Livingston 1962; Wescott 1967; Jablonski & Chapman 2004
Classification: Behavioural models.
Mnemonic: Social Behaviour
Specific Model: Intra-specific threat displays
Original Proponent(s): Livingston 1962; Wescott 1967; Jablonski & Chapman 2004;    
Assessment: Popularity: Behavioural Models were ranked 4th (out of 9) most popular in the texts reviewed. 25% referred to this idea specifically.
Simple: #33 (=3) / 42 (49%)
Detailed: #12 (=3) / 42 (59%)
Basic Summary: Increased bipedality through intra-specific threat displays was adaptive due to reduced aggression and violence    
Discussion: A behaviour in apes that has been popularly associated with a change to upright posture is the kind of social display adopted by gorillas when performing their characteristic chest beating. It was this sort of observation that first led Frank Livingston to publish, in the journal American Anthropologist, a model…

… to reconstruct the transition from ape to man along the lines of the mountain gorilla. The fact that they sometimes stand up to beat their chests and charge bipedally is seen as a model for which the earliest carnivorous hominids began moving” Livingston (1962:301.)

One of the basic assumptions of his idea was “that an open grassland or woodland environment was a necessary prerequisite for the development of bipedalism” Livingston (1962:301.) He went on to be much more specific, suggesting that it was at the edge of these two ecosystems where the process of bipedalization was likely to have occurred. Livingston quoted extensively from Audy and I shall do so too as it is interesting from the point of view of another model of bipedalism to be discussed later:

“Tracts of vegetation are naturally uniform over very large areas except where (a) altered soil drainage, or exposure, (b) high altitudes, c) presence of water edge, or (d) extensive destruction by animals (including man), interrupt these tracts and produce patches of zones of different vegetation. The contact-line between two types of vegetation has a special significance to the ecologists and the associated effects on organisms are known by the general term edge-effects. . . . One type of edge-effect is simply due to the fact that an animal living at or near the edge has access to two different types of habitat and food. All edges are potentially richer than the vegetation on either side” Audy (1957:191).

Livingston took this and applied it to a gorilla-based model of human evolution, assuming that gorillas were “edge-specific - or at least an edge preferring - species”, although this assumption seems to be based only upon the presence of mountain gorillas, away from the tropical rain forests, and that “gorillas have also spread extensively along rivers in the tropical forest, but this too is a fringe-habitat” Livingston (1962:302.) It is a view that is contradicted by many, including, ironically, the chief proponents of this model today, Jablonski & Chapman (1993:270.)

Whether extant gorillas today are actually edge species or not, Livingston’s main point is this: Upright threat postures which ‘free’ the arms to act as weapons, or as threatening devices, postulated to have evolved in early hominids living in ‘edge habitats’ bordering on woodland and savannah would be “remarkably pre-adapted to the carnivorous, bipedal, open grassland ecological niche which the early hominids appear to have occupied” (Livingston 1962:304.) His argument ends with speculation that reductions in normal food supply on the edges of shrinking woodland might have driven such hominoids to adopt such charging displays in order to begin to procure food through a carnivorous life style.

Later, in a brief letter to the same journal, Robert Wescott supported this view. He wrote: “. . . early hominid reactions [to extra-specific threats] must have frequently been agonistic. Part of this agonistic response undoubtedly consisted of two-legged standing or running, probably accompanied by fist shaking or arm-waving, and possibly involving the seizing and brandishing of sticks and stones. Agonistic exhibitionism, of course, is unlikely to have been the sole source of hominid bipedalism. The habits of carrying tools or removing scavenged meat from carnivores' kills and of rearing up to peer over savanna grass were unquestionably contributory to the abandonment of habitual quadrupedalism. But so too, I would maintain, was the habit of two-legged threat-display under circumstances in which such display had to be both frequent and impressive.” Wescott (1967:738)

Tanner also supported the idea in 1981. She wrote “standing and walking bipedally would also contribute to the effectiveness of defensive displays, as observed for chimpanzees. Transitional hominids, when displaying bipedally, would appear large, their arms could be waved while they vocalised, and sticks and rocks could be thrown. As intimidation this would be quite effective.” Tanner (1981:151)

The model has recently been promoted most seriously by Jablonski and Chapman.

