Lovejoy 1981; Parker 1987
Classification: Behavioural models.
Mnemonic: Social Behaviour
Specific Model: Nupital Gifts
Original Proponent(s): Lovejoy 1981 ; Parker 1987     
Assessment Popularity: Behavioural Models were ranked 4th (out of 9) most popular. Lovejoy's "Provisioning" hypothesis was referred to explicitly in 50% of texts, more than any other.
Simple: #41 /42 (34%)
Detailed: #36 / 42 (44%)
Basic Summary: Similar to Lovejoy's "Provisioning" hypothesis but without any assumption of monogamy and proposing more on the sexual selection aspect instead.    
Discussion: Parker describes a model for “rapid evolution of bipedal locomotion as a male adaptation for nuptual feeding of females” (Parker 1987:235.) It starts with a comprehensive introduction to the subject of sexual selection, and a timely reminder that this was a major part of part of Darwin’s original thinking. The argument is developed that sexual selection has been implicated in speciation in primates elsewhere. “Hamadryas and anubis baboons are two closely related species, recently diverged from a common ancestor. Hamadryas males have diverged more from the behaviour and structure of anubis males than the hamadryas females have from anubis females.” Parker (1987:240) As this kind of sexual selection has been important in primate speciation in baboons, Parker suggests it might have also been a factor in ape-human divergence. However this argument is difficult to follow as there are no reports suggesting that either sex in humans are closer to either sex in chimpanzees or bonobos, as in the baboon analogy.

Parker’s sexual selection model for the origin of hominid bipedal locomotion is then laid out over the next four pages. It may be summarized thus:

If australopithecines (and A. afarensis specifically) were ancestral to humans, and there was a significant degree of sexual dimorphism in the species, and males would have better able to do more “wandering” for food than females. This would suggest that our ancestors were socially pre-adapted for greater male provisioning. As the smallest great apes, chimpanzees and bonobos, are most omnivorous, it is most parsimonious to assume that our ancestors, before the divergence from Pan, were too. The same kind of parsimony may be used to also imply that our ancestors were similarly characterised by promiscuous mating strategies. And, assuming that at around the time of the divergence some of that ancestral stock began living in more arid habitats, where food procurement was more difficult, it is likely that male hominids did proportionately more of that difficult procurement than females and would have used this to attempt to gain more sexual access to females. Equally, females would have selected males more on the basis of how much high energy, difficult to procure, foods they could provide.

Some of the assumptions in his line of reasoning may be challenged: For example: australopithecines may not be ancestral to Homo at all (e.g. Oxnard 1979;) Fossils attributed to Australopithecus afarensis may be from two species, not one sexually dimorphic one and our early hominid ancestors might not have lived in drier habitats at around the time of the divergence with the apes (e.g. WoldeGabriel et al 2001.) There is apparently a degree of overlap between Parker’s model and others. For example, like Lovejoy, it assumes that male provisioning was a key factor. “Bipedal carrying and presentation of nuptial gifts would allow them to accurately assess the size of the male and the size of the gift …” Parker (1987:243) And like Tanner and others, it assumes penile display could also have been important too: “…, bipedal locomotion would also allow females' to assess the size and tumescence of the male's genitals.” Parker (1987:244.) However, Parker himself distances his own model from that of Lovejoy’s on the grounds that it does not assume monogamous pairs.

(See also er (1987:243) And like Tanner and others, it assumes penile display could also have been important too: “…, bipedal locomotion would also allow females' to assess the size and tumescence of the male's genitals.” Parker (1987:244.) However, Parker himself distances his own model from that of Lovejoy’s on the grouSexual selection is a major pillar of Darwinism and this model reminds us of that. Possibly an improvement on Lovejoy's model because it does not assume monogamous pairs.
Weaknesses:/td> The baboon evidence on which the argument is made has no anologies in the great apes.    
1.1 Survival Value 3 (Poor) As female hominids are likely to have been able to provide sufficient food for themselves (contra Lovejoy's hypothesis) and their infants, it seems unlikely that relying on males to provide food bipedally could have provided much selection pressure for a new form of locomotion.    
1.2 Sexual Selection 3 (Poor)  Despite Parker's proposal that females would make sexual selection choices based on food provision, even without Lovejoy's monogamous pre-reequiste, this model is still judged "poor" on this criterion. The usual characteristics of features that have resulted in sexual selection are ones that distinguish between the sexes, appear only during sexual maturity and provide little other apparent selective advantage. Bipedal locomotion is conspicuous in failing all of these tests. Clearly, males and females, indeed even immature individuals, all practise bipedalism    
1.3 Not Teleological 6 (Fair) It can be argued that models invoking sexual selection provide ‘runaway' positive feedback loops that cannot be said to be teleological.    
2.1 Improved Food Acquisition 7 (Good) Assuming Parker is right that males could have found extra food to provide to females as 'nuptial gifts' then this model would have to be judged as 'good' by this criterion.    
2.2 Accounts for Predation 2 (Poor) As with Lovejoy’s provisioning model, this type of activity would appear to only make individuals more vulnerable to predation.    
2.3 Why Apes are not Bipedal 5 (Fair) This model was judged neutral by this criterion.    
2.4 Extant Analogues 2 (Poor) I could find no reports in the literature of bipedal male provisioning for females, for nuptual enticements, or any other reason. The model therefore appears to be entirely speculative.    
2.5 Applies to Both Sexes 2 (Poor) The very basis of Parker's model is male-centric and thus the model is judged 'poor' on this criterion.    
3.1 Hominid Anomalies 2 (Poor) No reference is made in Parker’s discussion about the australopithecine anatomy or how it might be better explained by this kind of behaviour.    
3.2 Fits Paleoecological Record 5 (Fair) This model was judged neutral by this criterion.    
3.3 Precursor to Strider and knuckle Walker 5 (Fair) This model was judged neutral by this criterion.    
4.1 Extended Explanatory Power 6 (Fair) Parker’s paper does set out, like Lovejoy’s, to explain other aspects of human evolution in addition to bipedalism    
4.2 Complimentary 7 (Good) This behavioural model very much overlaps with most carrying models and is therefore complimentary to them. Similarly it is rather contradictory to climbing models.    
4.3 Falsifiable or Testable 3 (Poor) Parker makes no such tests or predictions against which this model might be evaluated.    
References Lovejoy, C. Owen (1981). The Origin of Man. Science Vol:211 Pages:341-350.
Oxnard, Charles E (1979). The Relationship of Australopithecus and Homo: Another view. Journal of Human Evolution Vol:8 Pages:427-432.
Parker, S T (1987). A sexual selection model for hominid evolution. Human Evolution Vol:2 Pages:235-253.
WoldeGabriel, Giday; Haile-Selassie, Yohannes; Renne, Paul; Hart, William K; Ambrose, Stanley H; Asfaw, Berhane; Heiken, Grant; White, Tim D. (2001). Geology and palaeontology of the Late Miocene Middle Awash valley, Afar rift, Ethiopia. Nature Vol:412 Pages:175-178.