A quick perusal of the backs of breakfast cereal packets highlights our growing obsession with ‘essential nutrients’. But do animals naturally balance their diets? If so, how do they manage to do so without a nutritionist peering over their shoulders? And what about our ancestors? Have dietary choices influenced the course of human evolution?
Recent studies suggest that most animals are surprisingly good at choosing a balanced diet. Wild baboons living in Africa, studied by Andrew Whiten and Dick Byrne and their colleagues at the University of St Andrews, tend to eat plants, and even parts of plants, that are high in proteins and low in fibre and the toxins such as alkaloids that plants produce to discourage herbivores. The baboons manage a fine balance: they will eat plants that contain relatively high densities of the toxins only if the plants are also rich in proteins, and will ignore other plants that are lower in toxins but also lower in protein. But their choosiness is tempered by what is on offer. As food supplies deteriorate, they become more willing to tolerate poorer quality items. In other words, they try to do the best they can by balancing the nutritional costs and benefits of particular plants.
Evolutionary ecologists have always assumed there is a direct link between an animal’s ability to match an optimal diet and its evolutionary fitness – that is, its ability to contribute descendants to future generations. But the timescale involved and the complexity of biological life histories has always made it difficult to prove a relationship between the two. Few have been tempted even to try. However, a recent study by Stuart Altmann of the University of Chicago suggests for the first time that such a relationship does exist.
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Fifteen years ago, Altmann began a long-term study of a group of infant baboons living in Kenya’s Amboseli National Park. He began by recording their diets at about 12 months of age when the infants start to feed themselves. Using optimality modelling, he identified the diet that simultaneously maximised energy intake while remaining above the minimum for other nutrients and below the tolerable maximum for plant toxins. He then compared the extent to which each of his infants optimised its diet with their subsequent histories over the next 13 years, by which time all but two of his sample of female infants had died.
It turns out that the best predictor of several different measures of genetic fitness in females – including length of reproductive lifespan, total number of births and the number of infants born that survived to one year – was how close the animals came to optimising energy and protein intake when they were just 12 months old.
Altmann remains uncertain about the reasons for this. ‘Do some animals have higher fitness because they eat better,’ he asks, ‘or do they eat better because they are more fit?’ He suspects that it is a bit of both because both optimal diet matching and lifetime reproductive performance may be related to a common biological factor. In particular, he notes that the rate at which a female has babies is not linked to her diet as a yearling. Indeed, in this population at least, lifetime reproductive output is most directly related to longevity. So an animal that starts off badly, either through accidents of illness or through inadequate maternal nutrition, may not only have a less than optimal diet when it is 12 months old (because it cannot feed as effectively) but may also have a shorter life expectancy (because it is more prone to disease or predation), so having lower genetic fitness.
Such studies remind us just how fine the thread of life can be. That we are here at all is, in part, a tribute to our ancestors’ abilities to get it right. Attempts to understand the diets of prehistoric humans tend to underline this. At the University of Cambridge, nutritionist Stan Ulijaszek has been trying to model the diets of humans at around the time of the transition to agriculture, at the start of the Neolithic period between 7000 and 10 000 years ago.
We know from the archaeological record that during the transition to agriculture people were under substantial nutritional stress. A comparison of various skeletal indices from burials in the Middle East that predate the transition with those from agricultural populations a few thousand years later reveal that the later populations were, on average, shorter (by about 10 centimetres), lighter (by about 7 kilograms) and more susceptible to skeletal diseases linked to nutritional stress such as osteoporosis. By modelling the nutrients in the diets suggested by the prehistoric refuse, Ulijaszek has been able to show that the people in the later agricultural period suffered chronic energy deficiency and ate little meat.
How could this have come about? And why should people shift from what seems to have been a perfectly balanced diet as pre-Neolithic hunter-gatherers to a less than perfect diet as agriculturalists? Prehistorians think that rising population densities may have been the critical factor. Only by shifting to the higher production rates possible from agriculture could these populations in the fertile Levant have maintained themselves. Clearly, they were just too successful as hunters and gatherers. What remains uncertain is whether it became difficult for a relatively small number of hunters to provide sufficient food for a rapidly growing population or whether the large human population had so reduced the natural wildlife so as to make hunting difficult.
The value of interdisciplinary studies of this kind has been further highlighted by the collaboration between Rob Blumenschine, an archaeologist at Rutgers University, and Tim Caro, an ecologist at the University of California, Davis. Interpreters of the fossil record have long struggled to distinguish ‘natural’ accumulations of bones (due to predators bringing carcasses back to their lairs, for example) from ‘artificial’ accumulations due to human hunting. Blumenschine and Caro studied the way in which carcasses are consumed by modern-day predators in Tanzania’s Serengeti National Park. They found that the types of bones at East African sites such as Olduvai gorge in Tanzania, where early hominids lived, closely match those left behind by large predators today.
