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To reproduce or not to reproduce, that is the question!

a review by Gert Korthof, 21 Sep 2003   (updated 24 Apr 2004)

Reproduction an incidental consequence of sex?

Exuberance Biological Exuberance. Animal Homosexuality and Natural Diversity.
by Bruce Bagemihl
St. Martin's Press, New York, 1999, 752 pages. Hardback.
many illustrations, index, references.


Homosexual behaviour occurs in more than 450 different animal species worldwide, and is found in every major geographical region and every major animal group, according to biologist Bruce Bagemihl. However, the existence of homosexual behaviour is a Darwinian puzzle because it seems to contradict the fundamental Darwinian principle that species cannot survive without reproduction. Uniquely for a critic of evolution, Bagemihl 'defends' homosexuality and 'attacks' heterosexual behaviour, thereby downplaying the importance of reproduction, which is incompatible with evolution.
It is a fundamental Darwinian principle that traits and behaviours cannot spread over evolutionary time if they reduce an individual's personal reproductive success. To be more precise, an imaginary species consisting only of individuals with an exclusive and life-long homosexual behaviour will be extinct in one generation. Parthenogenetic (virgin birth) species do exist in nature, they consist of 100% females, but a 100% homosexual species has never been found. Such a species cannot exist. Just as a sterile species cannot exist. What does exist in nature are species with up to 10 percent of homosexuals, according to Bagemihl. But even this is a puzzle. If those individuals do not reproduce, evolution theory predicts that the percentage in the population must decrease continuously down to a level that is produced by new mutations. But we observe more than that. So how should the excess be explained? One possibility is bisexuality, the combination of homosexual and heterosexual behaviour in the same individual. But this cannot be the answer either, because bisexual individuals will produce on average less progeny than full-time heterosexuals. So the existence is still unexplained. Bagemihl skilfully demolishes a variety of explanations for homosexuality proposed by biologists. This is one of the best, and interesting parts of his book. Probably few people have the encyclopaedic knowledge of homosexual behaviour in animals to be able to refute the variety of hypotheses to explain (away) homosexuality. In the end Bagemihl concludes, somewhat surprisingly, that homosexuality has no function, it just is. Homosexual behaviour has an intrinsic value.


Defending homosexuality, attacking heterosexuality

  Bagemihl defends homosexuality as a natural behaviour by documenting it extensively in mammals and birds. He successfully refuted claims that homosexuality is unnatural ('unnatural' defined as 'not occurring in nature'). However, he does more than that. Unexpectedly, he attacks the importance of heterosexuality. He starts by pointing out that in nearly every population there is a group of nonbreeders. I agree. For example in species with a harem, one male has access to the females in the harem to the exclusion of all other males. In other species, often only the highest-ranking male mates with females. But even in a heterosexual species without harem, males and females do not mate all the time. For a great number of species mating has never been observed despite hundreds or thousands of hours of observation. Often males and females live in separate groups. Young animals up to 3 - 5 years do not breed. Male baboons have been observed copulating a pregnant or lactating female, a non-reproductive behaviour. Reading the book, I started to realise that the group of non-reproducers is larger than the group of homosexuals (13). It is the unusual perspective on evolution that makes this book interesting. I could add to Bagemihl's list that in the human species, females are sexual receptive only 1-3 days a month (< 10% of the time) and > 90% of copulations are in fact non-reproductive (see also Jared Diamond: 11 ).

Bad things

  Bagemihl makes another step in attacking heterosexuality by listing a number of 'bad things' about heterosexual behaviour, such as the fact that males sometimes hurt females during copulation, things like rape and occasional heterosexual behaviour among live and dead animals. His wish to marginalize sexual reproduction is demonstrated by this amazing statement: "Although heterosexual mating can (and frequently does) lead to reproduction, this is often an incidental consequence rather than an overriding "goal" or ultimate purpose". Sexual pleasure is often a motivating force for heterosexual behaviour". Bagemihl concludes that homosexuality is not unique in the animal kingdom by virtue of its "failure" to lead to procreation.

