Five Blockbuster Evolutionary Insights and One Wild Life: Robert Trivers

Robert Trivers is the preeminent living evolutionist of our time.

By David P. BarashFebruary 13, 2016

Wild Life by Robert Trivers. Plympton. 238 pages.

“THE MASS OF MEN,” according to Thoreau, “lead lives of quiet desperation.” I always thought this an overstatement, and still do. Nonetheless, in a perverse way, this dictum might be true of many — perhaps most — great scientists, especially if we substitute “concentration” for “desperation.” After all, science is a demanding enterprise requiring intense focus, often to the exclusion of those niceties we designate the stuff of “normal life.” If Thoreau’s observation, with my modification, applies to great scientists generally, Robert Trivers breaks the mold.

He certainly qualifies as a great scientist, more precisely a great evolutionary biologist. With the deaths of William Hamilton, Ernst Mayr, John Maynard Smith, and George C. Williams, Robert Trivers is the preeminent living evolutionist of our time. What about Richard Dawkins, you ask? Much as I admire Dr. Dawkins and his work, I respectfully consider him a great writer, thinker, phrase-maker, and the finest popularizer around. And Edward O. Wilson? A brilliant and highly productive synthesizer, albeit one who (in my opinion) has gone off the rails with his recent espousal of group selection.

As Thomas Kuhn famously proposed, most scientists do normal science. Trivers didn’t do much of that. Instead, he was into paradigmatic revolutions. Indeed, he takes the prize when it comes to research of foundational consequence and originality. (He was in fact awarded the Crafoord Prize, an alternative Nobel.)

In an extraordinary burst of creative accomplishment during the first half of the 1970s, he established the basis for our current understanding of how evolution shapes an array of behaviors; his work from this half-decade alone comprises much of the backbone of today’s sociobiology (or evolutionary psychology, if you prefer the more current moniker). Thus, even among non-biologists, it is well known that a process called kin selection, or maximization of inclusive fitness, provides an evolutionary rationale for individuals acting “altruistically” toward others, in proportion as the cost to the actor is relatively low, the benefit to the recipient relatively high, and the interactants enjoy an adequate probability of sharing genes via common ancestry. This form of altruism is a direct consequence of natural selection acting on genes rather than on individuals, groups, or even species, with behavior that seems altruistic at the level of bodies actually being selfish from the perspective of the genes themselves.

In an eye-opening, paradigm-busting paper published in 1971, Trivers opened a window onto another route for altruism to evolve among genetically unrelated individuals, even members of different species. Known as “reciprocity,” it addresses the following problem: conveying benefit to a non-relative runs the risk of being literally altruistic — that is, self-abnegating and thus self-defeating in the biological sense. Such benefactors can end up giving more than they get, which sounds good to ethicists — but not to biologists who must reckon with natural selection’s hard-headed balance sheet. For reciprocity to work as an evolutionary process, the recipient must, with a certain degree of probability, reciprocate (when given the opportunity), rather than benefiting from another’s benevolence and then becoming conveniently hard of hearing, selectively blind, preferentially lazy, or just plain indifferent to the opportunity — in a sense, the obligation — to return the favor at some later time, thereby pocketing her advantage and suckering the original altruist who will end up less fit, in evolution’s calculus, than if she had simply behaved selfishly from the get-go. The young Bob Trivers, who was 28 in 1971, came up with an elegant demonstration of how reciprocity could in fact be evolutionarily stable. In the process, he introduced evolutionary biologists to the elegant albeit frustrating logic of the prisoner’s dilemma, which specifies the stringent conditions that must be met in order for mutual cooperation to evolve, such that both participants receive the potential payoff of mutual cooperation. It turns out that those conditions are so demanding that reciprocity has only been documented incontrovertibly among Homo sapiens. (And vampire bats!)

