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LESSON TWO – ALL YOU NEED TO KNOW BIOLOGICAL EXPLANATIONS OF EATING BEHAVIOUR Specification link: You will be able to outline and evaluate: Evolutionary explanations of food preference Outline and description Material for discussion, analysis and commentary Our feeding system is designed to identify the things we need and to reject things we either do not need or that might be dangerous such as toxins (poisons). During evolution we have evolved a digestive system suited to breaking down these foodstuffs into the nutrients we need so they can be absorbed into the bloodstream and metabolised by the body. Babies and young children show a range of taste preferences. In some cases this is very narrow. Our taste sensitivities are still influenced by our evolutionary past: Sweet – allows us to identify foods rich in carbohydrates such as sugars – a key source of energy Sour – identifies food that has gone off and may contain harmful bacteria Salt – critical to the normal functioning of cells in the body Bitter – associated with plant chemicals that might be harmful Umami – discovered relatively recently and represents a meaty or savoury quality indicating a good source of protein. The ability to distinguish between these tastes is adaptive as it is important for survival. There is a clear preference for sweet foods and these supply us with energy. Some of their dislikes are understandable; some vegetables such as broccoli and Brussels sprouts contain chemicals that can be toxic to the very young (Nesse and Williams, 1994). In adults taste preferences settle down and stay fairly constant with the exception of women in the early stages of pregnancy. Morning sickness is found in at least 75% of women and has a convincing evolutionary explanation – the embryo protection hypothesis (Profet, 1992). Surveys show consistently that the foods most avoided by pregnant women are coffee, tea, meat, alcohol, eggs and vegetables (Buss, 2008). Morning sickness is most severe in the early weeks of pregnancy, when the baby’s major body organs are developing and the baby is most vulnerable. Usually it then eases off and disappears as the baby becomes fully formed. Alcohol and coffee and tea (which contain caffeine) can damage the baby’s major organs. Meat and eggs are common sources of toxins such as bacteria, while some vegetables contain toxic chemicals harmful to the developing foetus. The sickness reaction therefore helps the mother to avoid foods that may be harmful, while vomiting prevents any toxins entering the mother’s bloodstream and affecting her baby. Babies can identify and distinguish between foods from an early age. They like sweet tastes. Furthermore, sweet foods are effective in reducing distress in babies (Benton, 2002). This suggests an innate (genetic) preference, possibly with adaptive purposes. Page | 1 Food neophobia is another evolutionary aspect of food preferences. Neophobia means ‘fear of the new’. Animals have a powerful tendency to avoid foods they have not come across before. Although this can lead to a dull diet, it means that we always eat food that we know is safe and avoid new foods that may be harmful. An aspect of neophobia is that we tend to show greater liking for foods as they become more familiar. Food preferences in babies, children and nonhuman animals are evidence for neophobia and the evolutionary advantage of eating food we know is safe. As we develop we learn to prefer those flavours that are associated with high energy foods. As Birch (1999) notes, this tendency would have been adaptive where high energy foods were in short supply (a common situation). Birch (1999) notes that at the point when a young child is just starting to eat solid foods, this neophobia response is minimal ant his may be because their access to food is largely controlled by adults. However, as they become increasingly independent (and can find food for themselves) increasing neophobia serves a protective function. The readiness to make quick This can be shown in humans, but has been associations with taste and dramatically demonstrated in animals. Garcia, pleasure/pain is also an evolutionary Rusiniak and Brett (1977) made wolves sick with advantage for us. Associative learning lamb’s meat contaminated with a mild poison means that unpleasant side effects of wrapped in a sheep’s skin. When allowed to eating something that does not agree approach live sheep the wolves left them alone. with us make it unlikely that we will They had learnt to associate the poison with sheep. eat that food again. Rats also show powerful taste aversion learning. If Associative learning makes us more poisoned at the same time as they are fed with both inclined to avoid foods that have a familiar and an unfamiliar food, they will in future made us ill in the past and also leads avoid the unfamiliar food, as their previous us to prefer flavours that have been experience tells them that the familiar food is safe. previously paired with a preferred taste. There is a clear preference for sweet foods and these supply us with energy. Humans are omnivores and are The shift to meat eating helps to explain the specialised to eat meat as well as selective pressure on brain evolution in order to fruits, nuts and plants. Humans cope with new technical and social skills. developed to eat meat about 6 million years ago (great apes such as chimps Fossil evidence on changes to our digestive system are usually vegetarian, but eat meat if supports hypotheses on the evolution of meat it is available). eating and the shift to hunter-gatherer society. Meat is a rich source of protein and However, there is a limited amount of fossil evidence, and evolutionary explanations are often this change in diet allowed the speculative. development of a larger brain, basic to the success of humans. Through evolution, other distinctive Biological explanations of eating behaviour eating behaviours have emerged: especially the evolutionary approach imply that Spices such as onions and garlic are much of our diet is driven by nature rather than used in cooking, especially in hot nurture (genetics rather than environment). This countries where food goes off quickly. ignores the cultural and social changes in food Spices contain chemicals that kill availability and choice over the years of human bacteria, so protect people from evolution. Although eating behaviour has a poisoning. biological and therefore ‘nature’ element’ it is also heavily influenced by environmental ‘nurture’ factors. Page | 2 Previous exam question June 2011 Outline and evaluate evolutionary explanations of food preference. (4 marks + 16 marks) Page | 3 AO1 = 4 marks Outline of evolutionary explanations of food preference There is a range of material available in this area. Candidates are likely to focus on taste preferences (preferences for sweet, salty and umami, avoidance of sour and bitter) and their adaptive significance. Other approaches might include the value of moving to an omnivorous diet and the significance of meat eating for the development for brain size and intelligence. Examiners must be alert to unfamiliar material that is in fact relevant to the question and creditable. Mark scheme AO2/AO3 = 16 marks Evaluation of evolutionary explanations of food preference Research studies on food preferences in babies, children and non-human animals would be a key source of AO2/AO3 credit. Methodological evaluation of studies may also be creditworthy, but may only earn marks beyond rudimentary if the implications for explanations are clear. Areas such as taste aversion learning and changes in food preference associated with e.g. pregnancy may be popular, but again can only earn credit if discussed in the context of evolutionary explanations of food preference. More general commentary may include changes in food preference with age and experience, and the possible role of associative learning, modelling and social/cultural factors. These must be discussed in the context of evolutionary explanations to earn AO2/AO3 marks. Generic evaluation of evolutionary explanations would also be relevant and creditable, while evolutionary explanations of eating disorders can earn marks if explicitly shaped towards the issue of food preference. Indicative issues/debates/approaches in the context of evolutionary explanations of food preference: evolutionary/biological approaches; behavioural approach (associative learning); social/cultural aspects; use of non-human animals in research; reductionism; free will/determinism. Such material must be used effectively to earn AO2/AO3 credit. AO2/AO3 material should first be placed in the appropriate band according to the descriptors. However, not all the criteria need be satisfied for an answer to be placed in a particular band. Weak performance in one area may be compensated for by strong performance in others. In order to access the top band, issues, debates and/or approaches need to be addressed effectively.