Eickoff (1988)
Classification: Feeding Models
Mnemonic: Hand to Mouth
Specific Model: Arboreal Predation
Original Proponent(s): Eickoff (1988)    
Assessment: Popularity: Feeding Models were ranked 2nd (out of 9) most popular in the texts reviewed.
Simple: #33 (=3) / 42 (48%)
Detailed: #26  / 42 (53%)
Basic Summary: Hominin ancestors became arboreal "site and wait" predators. This was a major factor in adopting bipedalism.    
Discussion: In a short, rarely cited, paper to the South African Journal of Science, in 1988, Renate Eickhoff suggested that arboreal predation was a likely precursive mode of locomotion for early hominid bipeds. It begins by reminding us that there are and have been many types of bipedal forms of locomotion which did not become human and suggests that bipedality may well have evolved in apes several times, and not just on the lineage leading to Homo sapiens. Consequently, it suggests that forms of bipedal locomotion may have first evolved much earlier than is currently thought “well before the Middle Miocene” (Eickhoff 1988:486.)

Several straightforward questions about the evolution of hominid bipedalism are posed in the title of the paper (“Origin of bipedalism – when, why, how and where?”) and perhaps the most relevant answer offered, as far as this work is concerned, to one of them is this one:

“Why did the bipedal mode evolve? Bipedalism frees locomotion from its dependence on forelimb participation; it renders hind limb locomotion autonomous. This may have been of selective advantage to sit-and-wait predators - slow moving animals that hunt from a stationary position. The shift to bipedal hunting would have allowed forward movement during the attack.” Eickhoff (1988:486)

However, Eickhoff notes that the bipedal posture cannot be maintained by non-human primates for extended periods as they do so in a flexed bent-hip, bent-knee posture. Therefore it is proposed that the maintenance of this upright posture was aided by the use of forelimbs, from overhead points of support in trees. It is proposed that early higher primates lived in the canopy of rain forest with an ecosystem with extremely dense vegetation serving a large enough community of primates for some to have specialised into a larger, more carnivorous role.

The postulated “sit-and-wait predator” (Eickhoff 1988:487) type, it is suggested, would have gained selective advantage from skeleto-muscular ‘redesign’ allowing for greater efficiency whilst maintaining upright posture during waiting phases. It is postulated that the precursive form of locomotion to bipedalism was “upright quadrupedalism” allowing apes to travel along “canopy highways” providing both substrates and superstrates in the form of branches, and that once populations of these apes had spread out of Africa to Asia and Europe, an abandonment of the ancestral, arboreal, niche led to a radiation into a variety of new niches, including terrestrial bipedalism.
 Strengths: The model has few obvious strengths but it could be argued that it does fit the recent paleoecological evidence placing early hominins in forested habitats.    
Weaknesses: The model can be criticised on several grounds.

Firstly, the best evidence of arboreal predation available to us in primates, that regularly practiced by Pan troglodytes on various species of primate, notably colubines, is characterised as anything but a ‘sit-and-wait’ type. ‘Hot pursuit’ is, perhaps, a more accurate description. Indeed it is difficult to imagine any form of animal taxa that would not be sensitive enough to the danger or arboreal predators to preclude them getting close enough to make a grab.

Secondly, the proposed advantage for bipedalism, that it allows the forearms to be used to grab prey during forward propulsion, is somewhat negated when that propulsion is being proposed in trees, so much that hanging from superstrates are required to maintain body posture.

Thirdly, there is clearly a negative feedback loop in terms of body size at work in arboreal animals. The larger the primate the more restricted is the available arboreal range. This effect would always act as a negative selection pressure on primate carnivory.

Fourthly, the fossil evidence of the earliest bipeds does not indicate a large degree of carnivory.
1.1 Survival Value 3 (Poor) The basis of the argument is that adaptations for a “sit-and-wait” arboreal predator would a) be selected for in early Miocene forests inhabited with many small primates to predate upon, and b) that these traits would somehow translate into a propensity to bipedalism. There is no evidence for either.    
1.2 Sexual Selection 5 (Fair) This model was judged neutral on this criterion.    
1.3 Not Teleological 8 (Good) This model cannot be seen as teleological as arboreal predation is not a normal human activity.    
2.1 Improved Food Acquisition 7 (Good) As the model theoretically proposes increased food procurement it was judged 'good'.    
2.2 Accounts for Predation 9 (Good) As the model is predicated upon the assumption that it occurred in arboreal predators, it is proposing that proto-hominids were the top predators in their arboreal niche.    
2.3 Why Apes are not Bipedal 4 (Fair) The model has ambivalent support here. As most great apes do not engage in arboreal predation and are not bipedal, it would appear to be supported. However, the ape most closely associated with that behaviour, Pan troglodytes, is no more prone to bipedalism than others, and is notably less so than Pongo and Hylobates.    
2.4 Extant Analogues 3 (Poor) Although there is good evidence that chimpanzees engage in arboreal predation, their mode of doing so is unlike the “sit-and-wait” model being proposed by Eickhoff and is not generally bipedal. Few other primate species have been associated with this behaviour.    
2.5 Applies to Both Sexes 3 (Poor) What evidence there is of extant ape predation (in chimpanzees), indicates that males are much more likely to engage in hunting behaviour than females. The model is therefore considered weak on this point.    
3.1 Hominid Anomalies 2  (Poor) Eickhoff makes no reference to the anatomy of the earliest hominids    
3.2 Fits Paleoecological Record 8 (Good) The recent trend in paleocological increasingly places early hominins in wooded habitats and so this model was judged highly here on that basis.    
3.3 Precursor to Strider and knuckle Walker 4 (Fair) Very arboreal apes, such as Hyblobates are amongst the most arboreal and so this model does gain some support on this criterion there. However, as predation is specifically not a good indicator of bipedality in such apes, the model was rated slightly worse than neutral.    
4.1 Extended Explanatory Power 2 (Poor) Eickoff does not attempt to explain anything other than bipedalism with her model.    
4.2 Complimentary 5 (Fair) This model was udged compatible with most other models, complimentary to those based on arboreality and contradictory to most based on open habitats or coasts.    
4.3 Falsifiable or Testable 4 (Fair) No falsifiable predictions were made  but the paper was framed in scientific way.    
References Eickhoff, R (1988). Origin of bipedalism - when, why, how and where?. South African Journal of Science Vol:84 Pages:486-488