2.4.1: EVASION/SENTINEL BEHAVIOUR (aka "Meercat Peering")
Reynolds 1931; Dart 1959; Day 1977; Ravey 1978
Classification: Behavioural models.
Mnemonic: Social Behaviour
Specific Model: Evasion/Sentinel Behaviour (aka "Meerkat peering")
Original Proponent(s): Reynolds 1931; Dart 1959 ; Day 1977 ; Ravey 1978.    
Assessment Popularity: Behavioural Models were ranked 4th (out of 9) most popular in the texts reviewed. Sentinel behaviour was the most popular of the behavioural models, being mentioned specifically in 44% of texts reviewed.
Simple: #40  / 42 (35%)
Detailed: #36 (=2) / 42 (44%)
Basic Summary: Standing upright in open habitats would be adaptive to help hominins avoid predators and/or spot prey.    
Discussion: Predator avoidance is a major idea around which models regarding bipedal origins have been proposed.

Rose originally separated this into two separate groups: ‘Evasion’ and ‘Vigilance’ but here they will be treated as one.

Reynolds (1931) was the first to articulate this idea in the literature and Washburn (1960) suggested that it was man’s ability to use and make tools that drove the change in locomotion and Dart (e.g. 1949; 1959) specifically argued that it was for the use of weapons that standing upright would provide a visual advantage to survey the surroundings (1959:p223.) The more recent literature from proponents of this idea turns out to be rather thin, apparently comprising of just two very short pieces: one attributed to a transcribed lecture by Day (1977) and another, a letter to the journal Science a year later.

Day’s lecture ‘Locomotor Adaptation in Man’ itself only mentioned the idea briefly: “The lifting of the head high above the ground permits the long view, of clear value in open country, and the sense of smell lessens in importance.” Day (1977:150.)

Ravey’s (1978) short letter in the journal Science also articulated the view that, for a relatively short hominid, upright posture could be adaptive for predator detection in open habitats as an “early warning system.”
 Strengths: If one assumes that early bipedal hominids lived in rather open habitats where vigilance from predators was an activity of high importance, then being upright is likely to have some selective advantage.    
Weaknesses: 1) Some authors have made the obvious counter-point that whereas being upright might indeed enable a hominid, out in open grassland, to see its potential adversaries (and potential prey) better, it will certainly make it more visible to them. For example Tanner (1981:150) noted that “Predators now [in her speculation that australopithecines had moved to more open, savannah habitats] posed a problem for this diurnal omnivorous primate, which was now obtaining a large proportion of its food in the relatively open African savannah where predator pressure was high. Their upright posture and developing bipedal locomotor pattern meant that the transitional hominids were more visible to predators.”

2) A number of workers have expressed scepticism that a sporadic behaviour like a threat display or even wielding a club would become the habitual mode of locomotion. For example, Stanford asked “why is it important to be permanently upright? Standing upright for just a few seconds would achieve the same results,” Stanford (1999:45). Others have cited evidence that no other species, living in open habitats, have adopted bipedalism as an evolutionary strategy for this reason. Hewes (1961:695), for example, suggests that “it is curious, however, that the [vigilance] advantages of bipedalism have not led to a more widespread adoption of this habit among both predators and their prey in grassland environments.” This is a point that could be echoed against almost every model of hominid bipedal origins but, it would seem that in this case, the answer to that particular quandary is simply that when it comes to escaping a predator, four legs is better than two. Also, if early hominins evolved in more closed woodland it is difficult to see how this model might work at all.
1.1 Survival Value 4 (Fair) Being upright may only provide selective benefit to hominids in certain, rather open, habitats. If bipedalism evolved elsewhere this argument would be far less important.    
1.2 Sexual Selection 5 (Fair) This model was rated neutral by this criterion.    
1.3 Not Teleological 6 (Fair) As this kind of upright posture is a potentially beneficial behaviour in its own right, it does at least provide a plausible means for hominids to start adopting those postures in the first place. Where it has difficulty is in how it might explain how such posture might become the normal mode of locomotion    
2.1 Improved Food Acquisition 5 (Fair) This model was rated neutral by this criterion.    
2.2 Accounts for Predation 3 (Poor) The model is somewhat self defeating because whatever benefit, in terms of seeing predators, might be derived from hominids being upright, would surely be lost by making them far more visible to those same predators    
2.3 Why Apes are not Bipedal 4 (Fair) Assuming that early hominids split from the African great ape ancestors through moving into open grassy habitats, this model does provide a reasonable answer as to why other apes might not have become bipedal too. However, chimpanzees, which live in the most open of all the habitats of the apes might therefore be expected to be the most bipedal. This does not appear to be the case.    
2.4 Extant Analogues 4 (Fair) Upright posture for extra vigilance has not, as a general phenomenon, been observed in extant apes in studies (e.g. Hunt 1994.)    
2.5 Applies to Both Sexes 8 (Fair) This model works equally well for both sexes.    
3.1 Hominid Anomalies 3 (Poor) As the paleohabitats associated with A. afarensis and their putative ancestors are generally “wet and wooded” and not specifically open, it is difficult to support this model from the perspective of the fossil evidence. Also, the specific traits of australopithecine postcranial anatomy do not appear to be adaptive to this particular behaviour.    
3.2 Fits Paleoecological Record 3 (Fair) The vigilance/predator avoidance models are largely based on open, more savannah-like, habitats. It has increasingly become apparent that this is not supported by much evidence.    
3.3 Precursor to Strider and knuckle Walker 3 (Fair) This model does not propose any particular precursive mode of locomotion that is compatible with human walking and knucle walking.    
4.1 Extended Explanatory Power 4 (Fair) As this model does postulate that much of the predator avoidance behaviour would have been co-operative it is, at least, compatible with models of human evolution which posit greater altruism and sociality.    
4.2 Complimentary 6 (Fair) This model was judged compliemntary to those that also promote open habitats and contradictory to those that promote wooded habitats.    
4.3 Falsifiable or Testable 2 (Fair) None of the proponents of this model have attempted to make testable predictions and, as already mentioned, it would appear to fail a simple one: that chimpanzees should be more upright than bonobos.    
References Dart, Raymond A (1949). The Predatory Implemental Technique of Australopithecus. American Journal of Physical Anthropology Vol:7 Pages:1-38.
Dart, Raymond A; Craig, Dennis (1959). Adventures with the Missing Link. The Institution Press (Philadelphia).
Day, Michael H (1977). Locomotor Adaptation in Man. Biology and Human Affairs Vol:42 Pages:149-151.
Hunt, K. The Evolution of human bipedality: ecology and functional morphology. Journal of Human Evolution 26:183-202, (1994).
Ravey, M (1978). Bipedalism: An Early Warning System for Miocene Hominoids. Science Vol:199 Pages:372.
Reynolds, E. (1931). The evolution of the human pelvis in relation to the mechanics of the erect posture (Rose). The Museum (Cambridge).
Washburn, Sherwood L (1960). Tools and Human Evolution. Scientific American Vol:203 Pages:63-75