As Blumenschine observes, this implies that early hominids scavenged from the kills of other carnivores. But whereas we have always assumed this to mean that they stole meat from the mouths of sabre-toothed cats, Blumenschine argues it is more likely that they simply picked up the unbroken long bones modern-day large carnivores find too difficult to deal with. These bones are an extremely rich source of marrow. The marrow from the limb bone of an adult wildebeest in good condition is about 94 per cent fat: a single long bone thus provides around 3000 calories. The way the fossil bones are broken suggests that these early hominids were after marrow fat.
Because we can recognise stone tools only by the fact that they have been artificially prepared, the sudden appearance of worked tools in the archaeological record has always posed two conundrums for palaeontologists. One is how to explain the sudden transition from a vegetarian diet that did not need stone tools to a meat diet that did; the other is how to bridge the ‘cognitive skill’ gap between the earlier populations that had no need to make tools and the later populations that were apparently suddenly able to manufacture the tools they needed to survive.
Blumenschine’s solution to the problem posed by the origin of the bones neatly solves both problems at a stroke. It provides an intermediate stage in which the hominids used natural rocks to crack open marrow bones. This would allow time not only to develop a taste for meat, but also for experimenting with making tools before they needed them to survive.
The suggestion that hominids were scavenging marrow-rich bones also tells us that they must have been very hard-pressed in nutritional terms. In baboons (and possibly chimpanzees) the frequency of hunting is inversely related to habitat quality. In other words, for these primates, meat is a fall-back resource, something you eat only when things get really tough.
For most primates, hunting is not the obvious first option: prey animals are not enthusiastic about standing still long enough to be caught. At best, hunting is a time-consuming and energetically expensive way of living. Instead, most fruit-eating primates such as baboons and chimpanzees prefer something as their ‘keystone’ resource – the resource they normally fall back on when few trees are in fruit. Both Nancy Lou Conklin from Harvard, studying chimpanzees in Uganda, and Caroline Tutin and her colleagues from Stirling University and the International Centre for Medical Research in Franceville, studying chimpanzees and gorillas in Gabon, found that these apes use high-fibre foods such as pith as their keystone resource.
Conklin contrasts the African apes’ use of pith with its absence from the diet of the Asian great ape, the orang-utan. She suggests that this difference in diets may reflect the forests that typify central Africa, where pithy plants are abundant, and Southeast Asia, where they are rare. This may explain why orang utans have thick enamel on their teeth which allows them to pulverise hard seeds and fruit casings, whereas the African apes (including ourselves) have thin enamel. As it wears, thin enamel forms a sharp cutting edge suitable for stripping plants to expose the pith. Pith turns out to be a valuable source of energy – at least when high-energy fruits such as wild figs are not available – even though it tends to be low in protein.
This is interesting, as it suggests that these animals may be limited by energy rather than protein. Each is important for a different reason: energy fuels activity and lactation; protein is essential for tissue growth. Although it is possible to convert proteins into energy, it seems that most species prefer to rely on natural energy sources such as fats and carbohydrates to provide their energy requirements. A particularly interesting finding in this respect comes from analyses of the nutritional content of the natural foods of African hunter-gatherers carried out by David Southgate of the Agricultural and Food Research Council’s Institute of Food Research at Norwich. His findings suggest that if hunter-gatherers can balance their energy budgets they will automatically meet all their other nutrient requirements. This implies that energy is likely to be the critical resource.
Southgate’s analyses reveal something else of interest. Most foods provide some quantities of all nutrients, but the relative proportions can differ widely. Leaves, for example, tend to be low in energy but high in protein, whereas sugary fruits such as plums or figs tend to be rich in energy and relatively poor in protein. Despite this, it is always possible to balance one’s nutrient budget by feeding exclusively on a single item; it is simply a matter of processing enough of it to obtain the required quantity of nutrients.
Biochemical evidence of this kind meshes well with my own work on the time budgets of baboons. Studies of baboons living in different parts of Africa suggest that most populations could always balance their nutrient budgets simply by spending more time feeding. But there are other demands on their time: notably, the time required for social interaction to maintain the cohesion of their groups and the amount of time that the hot climate forces them to spend resting. These demands prevent them from occupying certain habitats. The nutrients are out there in the environment in substantial quantities; the problem is that the animals cannot process them fast enough to meet their daily requirements. In effect, animals may often be time-limited rather than nutrient-limited.
So how do modern hunter-gatherer peoples compare? Kerin O’Dea, a biochemist at Deakin University, Victoria, has been collaborating with anthropologist Neville White in a study of the diets of Australian Aborigines following a traditional way of life in northern Arnhem Land. Despite being much thinner than the minimum levels set by the World 91É«ÇéƬ Organization, these people show no biochemical signs of malnutrition. One reason may be that their diets are based mostly on fat. It is not unknown, for example, for an adult to consume 2 kilograms of kangaroo meat in an evening. The favoured parts are high in cholesterol: liver, the main fat depots, and the brain. Indeed, the hunters show a fine-tuned appreciation of the natural life cycles of the many species they hunt, taking each only when at its fattest.
