2.2.1: INTER-SPECIFIC THREAT DISPLAYS Kortland 1980		Home
Classification: Behavioural models. Mnemonic: Social Behaviour
Specific Model:	Inter-Specific Threat Displays
Original Proponent(s):	Kortland 1980
Assessment	Popularity: Behavioural Models were ranked 4th (out of 9) most popular in the texts reviewed. However only 1 (out of 36) referred to this idea specifically. Simple: #19 (=4) / 42 (57%) Detailed: #14 (=4) / 42 57%
Basic Summary:	Bipedalism evolved through the need for hominins to exhibit inter-specific threat displays.
Discussion:	Kortland reported a study of the use of thorny branches and used it to support the idea that the use of such weapons in early hominids could have helped defend themselves effectively against their natural enenmies. As early hominins were "too small and too weak for throwing big stones or weilding heavy sticks in the chimpanzee manner" (Kortland 1980:79) it was suggested that thorny branches could have been suitable. Empirical data was presented to support the idea and Kortland used it to argue that the evolution of this behaviour may have started among hominins living in dry woodland and would have promoted bipedalism.
Strengths:	Provides a plausible means of predator avoidance in conjuction with bipedal posture. Based on an empirical study.
Weaknesses:	The idea is based upon relatively short-term behavioural response to predators. It is difficult to extrapolate this into a mode of locomotion. Predicated on savannah habitats.
Evaluation:
1.1 Survival Value	6 (Fair) This model proposes a means of survival that is relatively unimpressive (compared to some others) and predicated on very small, vulnerable hominins. It is consequently rated only slightly better than neutral.
1.2 Sexual Selection	5 (Fair) This model is rated neutral on this criterion.
1.3 Not Teleological	6 (Fair) The use of thorny branches is a plausible intermediate weapon so this model is rated higher than neutral on this criterion.
2.1 Improved Food Acquisition	5 (Fair) This model is rated neutral on this criterion as food acquisition is neither helped nor hinderered by it.
2.2 Accounts for Predation	8 (Fair) This model is rated strongest on this criterion as predation avoidance is the most significant proposed problem it is claimed to ameliorate.
2.3 Why Apes are not Bipedal	2 (Poor) As extant apes have often been observed using branches in threat displays and yet have remained quadrupedal, this model is rated poor by this criterion.
2.4 Extant Analogues	5 (Fair) This model is rated neutral on this criterion.
2.5 Applies to Both Sexes	7 (Good) This model is rated good on this criterion as both females and males could have used thorny branches as weapons.
3.1 Hominid Anomalies	5 (Fair) Kortland does base his idea on the anatomy of early hominin bipeds but does not go into much detail of anatomical anomalies.
3.2 Fits Paleoecological Record	6 (Fair) This model is based upon the paleoecological record of early hominins so is rated better than neutral.
3.3 Precursor to Strider and knuckle Walker	2 (Poor) The model does not provide a realistic precursor to human walking and so is rated poor.
4.1 Extended Explanatory Power	4 (Fair) This model is rated lower than neutral on this criterion.
4.2 Complimentary	7 (Fair) This model was judged to be complimentary to most carying models and also compatible with most arboreal ones too.
4.3 Falsifiable or Testable	6 (Fair) The paper on which Kortland's model is based reported empirical data that was supportive, however it lacked falsifiable predictions.
References	Kortlandt, Adriaan (1980). How might early hominids have defended themselves against large predators and food competitors? Journal of Human Evolution Vol:9 Pages:79-112