Jablonski & Chapman: Origin of habitual terrestrial bipedalism in the ancestor of the Hominidae 1993
By examining ecological transformations in the ‘late-middle’ and late Miocene of East Africa, Jablonski & Chapman have proposed that bipedal threat display-appeasement behaviours (i.e. “stationary bipedal displays, bipedal charges and bipedal mock fights and deference to these behaviours,” Jablonski & Chapman 1993:272) may be of importance in understanding the origin of habitual terrestrial bipedalism. Like Livingstone, they suggested that as habitats became more open and desiccated, food resources would have become more widely separated and increased intra-specific competition among pre-hominids for resources would have resulted. They proposed that such bipedal displays were therefore essential to the success of pre-hominids in this environment, in that they allowed for the relatively peaceful resolution of intra-group and intra-specific conflicts.

That climate change in East Africa acted as a driver to the genesis of the hominid clade, is a very popular notion and few would argue with Jablonski & Chapman’s assertion that this change was probably the ultimate explanation of the origin of bipedalism: “In connection with the evolution of bipedalism, the ultimate cause is the environmental change in equatorial Africa described above which led to a fragmentation of the habitats of some Miocene apes and the establishment of ecological barriers between some ape populations.” Jablonski and Chaplin (1993:261)

Their argument starts, on logical grounds, in putting forward a framework of “theoretical considerations.” They suggest that any theory purporting to explain the origin of habitual [hominid] terrestrial bipedalism must:

• Be in accord with available environmental, paleontological, anatomical and behavioural evidence.
• It should account for the evolution of positional behaviour equally well between the sexes.
• It should also attempt to explain the cause of lineage splitting that led to the Hominidae. (It is interesting to note that these three criteria were independently selected here in the framework to evaluate bipedal origin models.)

Few would have serious disagreements with such a starting point. However, when it comes to the proposed proximal cause of hominid bipedal origins, in terms of specific behavioural changes which might have resulted from this climatic shift, Jablonski and Chapman differ quite markedly with the majority of workers who have commented on it.

Their first argument was to contradict the energy efficiency model of Rodman & McHenry (1980), and others. Several arguments were offered against it, including the fact that the original chimpanzee data that Rodman & McHenry based their study on, and which showed much greater energy consumption than humans, were juveniles which according to Kleiber (1961) and Bruhn & Benedict (1936) have much higher metabolic rates than adults. They state: “As Taylor & Rowntree (1973) suggested, no conclusions should be drawn from energetics to why one form of locomotion was preferred over the other” (Jablonski & Chapman 1993:262.)

They go on to make the very important point that, for any pre-bipedal ape, it is likely to be more costly to adopt an upright posture than a quadrupedal one, especially considering that the human-like full extension of the hind-limbs would probably not have been anatomically possible. At this point we get to the crux of their argument: If upright posture was more costly for a would-be early hominid than the alternative quadrupedal posture, what pre-adaptive behavioural might provide sufficient adaptive advantage to induce it to be done? Jablonski & Chapman reject Robinson’s (1963) and others’ view that sentinel behaviour (peering over grasses to look for prey and/or predators) and propose, instead, that “among pre-hominids, the adoption of bipedal standing in specific social contexts resulted in a significant selective advantage by relieving morbidity and mortality due to intra- and inter-group aggression” Jablonski & Chapman (1993:263.) Unlike the papers in favour of this idea that preceded them, which seemed to support the idea based only on speculation, Jablonski and Chapman offered, instead, several strands of evidence.

They constructed an elaborate phylogenetic case that the common ancestor of Pan/Gorilla/Homo exhibited bipedal displays (Jablonski & Chapman 1993:266-270.) They cited behavioural observations from the field of Pan troglodytes, Pan paniscus, Gorilla gorilla and even Homo sapiens, where bipedal social displays are important, to indicate that it was therefore most parsimonious to assume the last common ancestor also performed bipedal displays (Jablonski & Chapman 1993:271.) As they put it: “That bipedal displays are a shared derived characteristic of the African great ape lineage is without doubt. How this ancestral behaviour was developed into a uniquely derived characteristic of hominids needs further elaboration” Jablonski and Chaplin (1993:272.) This is what they attempted to do in the rest of the paper.

