The Meat-for-Sex Hypothesis
The hypothesis that chimpanzees trade meat for sex has been frequently cited by anthropologists seeking to understand the evolutionary roots of human behaviour. In this essay I will explore the validity of this hypothesis and explore the link between chimpanzee behaviour to modern human male and female relationships. The meat-for-sex hypothesis, originally proposed by Tutin (1979), focuses on the suggestion that male chimpanzees exchange the meat they hunt and catch for sex from female chimpanzees. Mitani and Watts (2001) and Gilby (2006) oppose the meat-for-sex hypothesis in terms of short-term interactions after meat transfer, based on their data of group hunting and meat sharing. However, adopting a different approach, Gomes and Boesch (2009) support the hypothesis on a long-term basis, using data from a 22-month investigation from Tai National Park, where they showed that female chimpanzees copulate more frequently with males who share meat with them, forming a long-term exchange. If proven genuine, the hypothesis is able to shed light on the psychological nature of chimpanzees and further, expose the evolution of male and female relationships in human society today. The claim that chimpanzees trade meat for sex is important for human evolution because it raises the question of whether human meat-for-sex exchanges can be traced back to a pre-human ancestry (Gilby 2010).
For chimpanzees, meat is a highly valued resource because it is high in protein and micronutrients, and can often be hard to acquire. Although meat is clearly valued by all chimpanzees, and costly to obtain, possessors are often willing to share with their prize conspecifics. The question is, why do chimpanzees, often associated as as aggressive and egoistic species, cooperatively share such a valuable commodity? The meat-for-sex hypothesis was first proposed by Tutin (1979), who suggested that female chimpanzees showed a preference for males who spent the most time with them, and who showed the highest levels of affectionate behaviour, such as grooming and food sharing. Stanford (1994) also found that male chimpanzees hunted red colobus at least partially to obtain meat for use as a tool to attract sexually receptive (swollen) females. His investigation found, among social variables such as the hunting party size, the number of males in the hunting group, and so on, that the presence of one or more swollen females was the best predictor for chimpanzees to decide on monkey hunting. Despite the lack of meat-sharing evidence, this idea gained popularity under the name of the “meat-for-sex” hypothesis. The meat-for-sex hypothesis proposes that male chimpanzees share meat with females in exchange for mating access. Specifically, the meat-for-sex hypothesis states that “… the hunting performance of chimpanzees may be under sexual selection for capture and/or control of meat as a way to obtain additional copulations from females with estrous swellings in the hunting party” (Stanford 1996 p.101). If this is the case, that chimps base their hunting decisions on the potential for trading social favours, then they must possess a level of cognitive ability, by tracking relationships and anticipating the consequences of hunting and sharing. Meat-for-sex exchanges among chimpanzees is treated as a well-established, species-typical phenomenon, and provides the basis for various speculations about the evolution of human sexual behaviour (Gilby 2010). However, because of the lack of empirical support, the hypothesis that male chimps hunt in order to exchange meat for sex, has been widely disputed.
Mitani and Watts (2001) state that their investigations at Ngogo showed no impact of the presence of swollen females on hunting probability. Mitani (2013) argues that the meat-for-sex hypothesis lacks widespread empirical support, preferring the hypothesis that proposes that male chimpanzees share meat strategically with others in order to build and strengthen social bonds between them. He states that meat is more commonly shared reciprocally with males exchanging meat for coalitionary support (Mitani and Watts 2001). Chimps tactically create such bonds so that in times of need, their ally will offer joint aggression in defense against a rival. He records how Ntologi, a “particularly clever alpha male, shared meat non-randomly and selectively with other males, who in turn supported him in longer-term alliances.” (Mitani 2013 p.410). Making these alliances helped Ntologi maintain his position at the top of the dominance hierarchy for over sixteen years. Male chimpanzees are the most frequent participants in meat sharing periods; males swap meat methodically with specific individuals, and sharing is evenly distributed between pairs. Additional investigations indicate that males also trade grooming for meat (Mitani 2013). Mitani and Watts conclude that the observations at Ngogo provide support for the male social-bonding hypothesis rather than the meat-for-sex hypothesis.