True, but irrelevant

  Bagemihl does not distinguish between the biological and the human perspective. From the scientific perspective, reproduction is characteristic of life and evolution. According to the definition of life of evolutionary biologist John Maynard Smith:"Entities are alive if they have the properties of multiplication, variation, and heredity or are descended from such entities" (2). Without reproduction every biological species on earth is doomed to extinction. Viewed from that perspective, it does not matter how much time individuals spend on mating or how many individuals of a population engage in sexual reproduction.
It is true, but irrelevant that not every heterosexual mating results in fertilisation. It is true, but irrelevant that 'animal life and sexuality are not organised exclusively around procreation'. The point is that every individual animal, including the homosexual individual, is the product of sexual reproduction. Indeed, there is one big certainty and that is a fact about the past: each individual animal (except rare parthenogenetic species) has a father and a mother and therefore is the product of a heterosexual event (3). Every male got his Y chromosome from his father and his X chromosome from his mother. The essence of being a diploid organism is having pairs of chromosomes, one form the mother, one from the father (3). This is also valid in species with harems. Whatever unequal contribution individuals have to the next generation, every individual originated from the union of an egg and a sperm. Bacteria reproduce by cell division and higher organisms do it by meiosis and fertilisation. Deep in their bodies, homosexual individuals, just as everybody else, reveal the signature of heterosexual reproduction. It even shows on the surface: a navel.

Pleasure and DNA

  Sexual pleasure is often a motivating force for heterosexual behaviour, says Bagemihl. Of course is sexual pleasure a motivating force for heterosexual behaviour. But Bagemihl ignores the question why is sex fun? A proximate cause of sex is pleasure (1). The ultimate evolutionary cause is that the association of sex and pleasure increases the probability of reproduction. About the male body: Why does sperm contain the complete information for a new individual? The DNA in sperm is not necessary for an orgasm. If sex was only for fun, sperm could contain just proteins. About the female body: Why is the clitoris situated at the only place of the female body that enables sperm to reach eggs? (15). This makes sense from the point of view of reproduction. If sex was only for fun and had nothing to do with reproduction, then the clitoris could be anywhere on the female body (mouth, ears, nose, armpit, anus, navel). But it is not. Explain!

The lions of Serengeti

  The lions in the Serengeti plains of Kenya spend about 20 hours a day in rest or sleep, and only 1 hour a day on average in hunting (10). Following Bagemihl's logic, one could dismiss the importance of food for lions by pointing out that: (1) obviously lions are not preoccupied by hunting and getting a meal, (2) getting a meal is an "incidental consequence rather than an overriding goal or ultimate purpose" of hunting, (3) being built for the kill, the thrill of the kill is often a motivating force for hunting, (4) not every hunt is successful, (5) many members of the group do not participate in hunting, (6) young animals are often chasing each other for fun and this has nothing to do with getting a meal.
We learn from the Serengeti lions that food is crucial for survival (just as sex and reproduction are crucial for the existence of the species) despite the fact that they spend only an hour a day in hunting. Furthermore, there are always non-hunters in a group, but they depend existentially on the hunters, just like homosexuals existentially depend on the reproduction of heterosexuals.
It is important to distinguish between proximate (physiological) causes and ultimate (evolutionary) causes. The proximate cause of food consumption and sex is an anticipated agreeable sensation in the brain, the ultimate cause is an increase of the probability of survival and reproduction.

Points of view

  It is important to distinguish between the point of view of the individual and that of the species. An individual can be a nonbreeder, a species cannot. The point of view of an individual is a life's time. The time scale of a species is millions of years. That is what evolution is about.
    Furthermore, there is a distinction between individuals in a scientific theory (epistemology) and individuals in real life (ontology). As a person I can say that reproduction is not important in my life, but in the theory of evolution reproduction is crucial.
    Another point of view is that of the mortal body (soma) versus the potentially immortal germline. In fact the body of homosexuals and heterosexuals does not reproduce. All your body cells are an evolutionary dead end. They are mortal. Only sperm and egg cells (the germline) are involved in reproduction. They are potentially immortal (12). In fact, our body has a division of reproductive labor!
    There is still another point of view. Every human individual, and by definition every multicellular organism, is a community of cells. Each cell of our body originated by a process of cellular multiplication from one fertilised egg cell. This process is a form of asexual reproduction. It is basically similar to bacterial reproduction. The same process that single cell organisms use to reproduce, has built our bodies.

