Classification: Habitat Compulsion
Mnemonic: Snow and Mud?
Specific Model: Walking on Snow/Mud
Original Proponent(s): Köhler (1959)    
Basic Summary: moving on muddy substrate (or snow) lead to increased bipedality.    
Assessment Popularity: No texts reviewed mentioned this idea. Although it is categorised here under habitat compulsion no popularity ranking is offered under the "non-wading habitat compulsion" subcategory because all the texts that referred to those were discussing arboreal models.
Simple: #41 / 42 (29%)
Detailed: #41 / 42 (32%)
Discussion: One of the oddest-looking models listed in Rose’s (1991) classification was one labelled "walking on snow and mud". However, looking through the original literature for this source it would appear that Khöler’s only reference to anything remotely related to this was a report that amongst the chimpanzees he studies “upright walking (without brachial support) takes place when the hands are full, when the ground is wet and cold, or when the animals are excited in various ways” (Khöler 1959:313.)

A similar observation was also made by Van Lawick-Goodall:
“Chimpanzees frequently stand upright in order to look over long grass or other vegetation. Sometimes a branch or tree trunk is held with one hand, but often both arms hang down at the animal's sides. These apes frequently walk bipedally for short distances: (i) whilst moving through long grass when looking for an unusual object of searching for a companion, (ii) when it is raining hard and the ground is wet and (iii) when carrying food in one or both hands.”
Van Lawick-Goodall (1968:177)

It would appear that this observation would have to be interpreted in a grossly exaggerated way, if one was to take it as a potential model for hominin bipedal origins and it is not my intention to do so here. It is included only for completion sake, as Rose (1991) listed it.
 Strengths: This idea has few strengths other than some evidence for the proposed behaviour in extant apes.    
Weaknesses: There are many problems with this idea on human bipedal origins, notably a distinct lack of proposed improvment in fitness, difficulties in fitting with the paleoecological record.    
1.1 Survival Value 0 (Poor) The idea offers no improvemeent in survival value.    
1.2 Sexual Selection 0 (Poor) No increased sexual selction is proposed by this idea.    
1.3 Not Teleological 7 (Good) The idea is good here, in that it does not invoke a purely modern human response to the scenario being proposed.    
2.1 Improved Food Acquisition 0 (Poor) The idea offers no improvement in food procurment.    
2.2 Accounts for Predation 3 (Poor) The idea offers little extra in the way of predator avoidance.    
2.3 Why Apes are not Bipedal 3 (Poor) This idea is poor in explaining why only humans became obligate bipeds.    
2.4 Extant Analogues 7 (Good) Both Köhler and Van Lawick-Goodall reeported good evidence form chimpanzees in favour of this idea.    
2.5 Applies to Both Sexes 9 (Good) The idea applies equally to both sexes.    
3.1 Hominid Anomalies 0 (Poor) The idea does not account for any early hominin postcranial anatomical anomalies of bipedality.    
3.2 Fits Paleoecological Record 3 (Poor) Assuming that the model does propose 'snow' as part of the scenario beingg proposed it was judged poor by this criterion. If simply 'mud' or wet ground was taken, it would have been judged 'fair'.    
3.3 Precursor to Strider and knuckle Walker 5 (Fair) This idea was judged neutral by this criterion.    
4.1 Extended Explanatory Power 0 (Poor) No other explanation of Pan-Homo divergence was explained this way.    
4.2 Complimentary 4 (Fair) The model was judged complimentary to most models but contradictory to some.    
4.3 Falsifiable or Testable 0 (Poor) No falsifiable predictions appear to have been made to support this idea.    
RReferences Köhler W. (1959) The Mentality of Apes. London: Routeledge.
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).
Van Lawick-Goodall, J (1968). The Behaviour of Free-living Chimpanzees in the Gombe Stream Reserve. Animal Behaviour Monographs Vol:1(3) Pages:161-311