Baboons' tea-party

It is a difficult thing to know when an animal makes a mistake, simply because of our limited knowledge of the real constraints an animal is under. Much effort over the past two decades, however, has gone into producing models of optimal animal behaviour, particularly of foraging, in which essentially economic decisions are made about how long to search for certain foods or how long to stay in a food patch. The problem with models of optimality is that when animals do not conform with them, we are left with the question: which of the two is at fault?

Stuart Altmann's take on optimality is this: optimal behaviour is something achieved by few, but adaptive behaviour is anything that differs from randomness in a direction towards optimality. Furthermore, the variation that Altmann's definition of adaptedness recognises, far from an embarrassment, becomes the means to test models of optimality, by correlating distances from the putative optimum with differences in fitness.

Foraging for Survival presents a model of optimal foraging for, and data on, actual foraging by 11 yearling baboons. The book's premise is that baboons trying to eat well face a "packaging problem". While they want to consume nutrients, they are only able to ingest foods. The difference is not always apparent to ourselves, so successful have we been in breeding nutritious foods for our use, but the wild foods of the savannah are quite a different matter. First, no single food could ever fulfil an animal's nutritional requirements. This means a baboon must be eclectic. Second, the foods are defended, often physically by thorns, which the baboon's dexterity can overcome, but also by a chemical packaging of toxins, which it cannot. This means the baboon must be selective. An optimal diet must solve the packaging problem, and the packaging problem is complex - both for Altmann and his baboons.

Altmann, at least, has the benefits of a large computer that allows him to construct a model using constrained linear optimisation. The constraints are that the forager's diet must exceed their minimum for every nutrient and fall below their maximum for every toxin. The criterion for an optimal diet is then chosen. Altmann provides a menu of three: energy intake, protein intake and minimisation of time spent foraging. Finally, the available foods with their dietary compositions are plugged into the model. The equations tell you how much you should eat of what.

Altmann's subjects might benefit from a bit of constrained linear optimisation themselves, since they did a pretty poor job on their own. They consumed 40 per cent of the protein and energy that a different diet could have provided, and they took four hours to acquire the nutrients that a better-chosen diet would have given them in 50 minutes. How could they have got it so wrong?

The problem here is with the model. Altmann admits that he has disregarded some important foraging factors, such as the time required to find different foods and to harvest them. But there are others. For example, milk features highly in the yearlings' optimal diets, but milk provision is in the hands of the mother who has quite different ideas on optimality from her child, as it rudely discovers at weaning. Second, despite its number-crunching power, the linear optimisation technique is a severe simplification since it assumes that the benefits of nutrients and the deleterious effects of toxins exert their effect "all at once", at the crossing of some threshold. This is unlikely.

Perhaps the most serious problem with a model of this kind, however, is that it cannot incorporate social constraints. It is known, for example, that baboons will forgo rest and eat on the move in order to preserve time for the important social practice of grooming. Theoretically, there are also strong reasons for supposing that baboon fitness may be determined socially as much as nutritionally, and I doubt that any baboon ever made a foraging decision that was uncoloured by social considerations.

Undaunted by the poor fit of data to model, Altmann goes on to compare the baboons' performance to fitness. As he wryly notes, the advantage of taking 20 years to write up his data is that he has real, rather than projected, data on fitness - namely, total fecundity. Coupled with laudable care in his collection and treatment of data, Altmann's analysis boasts some dramatic effects. Ninety-six per cent of the variance in females' fecundity is explained by shortfalls in energy intake that they experienced as yearlings. Adding protein to the analysis brings the prediction to 99 per cent. There are also strong effects for which the basis is unclear: dietary shortfalls predicted age at menarche, but only with reference to mother's rank, hinting at hidden social mediation in life-history scheduling.

Still, Altmann's aim was to test an optimality model against real behaviour by correlating deficits from the putative optimum with deficits in fitness. Remarkable though the correlations are, they would be identical if expressed in terms of gains in fitness achieved with increases in energy and protein intake, thus making the model, and associated concept of deficit, somewhat gratuitous.

Perhaps, then, in judging Foraging for Survival it would be fairer to concentrate on its component parts. It is certainly a substantial contribution to biology to show how critical is the effect of childhood nutrition on fitness. The book also contains a mine of methodological resources. There is also a nice concluding chapter on baboons as whole animals, which many might benefit from dipping into. For, happily, the packaging problem is less severe in books than in the prickly acacia: you do not have to swallow the whole thing to get out the useful bits.

Thomas Sambrook is teaching fellow in psychology, University of Stirling.