Arguing in defence of the meat-for-sex hypothesis, Cristina Gomes and Christophe Boesch (2009), primatologists at the Ma Planck Institute for Evolutionary Anthropology in Germany, have published a detailed investigation aiming to prove that male chimpanzees do in fact exchange the meat they hunt for sex from swollen females. They argue that previous attempts to record the hypothesis have failed simply because researchers were looking for a direct exchange, where a male shared meat with a female and copulated with her immediately after. However, Gomes and Boesch have adopted a different approach, treating the meat-for-sex hypothesis as a long-term exchange. They found that males continue to share their catch with females even when they are not fertile, copulating with them when they are. Admitting that there has previously been an insufficient amount of evidence to support the meat-for-sex hypothesis, they demonstrate that female wild chimpanzees copulate more frequently with those males who, over a period of 22 months, share meat with them (Gomes and Boesch 2009). They tested meat sharing and copulation frequency in a group of wild chimpanzees in the Tai National Park between 2003 and 2006, in order to assess the validity of the hypothesis. The group consisted of 49 individuals, 5 adult males and 14 adult females. Across the 22-month period they found that male and female wild chimpanzees do exchange meat for sex; males who shared meat with females doubled their mating success, and females, who found it difficult to obtain meat on their own, increased their caloric intake, without suffering the energetic costs and potential risk of injury related to hunting. Moreover, when male chimpanzees shared with sexually unreceptive chimp, these females eventually repaid the generosity. Gomes and Boesch propose that in chimpanzee populations where female choice is present and hunters can usually control the sharing of their catch “male and female wild chimpanzees will exchange meat for sex over long periods of time”.
Conflictingly, Gilby (2006) argues that from observations of Gombe chimpanzees, groups containing swollen females were significantly less likely to hunt than those without swollen females, in fact, stating that males face a decision between hunting and mate guarding, signifying meat-or-sex rather than meat-for-sex. There was a similar negative trend at Kanyawara (Gilby and Wrangham 2007). Additionally, Gilby showed that Gombe males did not share preferentially with swollen females, nor did sharing increase the probability of mating during a meat-eating period. Furthermore, contradicting Mitani’s conclusion, no evidence was found that male chimpanzees swap meat for grooming or social allegiance. Gilby instead proposes that meat sharing is an example of ‘tolerated scrounging’ where the male chimpanzees observed choose to share rather than suffer the costs imposed by persistent beggars (Gilby 2006 p.961). At Gombe, harassment predicted the probability, amount and mode of sharing while copulation and grooming patterns did not. Therefore, with an understanding of the potential nutritional and social benefits of attaining meat, Gilby encourages us to conclude that there is no difference between the way chimpanzees and other social predators hunt and consume food in groups. There is no strong empirical evidence to support the claim that chimpanzees hunt for social benefits such as sex or grooming. Gilby acknowledges that Gomes and Boesch’s findings supposedly demonstrate long-term correlation across male and female meat sharing and copulations, yet finds their conclusions “mysterious” because it implies that the motivation of estrous females to copulate was not affected in the short-term by receiving meat, whereas it was affected in the long-term. Gilby prefers to only be concerned with the question of whether chimpanzees perform these exchanges within the immediate context of meat eating (Gilby 2010).