Next up for Trivers: understanding the biology of male-female differences, which would lead to his second intellectual blockbuster. Everyone knows about the biology of sex difference, right? Wrong. Biologists have long understood that maleness is not defined by beards, biceps, and penises, just as femaleness doesn’t depend on breasts, babies, and vaginas. Rather, the females of any species are those individuals who make eggs, and the males are those who make sperm. (That’s why we can confidently distinguish male from female sparrows, for example, even though both sexes have a common excretory/reproductive opening known as a cloaca.) But it took Bob Trivers — building on a then little-known research paper published in 1948 by A. J. Bateman — to dig deeper and explain why being an egg-maker or sperm-maker has such profound behavioral consequences. In short, sexual differences in behavior are a function of what Trivers labeled “parental investment” — defined as any expenditure on the part of a parent toward offspring that enhances the likely ultimate success of that offspring, but that occurs at the expense of the parent’s ability to invest subsequently. Given the large size and rarity of eggs (as compared to sperm) — compounded in mammals by the metabolically costly development of a placenta, and of nourishing offspring during pregnancy and especially lactation — females are typically (although not always) the purveyors of more parental investment. Trivers then pointed out that, as a general rule, individuals of the sex investing more become a limiting resource for the evolutionary success of those individuals investing less. Hence, the latter (typically, but not always, males) can be expected to compete for sexual access to the former (typically females).

“Why can’t a woman,” asked an exasperated Henry Higgins in My Fair Lady, “be more like a man?” The answer, of course, is that she can, just as men can be more like women. But, most of the time, each sex is predictably most like, well, itself. Thanks to Robert Trivers, we now understand why this is the case: it all comes down to differences in parental investment.

A year after his analysis of male-female differences, which became part of an edited volume celebrating the centennial of Darwin’s Sexual Selection and the Descent of Man, Robert Trivers came out with a third scientific blockbuster. This one addressed how parents adaptively alter the sex ratio of their offspring. Among evolutionary biologists’ proudest accomplishments had been — and, for the most part, still is — an understanding of why nearly all species, regardless of their breeding sex ratios, generate equal numbers of males and females. One might think that in a species such as elephant seals, in which a bull could well mate with 30 females, natural selection would produce on average one bull for every 30 cows. Not so. As pointed out decades earlier by the evolutionary geneticist and statistician R. A. Fisher, in such a case males and females are equally good biological bets, even though every cow seal is likely to breed, compared with only one in 30 bulls. This is because the successful harem-keeper’s breeding success (e.g., 30 pups) is precisely matched by the failures (zero pups) toted up by all non-breeding bachelors. As a result, even though boy bulls are a riskier evolutionary investment — since only one in 30 hits the jackpot — producing sons becomes, on average, precisely equivalent in payoff to producing daughters. Moreover, any deviation from an equal sex ratio (e.g., a slight excess of males) makes females a better investment on the part of parents, generating selection favoring females and thereby righting the ratio.

Again, along came Trivers, working this time with mathematician Dan Willard. They essentially upended our mathematically neat, game theoretic understanding of sex ratio equality by pointing out something that is obvious … once pointed out: the neat and nifty equality of sex ratios works insofar as parents have no prior information (not necessarily consciously known, mind you) as to the likely reproductive success of their offspring. But in a polygynous species, parents in especially good physical or social condition — and whose sons would therefore likely enjoy a higher-than-random success rate — can be expected to bias their offspring toward males, and vice versa for parents in relatively poor physical or social condition. Moreover, a mirror-image prediction can be made for polyandrous species, those rarities in which one female typically keeps a “reverse harem” of multiple males.

That insight appeared in 1973. A year later, Trivers published a fourth evolutionary blockbuster, which — once again — upended conventional wisdom while creating a huge intellectual and scientific opportunity, this time in developmental psychology (and genetics, and embryology, and physiology, and probably elsewhere as yet unrealized). Years of heart-warming nature films, plus no small dose of seeming biological wisdom, had suggested that parents and offspring should nearly always be on the same evolutionary page. After all, reproduction is the primary means whereby individuals maximize their fitness (not physical capacity, but rather their success in projecting copies of their genes into the future). Another, and more accurate, way of stating this: breeding is the main way genes promote their own success. This is quite simply the adaptive value of breeding. At the same time, no one gets to reproduce until he or she has been produced in the first place, not to mention being given an adequate start in life. Hence, parents and offspring should constitute one happy family whose only rough spots are generated by honest misunderstandings, largely due to the existential space that separates any two organisms.