Death, sex and reproduction

  Logically, the necessity to reproduce arises from the fact that every living organism dies sometime. There is no eternal life, at least on this earth. Species live for a few hundred million years, individuals only for a few decades. So we need reproduction. This logic is more fundamental than the logic of evolution. Even if one believes in fixed species and denies evolution (creationists!), one still needs reproduction. Death and reproduction characterise life on this planet.
Furthermore, this logic is independent of sex, because animals with external fertilisation (fishes, amphibia) do not need copulation to reproduce. And then there are asexual reproducing species (cloning). The most fundamental thing of life is not how we reproduce (sexual or asexual, internal or external fertilisation), but reproduction itself.

Darwinism and reproduction

  Darwin set out to explain the fact that organisms are so perfectly adapted to their environments. This was a well-known problem in his time and the answer in his time was that the perfect fit of organism and its environment was the work of God. Darwin's explanation was that adaptation was created by natural selection. By definition, natural selection is differential reproduction. All organisms have the potential to produce more offspring than can possibly survive, but some produce more surviving offspring than others. Reproduction is not inherently important, but it is an indispensable ingredient of Darwin's theory to explain adaptation. One of the core features of Darwin's theory, natural selection, simply does not work without reproduction. As a biologist, Bagemihl ought to know this.
    The second ingredient of Darwinism is mutation. Mutations are copying errors of DNA. Copies are made when a cell divides (reproduction). Without mutation, there is no heritable variation and natural selection can select nothing. Therefore, without reproduction no mutation and without mutation no natural selection, and without natural selection no evolution. As a biologist, Bagemihl ought to know this (14).

Sex and reproduction

  In humans sex and reproduction are disconnected by natural as well as by unnatural means. As Bagemihl rightly pointed out, in nature not every heterosexual act results in reproduction. Humans added medical interventions. Sex without reproduction is possible through the use of contraception. Conversely, reproduction without sex is possible by in vitro fertilisation. I hope nobody will conclude from this that there is no natural connection between sex and reproduction.

Purpose or effect?

  A great deal of confusion can be eliminated when 'goal' and 'effect' are more clearly distinguished. Reproduction is not 'the goal of life'. Life is the effect of reproduction. Animals do not have a conscious purpose to reproduce anyway. We all depend on that 'incidental consequence' of sex. It just happens to be so on this earth that organisms are mortal and organisms reproduce. But that does not make death and reproduction 'the goal of life'. Although death and reproduction are not 'the goal of life', they are crucial to explain the existence of life on earth. And that is what Darwinism does.

Science and values

  What is valuable? What is a meaningful life? Bagemihl suggests that we should evaluate homosexuality and other forms of non-reproductive behaviour not in terms if its biological usefulness, but in terms of its intrinsic value. However a scientific theory has nothing to say about values at all. Irrespective of the biological or evolutionary 'value' of behaviours, people value those behaviours as good or bad. No higher human goal is involved in reproduction. For humans there is no moral obligation to reproduce. Therefore, it makes no sense to defend or attack statements like "reproduction is the ultimate purpose of life". If Bagemihl would see this, he would not have the urge to attack heterosexuality. Evolutionary theories about non-reproductive behaviour do not say anything about the meaning or value of that behaviour. On the contrary. As evolutionary biologist George C. Williams noted: "With what other than condemnation is a person with any moral sense to respond to a system in which the ultimate purpose in life is to be better than your neighbor at getting genes into future generations." (4). Surprisingly, Bagemihl did not notice that Catholic priests value celibacy much higher than reproduction. Gregor Mendel, the monk in the monastery garden, the father of genetics, made an everlasting contribution to science, but he did not pass on his genes to the next generation.

Post-Darwinian evolution?

