Thursday, March 10, 2011
I haven't done a why-people-find-other-people-attractive story in a while. So I started looking through the literature, typing various search terms into journal databases that in any other context than science might have gotten me spam filtered. One of the first studies that caught my eye was from 2003, and it correlated facial attractiveness and semen quality in adult human males and tested how women rate facial attractiveness in various stages of their menstral cycle. Interesting. That study lead me to a more recent study out of Australia that sought to replicate the 2003 experiment and determine the generality of the relationship between attractiveness and semen quality.
But, as usual, let's take a step back. Not quite as big of a step as usual since you can click any of the links below and get a nice little sexual selection primer, but instead look at sexual selection in terms of honest signaling. Consider this: In a species where females choose the males, she can choose any male from a pool of individuals of varying qualities. In otherwords, she can choose a male with characteristics she likes. Those male characteristics are linked to attributes of the male that the female can use to assess his quality. This is where the honesty part comes in to play. If that characteristic, or signal, is honest that means that the quality of the male is what she thinks it will be. That means, on average, a certain degree of these signals must be honest for this system to work, otherwise females would just ignore them. So what are these signals anyway? Well, it depends on what characteristic you are talking about. Sometimes the signal is costly like the peacock dragging around that giant tail or a frog producing a mating call that will also tell predators his location. Signals like these can be handicaps - see Zahavi's Handicap Principle or Grafen's model - in which only very fit males can handle the cost of a handicap (very fit = good quality). Other signals fall into Weatherhead and Robertson's Sexy Son Hypothesis (or Dawkin's The Selfish Gene) in which a female chooses a particular mate because his genes will produce male offspring with the best chance of reproductive success. Of course, the female can't see the genes themselves but if she chooses an attractive male she will have attractive offspring and thus be more likely of passing on her genes.
In the field of animal behavior, mate choice is a popular and well studied subdiscipline. Typically these types of experiments are conducted with animals, called mate choice experiments. You give an individual, usually female, a choice between potential mates with different characteristics (looks, smells, displays, etc.) and see which one she chooses. Now I know that I've posted several studies on sexual selection including ones on fruit flies, mollies (fish), cichlids (also fish), seahorses, and even algae. I've also done a few stories about mate selection in humans, typically about why men find women attractive in a series of posts I like to call "Groundbreaking science - men like boobs." And today's study fits within that topic.
This is a study that looked at the phenotype-linked fertility hypothesis in humans. In the case of humans studies have shown that both face and body attractiveness contribute to overall attractiveness. Logical. So in this study they collected ratings of both and combined them into a single attractiveness component. So what did they do exactly? They picked 118 herterosexual, caucasian men between the ages of 18-35. They had each participant photographed, full body length and face close-up, wearing a white shirt and dark-colored shorts standing in a neutral position. Then the men filled out a questionnaire regarding lifestyle factors such as cigarette use, dietary habits, etc. (as they could potentially affect semen quality), and recorded self-measured testes volume. And, as this study evaluates semen quality, they took a semen sample. From this sample factors such as sperm concentration, motility, and morphology could be quantified. Then they had females rate each male face and body for attractiveness, in terms of a short-term sexual partner, on a seven-point scale (1 = not attractive; 7 = very attractive). A second set of females, not using hormonal contraceptives, were asked to rate the photographs at high and low fertile points in their menstrual cycle.
The researchers found no associations between attractiveness and semen quality. They also found no relationship between semen quality and measures of masculinity, symmetry, or averageness. So basically, this study does not replicate the 2003 study, which did find such relationships, and suggests that attractiveness in human males does not provide women with cues to their reproductive potential. Why the difference between studies? Who knows. It could be rating scale, including bodies and not just faces, how time in the menstrual cycle was determined, the number of men included in the sample, or any of a number of factors. So we have two studies that found different results. Why present this study at all? It is a good example for replication in science and an interesting study. I guess the jury is still out on whether or not being attractive is a signal for reproductive quality. However, coming from a female prospective - looks aren't everything but good looks don't hurt.
Read the study here:
Peters, M., G. Rhodes, and L.W. Simmons (2008) Does attractiveness in men provide clues to semen quality? Journal of Evolutionary Biology: 21, 572-579. (DOI: 10.1111/j.1420-9101.2007.01477.x)
And here is the study that they replicated:
Soler, C. et al. (2003) Facial attractiveness in men provides clues to semen quality. Evolution and Human Behavior: 24, 199-207. (DOI: 10.1016/S1090-5138(03)00013-8)
(image from corbis images)