Tuesday, April 6, 2010

Why [Recreational] Sex?

Sex is a very costly process for females. On page 157, Jerry Coyne explains that while the male investment in sex is cheap, "for females it costs much more: the production of large, nutrient rich eggs and often a huge expenditure of energy and time". Thus, females are often very picky about choosing a mate. However, in some organisms, this is not the case. Among bonobos and dolphins for example, sex is a very common occurrence, engaging in 'intimate relations' very frequently. Dolphins become sexually active even before they become sexually mature, and bonobo sexual behavior has no regard for gender or age. How could this sort of behavior have evolved among these organisms? How can females afford to take the risk of investing in so many potential offspring?


  1. In the case of the Bonobo primates, it comes down to a matter of physiology. We humans are biased when we think of potential investment when it comes to childcare because we are bipedal. When ancient primates became bipedal the birthing canal rearranged so that it could accommodate walking on two feet. This is why female humans hips widen during puberty, so as to provide space for a viable birth. This rearrangement also causes the majority of the pains during childbirth which contribute to our idea of investment. Also with one hand occupied by a baby human, primitive women would have a much harder time providing food and shelter for themselves and their baby with only one hand which makes the apparent investment that much larger. Quadruped primates have no such dependence on outside aid because the offspring are carried on their back and hold on to their hairs with the ancient reflex that we can see babies still retaining today, the ability to close their hands into fists that are miraculously strong willed. The same applies for dolphins in that they do not have to provide as much parental care because their young are born with the ability to move independently of their mother and the ability to catch their own meals from an early time in development. While certainly the investment of female animals in these recreational sex species is very great, especially in relation to males of the same species, perhaps the investment is not as severe or debilitating as it may initially appear given our bias.

  2. BKim, you pose a great question. First, dolphins.
    It is interesting to note that many juvenile humans are also very sexually active before they become sexually mature. Remember preschool kissing?
    "In fact, sexual development and sexual play are natural and healthy processes in children, from toddlers through childhood and into adolescence. For infants and toddlers, this usually involves body sensations, cuddling and touch, and playing with toys." (Rich 1) It would be an evolutionary advantage to those members of a species who had sex on their mind more often, because there is a greater chance that they will reproduce. This explains why thoughts of it occur in our mind in such abundance, because from a biological standpoint, our brains our wired to make us have sex, in order to pass on our genes. This is simply a similar occurrence in the young dolphin mind.
    As for bonobos, one of the most sexual monkeys extant today, one current theory is that their increased sexual behavior decreases social tension.
    "These contacts appeared to reduce the tension an to allow for food sharing. [...] The interaction could even take the form of an exchange, e.g., a female presents to a male who is holding a large bundle of branches and leaves and takes the entire bundle out of his hands immediately following sexual intercourse." (Feierman 69-90) It could be that the benefits of this decreased social tension, like increased food sharing, outweigh the cost of so many opportunities for unwanted fertilization of the female egg. Another example: "The majority of instances of genital massage, for instance, followed aggressive incidents in which the adult male had chased one of the adolescent males. After a couple of minutes, the younger male would return to the aggressor to present his genitals." (Feierman 69-90)


  3. This reminds me of the video that Mr. E showed during bio over the summer to show that we are not the only creature that enjoy sex. This is because if sex was not enjoyable, people (or animals) would not engage in it, and eventually no one would want to waste their time and energy recreating. What I'm getting at is that this behavior evolved because of natural selection. The adaptations to enjoy sex is huge because no one would recreate. The point of a species is to create the most offspring, but not just a lot of offspring, it is to create those that will most likely survive, and reproduce. So as these creature recreated, something needed to evolve to help them keep recreating, otherwise they would stop, and their species would eventually go extinct. No creature would want to waste their energy that could be used for something completely different. Animals, mostly males, want to have as many children as possible, and spread their seed around. So in order for them to do this they need to have some drive or motivation to do this. Us humans have a libido to help drive our sex life. There are theories on how we develop our libido, and it depends on our age in life (see source below). Without this people would see no purpose in sex.
    Women take the risk of so many potential offspring because they too want to make sure that their family lives on from generation to generation. Women have a libido too, but not as strong as males. I agree with Jonathan when he states that the investment gets larger when a women has to hold her kid, and makes her life more difficult. Women go through all this pain to have a child. Bringing this child into the world has to come with great responsibility. You are insuring that your name lives on, and that your genes are passed on to future generations. It is worth it because you are enjoying sex, but knowing that your genes are passed on. Females are choosy, and this leads to sexual dimorphism (146). Females want to pass on their genes with men that they know will have good genes. They don't mind how many times they have children as long as they ensure that their children survive and are able to live life so they could find that one person to have children with.