However, as stated before, Gomes and Boesch maintain that “short-term exchanges alone cannot account for the relationship between sharing meat and mating success”. Throughout their paper they acknowledge that previous studies on other chimpanzee populations have shown that the presence of swollen females does not increase the probability of hunting more frequently (Ngogo) and in some cases actually decreases hunting probability (Gombe). Yet they still maintain that previous investigations might not have found a relationship between mating success and meat sharing because they have only focused on more short-term exchanges. Previous studies may not have found a relationship between mating success and meat sharing because they focused on only short-term exchanges, or perhaps because in such groups female choice was rare, hence, exchanging meat-for-sex was not a viable option. They conclude that further studies on other chimpanzee communities, which take into account the degree of female choice in the group and the capacity of hunters to control the sharing of their catch, will determine whether such exchanges can be generalised to the species. Their investigation has been widely accepted by experts as conclusive, revealing the ever puzzling nature of chimpanzee relationships. The study was rigorous and complex; factors such as male and female rank, female gregariousness and female begging behaviour were statistically controlled. Anne Pusey, director of the Jane Goodall Institutes Center for Primate Studies, states that it’s the first study to show such a strong statistical correlation. She adds that it’s not just the female chimpanzees that are thinking long-term; males may also be investing in future offspring by making sure their mates are well-fed.
As Boesch points out, social interactions are not random within chimpanzee society. Much of it is based on memory and long-term relationships. In the way that they can predict behavioural responses and act accordingly, chimpanzees have a coherent understanding of the psychological functioning of others. Gomes and Boesch have successfully provided evidence that chimps understand others as goal-directed agents who also perceive the world in order to devise behavioural strategies for meeting those goals. Melis, Hare and Tomasello (2008) performed an experiment in order to determine whether or not chimpanzees have the capability to remember and make use of the memory of past collaborative efforts in what they call ‘contingency-based-reciprocity’. The experiments were carried out on orphaned chimps at the Ngamba Island Chimpanzee Sanctuary. They involved locked rooms, a wooden key, empty food bowls, carefully selected ‘nice’ and ‘mean’ collaborators, as so on. The conclusions revealed that chimps do have the capability of contingency based reciprocity, in sometimes deliberately choosing collaborators who had been of help before. The remarkable co-operation displayed in chimpanzee society may provide useful insights into the origin of sharing in humans. Chimp communities, similarly to human societies, involve trade and complex relationships. Gomes and Boesch state that their findings “also sheds light on our current knowledge on meat sharing in humans”. They suggest that the increased reproductive success of accomplished hunters compared to unsuccessful hunters in forager societies could be driven by female choice, and be linked to direct exchanges of meat-for-sex between men and women.
Upon further research, it was interesting to discover the inclination to associate food with sex in exchanges of food for sex in male and female relationships, recorded in John M. Ingham’s book ‘Psychological Anthropology Reconsidered’. He explains the inclination to associate food with sex among hunter gatherers, for example, suitors and husbands often give food to their wives and parents-in-law. Marriage and sexual relations with one’s wife depends on sharing meat from the hunt. Janet Skiskind (1973) further observed that successful hunting implies virility in Eastern Amazonia. The successful hunter may attract extra wives whereas the unsuccessful hunter is often rejected by women. This follows the basic idea of an exchange of meat for sex. Thomas Gregor (1985) also found many associations between food and sex among the Mehinaku of central Brazil where sexual appetite is compared to hunger, and sexual intercourse to eating. Mehinaku women are said to “use their sexuality to secure food and support in exchange for intercourse” (Gregor 1985 p.33). Men secretly offer fish to their lovers as a symbol of their productivity and sexuality (Gregor 195 p.75). It is fascinating to discover that the sexual connotations of food, and food sharing, follow that of chimpanzees.
Experts agree that Gomes and Boesch’s investigation offers strong proof for the meat-for-sex theory, which has been previously significantly debated. We can therefore conclude that wild chimpanzees exchange meat-for-sex, and do so on a long-term basis. Gomes and Boesch state that similar studies on humans will determine if the direct nutritional benefits that women receive from hunters in foraging societies could also be driving the relationship between reproductive success and good hunting skills (Gomes and Boesch 2009). Moreover, further studies on other chimpanzee communities, which take into account the degree of female choice in the group and the capacity of hunters to control the sharing of their catch, will determine whether these exchanges can be generalised to the species. We can conclude however, that chimpanzees continue to serve as models of human behavioural evolution.