Tanner 1981
Classification: Behavioural models.
Mnemonic: Social Behaviour
Specific Model: Phallic Display
Original Proponent(s): Tanner 1981      
Assessment: Popularity: Behavioural Models were ranked 4th (out of 9) most popular in the texts reviewed. 6% of texts mentioned this idea specifically.
Simple: #41 / 42 (30%)  
Detailed #40 / 42 (35%)
Summary: Sexual selection from females for males due to phallic display was a major factor in the evolution of hominin bipedality.    
Discussion: One major difference between the genus Pan and Homo resulting directly from the way they generally move is the relative visibility of the sexual organs. In chimpanzees and bonobos, females in oestrus are easily identifiable by their tumescence, or exaggerated sexual swellings whereas in women any analogous, or any other visible signs of sexual receptiveness, are notable only by their absence. On the other hand, whereas male genitalia in Pan are usually obscured from view, whilst moving quadrupedally, a bipedal naked man has to take deliberate action if he wants to conceal his sexual organs from being displayed. That there is a clear difference between the orientation of the sexual organs in male and females Hominoidae directly dependent on their posture, upright or otherwise, is not in any doubt. The issue here, is one of cause and effect: Was this difference in sexual display a causal factor in the adoption of hominid bipedalism, or merely an unavoidable consequence?

Tanner is one adherent to the view that might have been the former, through some kind of sexual selection.

She wrote: “Much of the selection pressure engendered by the female choice of sexual partners was directed toward male social and communication behaviour, reinforcing the potential and capacity for sociability, social learning, and intelligence. Sexual selection also increased the contribution of genes from males who exhibited frequent bipedalism. A male's contribution to the gene pool necessitated his keeping up with the gathering females as they covered large expanses searching for food. Further, obvious visual cues such as a bipedal male's erect penis could have attracted female attention and action. [Even more directly, in the above context, sexual selection may have contributed to growth in the size of the male's penises. Homo sapiens males are quite well endowed] Such an image might appear amusing and improbable, but let us remember that these ancient forebears living in the warm African savannas had not yet invented clothing. As the female hormonal cycle and ovulation came to contribute less to timing of her arousal, it is not illogical that visual cues could become increasingly significant. If so, sexual selection for bipedalism would be yet another instance of natural and sexual selection together advancing the species adaptation farther along the same path for both females and males.”  Tanner (1981:165-166)
 Strengths: The contrast between female genital tumescence in quadrupedal chimpanzees and bonobos and increased penis size in bipedal human males and concealed ovulation in females is neatly incorporated into a model explaining the evolution of that shift in locomotion.    
Weaknesses: This model provides no benefit in survival value and the proposed advantage in sexual selection is, at best, speculative.    
1.1 Survival Value 0 (Poor) This model provides no additional survival value to individual hominins.    
1.2 Sexual Selection 9 (Good) Female selection of males due to penile display must be taken seriously in the context of this model. It was judged 'good' on that basis.    
1.3 Not Teleological 6 (Fair) This model was judged better than neutral.    
2.1 Improved Food Acquisition 0 (Poor) This model provides no possible mechanism of improved food acquisition.    
2.2 Accounts for Predation 0 (Poor) This model provides no possible mechanism of predator avoidance.    
2.3 Why Apes are not Bipedal 2 (Poor) As extant chimpanzees and bonobos have often been observed performing upright penile displays, it could be argued that this model does not satisfactorily demonstrate why upright posture could have been selected for only in the hominid lineage for this reason, and not in the apes.    
2.4 Extant Analogues 7 (Good)  As just stated, as some extent apes (in the genus Pan at least) have been observed performing upright penile displays, it does have some support in this criterion. However, even here, the penile displays have rarely been associated with movement, only posture
2.5 Applies to Both Sexes 2 (Poor) In terms of this criteria, this model must be rated poorly as it mainly favours female selection over males and not the other way around.
3.1 Hominid Anomalies 2 (Poor) The proponents of this model have not made any suggestion that this kind of behaviour might explain the ambiguous traits in australopithecines.    
3.2 Fits Paleoecological Record 5 (Fair) This model would appear to work in any ecological context and is not really helped by the commonly assumed notions of East African niches becoming more open and seasonal in terms of rainfall.    
3.3 Precursor to Strider and knuckle Walker 2 (Poor) This model does not propose any precursive mode of locomotion to human walking or knuckle-walking.    
4.1 Extended Explanatory Power 4 (Fair) As this model does imply changes to the socio-sexual system, from alpha male dominant (harem type) systems towards ones where females have more choice in their sexual partners, it does at least contribute more than just a proposed mechanism for bipedal change.    
4.2 Complimentary 4 (Fair) This model was judged compatible with most models, incompatible with wading models and complementary to models based on male provisioning.
4.3 Falsifiable or Testable 2 (Poor) There is no argument in the literature to  suggest that this model may be tested in any realistic way.    
References Tanner, Nancy Makepeace (1981). On Becoming Human. Cambridge University Press (Cambridge)