  In chapter 6 Bagemihl explains his new paradigm: biological exuberance. His new paradigm is 'post-Darwinian Evolution' and is a mix of Gaia, chaos theory, indigenous cosmologies, and Georges Bataille. I found it unconvincing, vague, and irrelevant. Bruce Bagemihl is the first critic of evolution who rejects the all-importance of heterosexual reproduction, and thereby implicitly reproduction. The existence of sexual reproduction, in contrast to asexual reproduction, is an outstanding, unsolved problem in evolutionary biology (8), but the importance of reproduction itself is unquestioned. Bagemihl did not make clear what exactly 'post-Darwinian Evolution' means. Does he reject the core evolutionary concept natural selection? Selection is 'differential survival and differential reproduction'. Without selection no evolution. Selection implies reproduction. Bagemihl did not explicitly reject evolution. Therefore, to be logically consistent, he must accept natural selection and reproduction too. In fact, Darwin stressed that there is an over-reproduction in nature, which causes a struggle for life. Bagemihl could have defended homosexual behaviour as 'natural', without attacking the logical necessity and the fact of reproduction. Bagemihl would be surprised to learn that modern sociobiology has refuted the old idea 'that sex is a fundamentally benign, cooperative venture designed to perpetuate the species' (5).


  Bagemihl says cognitive adherence to Darwinian theory blinded observers to common homosexual and transgender behavior in the animal kingdom (6). This is undoubtedly true (9). But the solution is not to reject evolutionary biology, but to eliminate prejudices. It is a distortion of reality to say that homosexual behaviour is unnatural (defined as 'not occurring in nature'), but it is also a distortion to say that reproduction is unimportant in biology. To explain life, evolutionary biologists are fully justified to give reproduction a central place. For example, we could not understand the devastating effects of the Human Immunodeficiency Virus (HIV), without understanding its awfully efficient reproductive power (7).


  Strong parts of the book are the encyclopaedic documentation of homosexual behaviour and the use of those data to evaluate hypotheses and prejudices about homosexual behaviour. Weak points are biased descriptions and interpretations of homo/heterosexual data and a failure to acknowledge that one cannot explain the existence of life on earth without reproduction, and that one cannot explain biodiversity and adaptation without differential reproduction.


  1. On the other hand, birth is painful (at least in humans). Why is urination not associated with pleasure contrary to the release of sperm? Technically it could be done.
  2. John Maynard Smith & Eörs Szathmáry (1999) The Origins of Life, p.3. Hardback, OUP. p. 5. However, the definition is somewhat tricky. Sterile animals are living creatures too. Therefore a clause must be added to include those individuals in the definition. Tibor Ganti (2003) lists 5 absolute criteria of life and reproduction is not one of them. Reproduction is in the list of potential life critieria. In this context a more useful criterion is this one: "A living system carries within it the information necessary for its origin, development and function", which captures the potential to reproduce. "Reproduction is neither necessary nor sufficient for the individual living state" (Gánti,2003, p.159).
  3. This is what Mendel is said to have discovered (review). Critics like Senapathy (review, another review) have great difficulty to see that diploidy and sex are two fundamental and connected properties of animal and plant life.
  4. Quoted in John Alcock (2001) The triumph of Sociobiology, p. 215.
  5. Ibid, p.104.
  6. http://www.datalounge.com/datalounge/news/record.html?record=3992
  7. It is the exuberance of replication that makes HIV a killer: it can produce a billion copies of itself in a day. HIV is absent from the index of Bagemihl's Exuberance. There is a very good chapter about the evolution of HIV in the human body and the way drugs interfere with HIV's reproduction in Stephen Palumbi (2001) The Evolution Explosion, chapter 5.
  8. Mark Ridley (2004) Evolution, third edition, Blackwell, p. 314-327.
  9. Frans de Waal (2001) The Ape and the Sushi Master. He quotes Bagemihl approvingly.
  10. Mark Ridley (2004) Evolution, third edition, Blackwell, p. 293
  11. Jared Diamond (1997) Why is sex fun? chapter 4.
  12. William R. Clark (1996) Sex & the Origins of Death. In single-cell eukaryotes (Paramecia) for the first time the generation of DNA that is not transmitted to the next generation. (p.72)
  13. see also my review of Berman's book (table with positive and negative factors in reproduction). Add to the nonbreeders the group of intersex. (Total number of people whose bodies differ from standard male or female is one in 100 births!)
  14. In modern language: Mutation and Natural selection are nature's way to explore genome space.
  15. According to Elisabeth Lloyd (2005) The case of the female orgasm - Bias in the Science of Evolution the clitorus and penis have the same embryological origin and that explains the position of the clitorus (see review).