Wait a minute, protested Trivers in 1974. Not so fast! Granted that in every sexually reproducing species, a gene present in either parent has a 50 percent chance of ending up in the sperm or egg that eventually finds itself ensconced in a given offspring, it also must contend with a 50 percent chance of not being so fortunate. On balance, therefore, parents aren’t genetically identical to their offspring; rather, they are only one-half related. And vice versa for offspring vis-à-vis each of their parents.

The results are staggering. Parents can only be expected to be one-half as interested in the welfare of their offspring as the latter would themselves prefer, and once again, vice versa for the offspring. After all, each offspring shares a 50 percent genetic coefficient of relationship (a measure of genetic identity due to recent common inheritance) with its parents, whereas it is 100 percent “related” to itself. This asymmetry yields numerous important consequences. For example, parents and offspring can be expected to disagree over the timing of the next reproductive attempt (parents wanting it sooner than do offspring), as well as over the actual amount of parental investment to be transferred (offspring having been selected to want more than parents are inclined to provide). Not only that, but parents and offspring can be expected to disagree over the manner in which those offspring interact with each other, since each individual can be expected to devalue by a factor of two any costs (or benefits) received by a full sib. Recall that, like parents and offspring, full sibs are related to each other by 50 percent, whereas each sib is entirely “related” to itself. At the same time, since a parent is equally related to each of its children, Trivers noted that parents should in theory want their offspring to play together more nicely, and to share more — precisely 50 percent more, in fact — than the offspring themselves would be predisposed to do. Sibling rivalry, anyone?

It gets even more intriguing. We know that offspring are considerably less powerful than their parents (as Trivers noted, an infant cannot fling its reluctant mother to the ground and nurse at will), and yet both parties retain a significant interest in each other’s well-being. Add the fact that parents must be at least somewhat sensitive to legitimate offspring needs, and it becomes predictable that infants especially use psychological rather than mechanical mechanisms of coercion, which parents must then distinguish from expressions of genuine need. “The course of true love,” we learn in A Midsummer Night’s Dream, “never did run smooth.” In this case, Shakespeare was concerned with romantic love. Thanks to Robert Trivers, we finally understand why this seemingly cynical but troublesomely apt observation applies equally to the love between parents and offspring.

At this point, if you are inclined to note that most of these insights are obvious, I can only agree. Indeed, upon reading each of Robert Trivers’s seminal papers in the 1970s, my own experience was nearly always to emulate Thomas Huxley upon reading Darwin’s Origin: “How stupid of me not to have thought of that!” But Darwin’s brilliance lay not only in his perseverance but also in seeing something that is so important, and also so obvious — once pointed out — that it then seems blindingly obvious even to the rest of us. Ditto for the immense achievements of Robert Trivers.

In 1976, Trivers — this time working with graduate student Hope Hare — made his fifth blockbuster research contribution, which is perhaps the most persuasive empirical support for William Hamilton’s key insight about inclusive fitness as applied to the social insects, specifically the gene-based explanation for why among the “eusocial” bees, ants, and wasps, entire castes of workers labor for the success of their mother the queen, while remaining non-reproductive themselves. Even the long-form book reviews so wonderfully indulged by LARB don’t allow for adequate explanation of these findings, but trust me, they were impressive. And still are.

Trivers has subsequently done important work on intra-genomic competition, the evolutionary biology of self-deception, and the implications of human body symmetry, but his five earlier key papers are more than enough to guarantee enduring fame and the admiration of succeeding generations of evolutionary biologists who — along with increasing numbers of social scientists — will be kept busy long into the future, exploring the implications of his work.

