Like the author of this intriguing book, I grew up with an alcoholic parent and understand the devastating effects it can have on both sufferers and their families. But unlike its author, and despite being an evolutionary biologist myself, I had never considered viewing alcoholism as an evolutionary problem. Perhaps naively, I thought that all addictions were much alike and triggered by similar neural mechanisms ¨C in other words, some people are born with an ¡°addictive personality¡± ¨C and the precise manifestation of the addiction was more a result of circumstance and environment than evolutionary history. But in The Drunken Monkey, Robert Dudley argues that alcoholism is different from other addictions, in that it taps into hard-wired evolutionary pathways, primarily designed to increase foraging efficiency for highly calorific fruits, but which in modern human society predisposes some to find the reward stimuli triggered by alcohol too powerful to resist.

If one is to argue that a certain trait has been under positive selection so as to increase in frequency within a population, first one must identify how that trait will benefit an individual so as to outweigh any potential associated costs. Although drunken behaviour is unlikely to provide any benefits in the current environment, and more importantly in the environment our primate ancestors lived in, Dudley argues that it is not drunkenness that has been under selection, but the search for food with the greatest number of calories. The majority of primates have fruit-heavy diets, and fruit-bearing plants invest in many adaptations ¨C such as sugar-rich flesh and vividly attractive colours ¨C that make their bounty more appealing, so that passers-by looking for a meal will inadvertently aid dispersal of their seeds.

Dudley delights in telling comical stories, drawn from experiments and anecdotal encounters, of animal drunkenness

As primates, we still exhibit behaviour that points to our historic relationship with fruit. We will touch, squeeze, smell and observe fruit before consumption, making sure to pick those most vibrant in colour and sweetest in scent. Other primates and frugivores exhibit similar behaviour, and it has been shown that our olfactory senses, compared with those of other mammals such as rodents, are particularly well tuned to detect high sugar and alcohol concentrations. If we eat a fruit that is not sufficiently ripe, we are left with a sour and unpleasant aftertaste; if the fruit is too ripe, its decomposition by microbes will have digested the calories and risks making us sick. Therefore, our fruit ripeness detection sensors are programmed to what you might call Goldilocks mode, pinpointing just the right ripeness to satisfy our calorific demands without causing illness. Dudley describes how fruit at the optimum state of ripeness will have been colonised by yeast spores and therefore will have begun to ferment and contain some low levels of alcohol. The additional benefits of alcohol he notes include increased lifespan (at least when it is consumed in small, regular doses) and increased appetite, resulting in more calories consumed per food sitting. What remains undetermined, however, is whether exposure to these low levels of alcohol in ripe fruit is enough to influence the evolution of alcohol-degrading enzymes ¨C but Dudley makes a strong case.

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He delights in telling comical stories, drawn from experiments and anecdotal encounters, of animal drunkenness. And while his accounts of ¡°cavorting groups of inebriated baboons, sozzled chimps falling out of trees and birds too drunk to fly¡± may sound frivolous, there is an important underlying message. To date, the majority of rigorous experimental work regarding the effects of alcohol has been conducted on fruit flies; the only other accounts we have are local tales of drunken animals causing mayhem. There is a pressing need for empirical data about animals from which we can draw closer parallels. That is not to say that current experimental data on fruit flies isn¡¯t interesting or insightful, and there have been some surprising discoveries made ¨C for example, it has been shown that male flies deprived of opportunities to mate exhibit an increased preference for alcohol. But Dudley argues that if we hope to gain a better understanding of our own association with alcohol, we need to look at examples a little closer to home, and conduct controlled field trials that can help us find answers to long-standing and basic questions regarding our evolutionary history with alcohol, such as: What are typical fruit alcohol concentrations? How frequently are frugivores exposed to alcohol? How does low-level natural exposure translate into addiction? And to what extent is alcohol preference and detection genetically determined?

There are also many questions in our own human populations that need addressing. Dudley carefully and concisely elucidates why we see geographic divides between different cultures¡¯ tolerance to alcohol. In East Asia, for example, there appears to be a markedly lower tolerance and Dudley explains the genetic reasons for this. In these populations, the genes responsible for enzymes that metabolically degrade alcohol are variants that result in the rapid breakdown but subsequent accumulation of toxic products ¨C making alcohol consumption often less than pleasurable. Interestingly, genetic analyses of the emergence of these variants have placed them at about the same time as the widespread cultivation of rice, and Dudley ponders whether selection against the adverse effects of alcohol became the trait under selection in these populations.

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Given what we know about alcohol consumption in modern society, what data are lacking that would help Dudley¡¯s case for our ¡°evolutionary hangover¡±? He proposes an interdisciplinary approach that draws on tools from modern behavioural biology, comparative evolution and genomic analyses. Here, he not only points out holes in our knowledge but proposes experimental methods that might help acquire such data. A key assumption of Dudley¡¯s hypothesis is that fruit eaters in the wild will preferentially select fruits with low but non-negligible levels of alcohol. If a carefully conducted field trial could provide direct evidence for such behaviour, I think Dudley would be jumping for joy, but as it stands there is something left to be desired in the experimental support for his case.

Why do we as humans have such a fascination with alcohol? In many social contexts we seem to rely on it, and its health benefits in small, regular quantities are known. However, this relationship is a conflicted one, and excessive drinking can ruin lives. Perhaps Dudley¡¯s approach will offer hope for future treatments for alcoholism, and through understanding the genetic association to the disease we may begin to develop more sophisticated drugs that block or suppress neural reward pathways. However, although The Drunken Monkey is built on a carefully laid out hypothesis, it is one that relies on a lot of guesswork and musings. This state of affairs is through no fault of the author, and Dudley is indeed brutally honest about the many gaps in the data and our knowledge of human historical and primate association with alcohol.

The book¡¯s final few pages bring some welcome humanity to the scientific perspective. Dudley is, after all, the son of an alcoholic, and despite the unbiased viewpoint presented throughout the book, he often reveals glimpses of his own personal battles. Perhaps by taking a look at alcoholism from an evolutionary perspective, we may be able to shed light on the genes and mechanisms responsible for our sometimes inexplicable behaviour; it is here, perhaps, that hope for future innovative treatments lies.