would show that postural feeding from trees was the most likely scenario to induce such bipedalism, as it is rather difficult for a large primate to obtain food from trees in any other way. Strictly, by Hunt’s definition, even instances of normal terrestrial quadrupedal knuckle-walking might be classed as bipedalism if ever the centre of mass became closer to the hind limbs than the forelimbs such that more weight was placed on the hind limbs than the fore. It is obvious that Hunt did not classify such behaviour as bipedal but it begs the question: on what basis were such categorisations made? Furthermore, the vast majority (>90%) of the incidents of bipedalism reported by Hunt were postural and/or supported by the upper limbs. Almost none were purely for locomotion and unsupported. One is tempted to wonder exactly what reaching across for a fruit whilst holding on precariously to a branch in a relatively small tree has to do with human locomotion in the first place. It may therefore be argued that it might be more useful to look for even rarer behavioural contexts of bipedalism, but where that bipedalism is more likely to be unsupported and used during locomotion. If such behavioural contexts were found, Hunt’s premise that behavioural contexts that illicit bipedalism could offer clues as to the evolutionary pressure involving early hominid bipeds might be more illuminatingly applied.
1.1 Survival Value 6 (Fair) Hunt makes a good case that food procurement was a significant component in favour of survival.    
1.2 Sexual Selection 5 (Fair) This model was judged neutral by this criterion.    
1.3 Not Teleological 9 (Good) The postural feeding model is judged very good on this criterion as it is based on plausible primate behaviour rather than a human use for bipedalism.    
2.1 Improved Food Acquisition 9 (Good) As the model is based on food acquisition, it was judged good by this criterion.    
2.2 Accounts for Predation 7 (Good) As the model is postulating a fair degree of arboreality in the lives of early hominid bipeds, the pressure from predation is largely evaded.    
2.3 Why Apes are not Bipedal 7 (Good) Hunt’s hypothesis is that human ancestors inhabited forest edge habitats where smaller trees bearing smaller fruits were in greater numbers. It is postulated that this habitat shift was sufficient to lead at least one lineage to adopt bipedalism. However it does not meet the prediction that chimpanzees living in such habitats today should, consequently, be more bipedal than those living in denser woodland, such as bonobos.    
2.4 Extant Analogues 8 (Good) Hunt’s evidence is probably the best for extant apes. 80% of all bipedal instances were attributed to postural feeding. However, it should be noted that this accounted for only 3% of observed behaviour and was largely postural.    
2.5 Applies to Both Sexes 9 (Good) Hunts’ model applies equally to both sexes. Though large males are less arboreal than smaller juveniles and females, they are reported as being bipedal for different reasons.    
3.1 Hominid Anomalies 8 (Good) Hunt goes in to some detail on australopithecine anatomy to justify his model. He is one of the few authors to do so    
3.2 Fits Paleoecological Record 8 (Good) As most early hominid sites are associated with gallery forests bordering on grasslands the paleoecological evidence is supportive of his model.    
3.3 Precursor to Strider and knuckle Walker 4 (Fair) Although postural feeding does at least get early hominids into an orthograde posture and occasional movement on their hind limbs, a great deal of it must have been in trees and therefore much of the bipedalism most have been postural only.    
4.1 Extended Explanatory Power 5 (Fair) Hunt extends his model to a few areas beyond bipedalism. As with Jolly, some cranio-dental features are also expained this way.    
4.2 Complimentary 7 (Good) Hunt (1998) goes to some lengths to stress how his model is compatible and supportive of other feeding models. However it is somewhat contradictory to energy efficiency models and those involving threat display and carrying.    
4.3 Falsifiable or Testable 4 (Fair) Hunt’s model is based on empirical observations in extant apes and the evidence although it doesn't make falsifiable predictions.    
References Hunt, K D. (1994)The Evolution of human bipedality: ecology and functional morphology. Journal of Human Evolution 26:183-202.
Hunt, K D. (1996). The postural feeding hypothesis: an ecological model for the evolution of bipedalism. South African Journal of Science Vol:92 Pages:77-90.
Hunt, K D. (1998). Ecological Morphology of Australopithecus afarensis: Traveling Terrestrially, Eating Arboreally. In: Strasser, Elizabeth; Fleagle, John G; Rosenberger, Alfred; McHenry, Henry (eds.), (1998). Primate Locomotion: Recent Advances. Plenum Press (New York).
Jolly, C J. (1970). The Seed-Eaters: A New Model of Hominoid Differentiation Based on a Baboon Analogy. Man Vol:5 Pages:5-26.
Jolly, C J and Plog F. (1987) Physical anthropology and archeology. Alfred A Knoopf. New York.
McGrew W C, Baldwin P J and Tutin C E G. (1981) Chimpanzees in a hot, dry and open habitat: Mt. Assirik, Senegal, West Africa. J Hum Evol. Rose, M. (1991). The Process of Bipedalization in Hominids. In: Senut, B., Coppens Y; (Eds) Origine(s) de la bipedie chez les hominides. In: Coppens, Y., Senut, B. (eds.), (1991). Origine(s) de la bipedalie chez les hominides. CNRS (Paris). Tuttle, R. (1981) Evolution of Hominid Bipedalism and prehensile capabilities. Philosopical Transactions of the Royal Society London 292 (1057):89-94. Wrangham, R. (1980). Bipedal locomotion as a feeding adaptation in gelada baboons, and its implications for Hominid Bipedality. Journal of Human Evolution Vol:9 Pages:329-331.