Isaac Newton famously wrote that if he has seen far, it is because he has stood on the shoulders of giants. To be sure, Trivers has stood on the shoulders of such oversized intellects as R. A. Fisher, J. B. S. Haldane, Theodosius Dobzhansky, and — of course — Charles Darwin. But he has also shown himself to be a giant, permitting the rest of us (comparative midgets) to see further ourselves.

All of which is prelude to the fascinating revelations provided in Trivers’s most recent book, appropriately titled Wild Life: Adventures of an Evolutionary Biologist. The double entendre is appropriate, for although Trivers has spent time studying the lives of animals in their natural state (more so than I had realized, given his towering accomplishments as a theoretician), his book leaves no doubt that he has led something of a wild life indeed, definitely not one of quiet desperation!

A notable characteristic of Wild Life is the author’s fearless, painfully honest willingness to share many aspects of his life. We learn, for example, of his lifelong struggle with bipolar disease, including treatment while still a college student at Harvard, at McLean Hospital, “a well-known private hospital outside Boston with a very heavy Freudian bias that specialized in keep­ing wealthy people in their care for long periods of time.” Trivers’s sardonic sense of humor, apparent here, is visible throughout Wild Life:

I once grew particularly close to a blue lizard (Anolis grahami) that I had trained to share an afternoon’s drink with me. This is easy to achieve. Blue lizards like sweet drinks, and if you simply set out one for him at the same time every day he will soon enough search it out. Once he tastes Stone’s ginger wine, he is gone — he wants it every afternoon. Now you both drink together each af­ternoon. He turns a bright blue-purple-yellow color, which seems in general to be a sign of arousal and personal happiness. You are turning whatever col­ors you turn when you drink. The key is that you are in synchrony with the lizard. Show up at four p.m. and he will be waiting, please bring two glasses.

We are also made privy to painful aspects of Trivers’s wild loves, not least his long-standing fondness for ganja (Jamaican for marijuana), and for women:

One day I received in rural Jamaica my “Dear John” letter from the wom­an I had loved non-stop since the day I met her as a twenty-year-old undergraduate at Harvard. She made it clear that the relationship was over — over — over! — it was OVER. After a while you got the full pressure of her point; she wanted to make sure that even someone as self-deceived as myself would realize things were over, and she succeeded at that. I thus traveled early to the ‘back-a-rock’ area near Barbary Hall where you could smoke your ganja in reasonable certainty that you would not spend eighteen months in prison at hard labor for doing so.

Robert Trivers has in fact spent time in a Jamaican prison, and has had several near-death experiences in that tropical other-than-paradise, occasioned at least partly by his fondness for ganja, and his hard-drinking, bar-visiting inclinations, along with his deep personal involvement with the people of Jamaica, at least some of whom seem less than salubrious. A few readers might also be surprised — although at this point, perhaps not! — to learn that, although Caucasian, he was for a time a Black Panther, and a close friend of Huey Newton. We also learn of his more traditional associations, including with such other greats of what I call revolutionary biology, notably Ernst Mayr, William Hamilton, Richard Dawkins, G. C. Williams, and Irven DeVore, as well as a much-deserved skewering of Stephen Jay Gould.

I would love to wrap up this review with a compelling account of how Trivers’s wildly creative research contributions have been reflected in his comparably wild personal life, but frankly I can’t seem to manage it — although Dr. Trivers clearly has done just that! The Thoreau quote with which I began this essay goes on to mourn that most people “go to their graves with the song still in them.” All of us — and not just biologists — are fortunate that Bob Trivers has shared his song with us.


David P. Barash is an evolutionary biologist and professor of psychology at the University of Washington. His most recent book is Buddhist Biology: Ancient Eastern Wisdom Meets Modern Western Biology (2014; Oxford University Press), with Out of Eden: Surprising Consequences of Polygamy forthcoming from Oxford in early 2016.

LARB Contributor

David P. Barash is an evolutionary biologist and professor of psychology at the University of Washington. His most recent book is Buddhist Biology: Ancient Eastern Wisdom Meets Modern Western Science (2014, Oxford University Press).


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