    Coyne, Why Evolution Is True

    Jonathan's comment above



  4. In order to consider the question of how these animals have such high frequencies of sex, it’s important to establish a parallel between one of your examples, bonobos, and humans. Similar to chimpanzees, “bonobos (Pan paniscus)…are the closest living relatives to humans” (D’Août et. al). Coyne supports this claim by saying that in terms of genetic similarity, humans and bonobos have a 98.5% similarity (Coyne 210). The direct implication that can be drawn from this information is that most of the evolutionary pressures that have affected bonobos, chimpanzees, and humans are the same. Therefore, we can predict that many of their behaviors are very similar. Coyne exemplifies this in the example of the vestigial trait in having a propensity to grab on to a hand that touched their hand, called the “grasping reflex”, for a short time after birth (Coyne 80). Specifically to sexual behavior, we see a striking similarity between bonobos and humans: according to The Kinsey Institute describes the immense amount of sexual tension in humans by citing a statistic that say that 89% of males have had intercourse by the age of 22-24 and 92% of females have done so in that respective age group (Kinsey Institute). The conclusion that this information comes down to is that a frequent sexual behavior most likely evolved in the common ancestor (or earlier) of bonobos and humans. However, this begs the question concerning why this similarity may have come about. I predict that this must have come about as a selective advantage to that ancestor. For instance, that ancestor must have been similar to both the bonobos and humans, implying that it was a K-selected species. K-selected species following a Type III Survivorship Curve, limiting the amount of offspring produced during intercourse and increasing lifespan (Campbell, Reece 1177-1178). So an evolutionary advantage may have been to have a greater intrinsic desire to have sex in order to increase the size of a population more like an r-selected species than a true K-selected species.
    However, BKim, you ask a wonderful question as to why females would bother to put up with the risk of investment, energy costs, and more troubles of having a higher sex frequency. I believe that our Biology book points out the answer to your question in the economic model of game theory, which “evaluates alternative strategies in situations where the outcome depends on the strategies of all the individuals involved” (Campbell, Reece (1137-1138). In the context of evolutionary pressures, game theory is complemented by the development of the Red Queen Hypothesis in the 1980s, which proposes that “sex is needed to fight disease” (PBS 1). As diseases evolve at a much faster rate than the K-selected species that you discussed, game theory would suggest that the rate at which these animals have sex would increase to increase the chances of surviving. This is exemplified in another PBS article in the following sentence: “Sexual reproduction thus increases genetic variation, which increases the raw material on which natural selection operates” (PBS 2). With an increase of this “raw material”, or genetic variation, animals have a higher success rate of surviving against other players that would, according to game theory, put an evolutionary pressure on other organisms to evolve. In retrospect, the increased cost of sexual behavior that you questioned can be seen as marginal compared to the cost effectiveness of evolving more effectively around the evolutionary pressures that act as obstacles to these species.

    http://www.pbs.org/wgbh/evolution/sex/advantage/page03.html (1)
    http://www.pbs.org/wgbh/evolution/library/faq/cat01.html (2)
    Niel Campbell, Jane Reece: Biology: AP Edition, 8e
    Jerry Coyne: Why Evolution is True

  5. It is true that sex can be a very costly for females in terms of energy if that occasion leads to reproduction of offspring. However, if this action were not enjoyable then people would scarcely do it. Therefore, enjoyable sex is a selective advantage as it ensures reproduction and the passing of traits to the next generation. Sexuality overall is a very important part of life. Sigmund Freud claimed that sexual desires (and aggression) were the root of our actions/behavior. Carl Jung stated that people have libido, a sexual drive that attracts one human being to another. Apparently it is not only humans that exhibit these behaviors. Both bonobos and dolphins are known to have sex for pleasure rather than just reproductive intentions. One reason for this may be due to how social each species is. The bonobo monkeys have sex constantly, not only between male and female but also between those of the same sex and those of different ages (old and young). This sexual activity is said to be a way the bonobos solve conflict: when a coupld of bonobos have a conflict they may make up with sex. Overall they are a peaceful species. Sex may be a contributing factor as a gives them a bond. Dolphins also have sexual relations with many members of their schools/pods. They often engage in sexual activity before they have even sexually matured. This is not a bad thing because it instills that sexual desire in them. That desire is important because it will encourage them later in life to reproduce and continue the growth of the population.
    There is still the issue of how taxing reproduction is on the female's body. Yet when the costs and benefits are compared between lots of sex and painful/energy costing reproduction, the benefits may outway the cost. These benefits include genetic variation. Sexual reproduction allows for genetic variance and possibility of mutation. With a variety of genes and few mutations, evolution is greater. These different genes respond differently to environmental pressures. Therefore, natural selection may select for certain genes and not others. With constant sexual activity and reproduction, that evolutionary process will be much faster allowing for that species to become more advanced (which is a huge benefit).

    Why Evolution is True
    Psychology Themes and Variations