Their main argument was that “climatic deterioration of the late middle and late Miocene led to the gradual fragmentation of the forests of equatorial Africa and the fragmentation of the ranges of several species of ancestral hominoids. In the most marginal habitat of all evolved the ancestor of the Hominidae.” Jablonski and Chaplin (1993:272-273) How does this climatic deterioration translate into increased bipedal locomotion? According to the authors, the resulting habitats would have had food and water resources which were sparsely distributed and this would have lead to larger ranges being covered by their early hominid inhabitants. From this, “increased competition for resources probably ensued,” (Jablonski and Chaplin 1993:273) and this competition would have led to greater adoption of bipedal displays. The authors “suggest that bipedal displays and increased deference to them were the behaviours essential to the success of pre-hominids in this environment in that they allowed for the relatively peaceful resolution of intra-group conflicts, including those over scarce resources” (Jablonski and Chaplin 1993:273.) They conclude their argument with some speculations. Perhaps occasionally bipedal displays won an argument, without a fight, purely “by dint of their sheer novelty” (Jablonski and Chaplin 1993:274) and that this could have been followed by greater mating success in those that had performed them. The authors further speculate that female inter-group migrations would, having resulted from the off-spring of such encounters, act to spread the behaviour and associated traits across the wider population. And, that once this behaviour reached a critical mass, other benefits of bipedalism, cited here as alternative models in its origin, would have naturally resulted, further reinforcing the behavioural switch.