Analysis of their preferred plant foods reveals that they invariably opt for the most nutritious types available. Their favourite species of yam prove to be 30 per cent starch (compared to a mere 18 per cent in potatoes) and to be rich in fibre and essential trace elements such as iron, copper and potassium. Yet the Aborigines also reveal a remarkable sensitivity to the needs of a balanced diet, never eating meat (protein) without also eating a carbohydrate (usually either yams or honey). After all, they observe, meat alone makes your stomach go sour.
There is growing evidence that a high-protein diet may actually be bad for humans. A reanalysis by David Rush of the data from a recent study of the nutrition of a large group of Glasgow women has suggested that although increased protein in the diet during pregnancy does lead to higher birth weights for infants, this is true only as long as proteins provide less than a quarter of the calories in the diet. At values greater than that, more protein in the diet has precisely the reverse effect – smaller birth weights. Some evidence to support this comes from the records of early expeditions to the Canadian and American hinterlands. John Speth, an archaeologist at the University of Michigan, discovered that expeditions came to grief when they were forced to live on a meat-only diet after their flour and grain stocks ran out. Men died even when they were consuming unlimited quantities of fresh meat. As Elsie Widdowson at Cambridge points out, these findings are consistent with the results of experiments carried out on rats as long ago as the 1960s. Although their significance was not appreciated at the time, these studies likewise showed that high-protein diets increase mortality rates.
Hunting and gathering generally seems to impose a feast-and-famine economy, to which those who pursue this lifestyle often seem to become both culturally and biochemically adapted. The Aborigines studied by O’Dea, for example, appear to avoid problems with chronic diabetes that would normally be experienced by Europeans under a regime of frequent fasting. At the same time, they have both a specific hunger for fatty foods (so maximising energy intake whenever these are available) and a cultural aversion to exercise (so avoiding unnecessary energy expenditure). Unfortunately, this combination of traits seems to predispose the Aborigines to obesity when they adopt a Western lifestyle, and the associated medical problems are particularly prominent among those individuals that have settled in towns. Their ability to readjust is, however, remarkable. When a group suffering from diseases related to a Western lifestyle were taken to live a traditional life for seven weeks, all showed striking improvements in their conditions.
But factors other than simple nutrition can sometimes play an unexpectedly important role in determining the ‘natural’ diets of human groups. Katherine Milton of the University of California, Berkeley, has been studying four different tribes of indians living in different Amazonian forest habitats. She found that the tribes differed markedly in their dietary preferences; yet, there was no common ecological theme that might explain these differences. Instead, it seems that historical cultural differences had given rise to what amounted to separation into distinct ecological niches, particularly when it comes to those dietary items that the tribes relied on during the most impoverished times of the year. Part of the cultural process seems to involve refusing to eat anything eaten by neighbouring tribes simply because the members of these tribes are not considered to be human beings. This kind of cultural ‘badging’ has often been noted among hunter-gatherers, although in most cases it is concerned with the maintenance of group identity. Milton seems to have identified a nice example of the way in which culture is used to partition ecological resources.
A particularly intriguing example of how factors other than nutritional considerations can influence human foraging behaviour is provided by a study of hunting performance among the Hadza, a small tribe of East African hunter-gatherers. Kristen Hawkes and her colleagues at the University of Utah have used game theory to evaluate the costs and benefits of different types of hunting for Hadza men. This analysis suggests that the preferred style of hunting (solitary pursuit of large prey) is the most profitable in terms of the quantity of meat brought back to camp per hour spent hunting. But the meat from large animals is widely shared among everyone in the camp. So this is not the strategy that provides the greatest advantage to the hunter and his family. Yet considerable esteem may be attached to successful hunters. What benefits could possibly accrue to the men to make the effort of hunting so worthwhile?
Because appeals to the greater benefit of the group as a whole are not plausible on evolutionary grounds, and appeals to customary practice invariably prove inadequate on close inspection, we must look elsewhere for the persistence of such practices. One possible answer comes from a study of the Ache of Paraguay. In a detailed study of this tribe, Hawkes and her colleagues found that the most successful hunters were more often named by the women as extramarital lovers and as the fathers of illegitimate offspring. Their children also had a better chance of survival. In an economy like that of the Ache, which is heavily dependent on hunting, persuading good hunters tostay with the group by giving them a larger than average stake in the offspring seems to be a crucial component of the women’s long-term reproductive strategies.
Robin Dunbar is a reader in biological anthropology at University College London. The article is based on a recent meeting of the Royal Society on foraging strategies and natural diets in primates.