Further Reading

  • Jared Diamond (1997) Why is sex fun? The evolution of human sexuality, Weidenfeld & Nicolson, hardback, 168 pages. Chapter 4 is about the evolution of recreational sex in humans. He gives also examples of sex in other animals that is at the right moment of the female cycle (contra Bagemihl's view). Recommended book, good supplementary reading.
  • Henry Gee (2000) Why have sex? About the growing body of evidence that sex is a good way of maintaining genetic variation as a hedge against disease.
  • John Alcock (2001) The Triumph of Sociobiology. Alcock clearly explains the difference between proximate and ultimate causes of behaviour. Accessible. Recommended book.
  • John Alcock (2001) Animal Behavior, seventh edition (this is a well-known textbook about animal behavior and evolution). On page 472-474 Alcock explains how male/female patterns of homosexual behaviour follow those of heterosexual behaviour.
  • Olivia Judson (2002) Dr. Tatiana's sex advice to all creation, Metropolitan Books, hardback, 308 pages. Chapter 9 is about homosexuality. Entertaining, but too funny book about evolutionary biology of sexual behaviour.
  • Paul Harvey (1999) "A bestiary of chaos and biodiversity", Nature, 397, 6718 pp.402-403. Review of Biological Exuberance. Please note that a review of a book in Nature means recognition. '...this book should surely become the standard reference work...'. The reviewer dismisses Bagemihl's alternative theory.
  • Gert Korthof (2002) Bas Haring en de kern van de evolutie theorie (Dutch). Van belang omdat Bas Haring duidelijk uitlegt wat er gebeurt als dieren geen sex meer hebben (de beren in het plaatje!).
  • Louis A. Berman (2003) The Puzzle. Exploring the Evolutionary Puzzle of Male Homosexuality is a book review on this site.
  • Niles Eldredge (2004) Why we do it: rethinking sex and the selfish gene. Those we agree with Bagemihl, will agree with Eldredge. There is a review Sex under pressure I could have written in Nature, 430, 613-614, 5 Aug 2004 by Robert Foley. See also my review of his book Reinventing Darwin, section about sex. See also creationist Walter Remine about sex.
  • Gregory Cochran An Evolutionary Look at Human Homosexuality. Cochran argues that human male homosexuality is caused by a bug that acts on a specific part of the brain and thereby changes sexual interest. The bug is not HIV. This hypothesis does not say anything about the cause of AIDS, because it is not an explanation of AIDS but of homosexual behaviour. Homosexuality is not caused by HIV (that would be a circularity), but by some other infectious agent and has probably a non-sexual transmission. There is no direct evidence for the hypothesis. 23 Jan 2005
  • Vincent Savolainen and Laurent Lehmann (2007) 'Genetics and bisexuality', Nature, 11 Jan 2007. A population-genetic model indicates that if there is a gene responsible for homosexual behaviour it can readily spread in populations. The model also predicts widespread bisexuality in humans. 11 Jan 2007
  • N. J. Peters Conundrum The Evolution of Homosexuality (Paperback), AuthorHouse (February 16, 2006)
  • Nathan W. Bailey and Marlene Zuk (2009) 'Same-sexsexual behavior and evolution', Trends in Ecology and Evolution July 2009 TREE-1113; (free pdf)
    "highlights the importance of not just focusing on the origins of same-sex sexual behaviors but identifying the influence of such behaviors as agents of evolutionary change."


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Copyright ©G. Korthof 2003 First published: 21 Sep 2003 Updated: 22 Apr 05 Notes/Links: 28 Aug 2013