Jablonski & Chapman have recently continued to support this hypothesis with a chapter evaluating three theories of bipedal origins. They conclude: “The results of this study suggest that two of the most strongly favoured hypotheses for the evolution of habitual bipedalism, the food carrying hypothesis of Lovejoy (1981) and the arboreal feeding hypothesis of Hunt (1994, 1996) are untenable because they are not compatible with the anatomical evidence of the fossil record and because they propose activities that are extremely costly in terms of energy expenditure.” Jablonski and Chaplin (2004:290-291)
 Strengths: • This model offers a reasonble explanation as to why only hominids became bipedal, as it is postulated that the ancestors of Pan and Gorilla lived in relatively food-rich habitats.
• It is also certainly possible that behaviours which reduce fight-related morbidity could be selected for and, consequently, the traits which favour those behaviours.
• There is evidence in extant apes that this behaviour does, indeed, induce upright posture quite predictably.
• No anatomical change is required to elicit the proposed behavioural change, evidenced by the fact that all species of extant apes have been seen to exhibit postural bipedalism to some degree.
• These kinds of models tend to postulate the kind of ‘positive feedback loops’ which are often thought to underpin adaptive changes.  
Weaknesses: • The kind of behaviours being postulated are, at best, rather transitory. A number of workers have expressed scepticism a sporadic behaviour like a threat display or even wielding a club would become the habitual mode of locomotion. (e.g. Hewes 1961:696)
• One strong counter-argument to postural theories is that they do not really invoke much locomotion. In this model, at least some of it does include bipedal charging but, as the authors admit, most of it does not. Anatomical shifts to upright posture can be argued to aid bipedal locomotion but bipedal movement is bound to do so better. The question really is: How long do they stay on 2 legs after the behaviour?
• One direct challenge to the Jablonski & Chapman model is that it predicts that greater sparcity of resources should invoke a greater tendency to bipedalism. However, primate species which inhabit the harshest, most arid regions (e.g. patas monkeys, langurs) appear to be some of the most committed terrestrial quadrupeds in the order. More specifically in this regard, a comparison of chimpanzees and bonobos should, therefore, show a greater tendency for bipedalism in chimpanzees than bonobos for they tend to live in more open habitats. This does not appear to be the case.
1.1 Survival Value 6 (Fair) If one can assume that such displays might reduce injurious inter and intra specific competition, then it could be a feature that would be adaptive. It appears likely, especially, in alpha male dominated socio-sexual systems, that males able to avoid injurious competition through impressive bipedal displays, could be the most successful in passing on such genes which favour such behaviour.    
1.2 Sexual Selection 8 (Good) As such threat displays have ben observed in extant apes, it is likely that such behaviour could have been under strong sexually selection.    
1.3 Not Teleological 6 (Fair) Another strength of this type of model is that it does not require any anatomical features in place to begin the proposed evolutionary trend. The difficulty, if there is one, resides at the ‘other end’ of the evolutionary process: How does this display behaviour translate itself into a new form of locomotion?    
2.1 Improved Food Acquisition 5 (Fair)  This model was judged neutral with respect to this criterion.    
2.2 Accounts for Predation 6 (Fair) The postural display model also works well when posited as an inter-specific display. Some predators apparently are intimidated by upright posture displays and this too is likely to have had selective benefit. However erect posture alone is unlikely to have been a sufficiently strong deterrents in evolutionary timescales against large predators which were also battling for survival. ;  
2.3 Why Apes are not Bipedal 2 (Poor) This is a difficult area for these models as they draw upon fairly common behaviour in the extant great apes. The question must be posed: Then why did not other ape species become bipedal for this reason too? The answers to such questions usually propose that the hominid lineage underwent a harsher environmental regime than did the other great apes, and this increased competition required novel responses in this area.    
2.4 Extant Analogues 8 (Good) One of the strengths of these models is their ability to draw upon many, easy to obtain, examples from extant great apes.    
2.5 Applies to Both Sexes 3 (Poor) Although females of all ape species do perform bipedal displays from time to time, it is largely a male phenomenon.    
3.1 Hominid Anomalies 3 (Poor) Generally these models do not have much to say about early bipeds, except to suggest that such behaviours are likely in animals that are not yet anatomically adapted to fully upright bipedalism.    
3.2 Fits Paleoecological Record 6 (Fair) Most of the social display models are based, to some degree, on the agreed shift in habitat in East Africa, from forested towards more open savannah and suggest that increased competition in such habitats would have been greater and therefore selected for more.    
3.3 Precursor to Strider and knuckle Walker 3 (Poor) This model was judged poor by this criterion as it does not propose a plausible mode of locomotion that could act as a precursor to both human walking and chimp/gorilla knuckle-walking.    
4.1 Extended Explanatory Power 4 (Poor) Postural displays are, by definition, rather ape-like and ‘primitive’ in the context of modern human language and altruistic social systems. It is difficult to see how such models can be used to infer the evolution of other human social traits, although some authors have attempted to do so.    
4.2 Complimentary 7 (Good) Models of social behaviour tend to be among the most compatible of bipedalism models and are not contradicted by any.    
4.3 Falsifiable or Testable 6 (Fair) Jablonski & Chapman (2004) argue that the postural display hypothesis is not only testable but supports those tests better than any other. However few authors of other theories have been convinced by such arguments    
References Audy, J R (1957) Mosaic vegetation in relation to medical ecology. Abstracts & Papers. 9th Pacific Science Congress. Bangkok. pp 191-192.
Jablonski, Nina G; Chaplin, George (1993). Origin of habitual terrestrial bipedalism in the ancestor of the Hominidae. Journal of Human Evolution Vol:24 Pages:259-280.
Jablonski, N., Chaplin, G. (2004). Becoming Bipedal: How do theories of bipedalization stand up to anatomical scruntiny?. In: Anapol, Fred C; German, Rebecca Z; Jablonski, Nina G (eds.), (2004). Shaping Primate Evolution. Cambridge (Cambridge).
Livingston, Frank B (1962). Reconstructing Man's Pliocene Pongid Ancestor. American Anthropologist Vol:64 Pages:301-305.
Wescott, Roger Williams (1967). The exhibitionistic origin of human bipedalism. Man Vol:2 Pages:630.
Tanner, Nancy Makepeace (1981). On Becoming Human. Cambridge University Press (Cambridge).