Did Darth Vader have a low sperm count?

Lilting voice? No need to look on the dark side. Bisgrafic

Few individuals in the Star Wars universe inspired more fear than the Sith Lord, Darth Vader. But beneath the dark exterior, the commander-formerly-known-as-Anakin-Skywalker might have been hiding a dark secret: poor semen quality.

The give-away? Vader’s iconic, deep voice.

New research by Leigh Simmons, Marianne Peters and Gillian Rhodes – published on Wednesday in the journal PLoS One – suggests men with low-pitched voices might have poor semen quality.

As with other exaggerated male features in nature – including the peacock’s train, the lion’s mane and deer’s antlers – the deep voice of the human male might be explained by female mate choice. But why should females be interested in what seem to be only aesthetic qualities?

The best-supported explanation is that these characteristics are indicators of some aspect of the male’s suitability as a reproductive partner. In other words, seemingly aesthetic characteristics are correlated with something far more important.

EPA/M.A Pushpa Kumara

So, what’s important to a female in a sexual partner? Two things mainly: the quality of the genetic material the male will contribute to her offspring, and his ability to help raise the kids. In monogamous species, both of these are important.

But does it work the other way round – do males select for exaggerated characteristics in females? They do to some extent – think of a low waist-to-hip ratio and permanently enlarged breasts in human females – but not normally nearly to the same degree that females do.

This is because females typically “invest” far more in reproduction than males. In the case of mammals, this investment includes pregnancy and lactation, making female reproductive output far more limited than that of males. In extreme situations, a male’s contribution to reproduction may be nothing more than sperm.

Females are the limiting resource in the reproductive market and can therefore do most of the choosing.

All of this may explain many of the differences between men and women – why men are, on average, bigger, hairier and more aggressive, and even why they have deeper voices.

The study by Simmons, Peters and Rhodes tests the idea that a deep voice is an honest signal of a man’s fertility. Previous studies have shown that women find masculine faces and deep voices attractive.

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Interestingly, the preference for deep voices is greatest when women are ovulating and when they are considering a short-term relationship. An attractively deep voice is also positively correlated with a man’s self-reported number of sexual partners, and in the Hadza hunter-gatherers of Tanzania a deeper voice was a reliable predictor of a higher number of living offspring.

The new study tests for a correlation between the attractiveness of a man’s voice and various measures of his semen quality. Some 54 men between the ages of 18 and 32 were recruited from the campus of the University of Western Australia for the study. After having their voices recorded, participants were presented with the same four explicit images and asked to deliver a semen sample to the laboratory, within one hour of collection.

The voice recordings were rated for attractiveness and masculinity by 30 women between the ages of 18 and 30. The semen samples were analysed for concentration of sperm and various parameters associated with sperm swimming speed.

The researchers found that, as expected, the women rated deeper voices as more masculine and attractive. But there were no correlations between voice attractiveness and the various measures of sperm swimming prowess. Contrary to expectations, the researchers also found that men with more attractive voices had lower sperm counts.

According to Simmons, Peters and Rhodes, the negative relationship between voice attractiveness and sperm count may be explained as a trade-off between investing energy in attracting a mate – by generating low-pitched sounds, among other things – and producing sperm. Although sperm are usually thought of as being “cheap” to produce, there is evidence showing it takes considerable energy to produce the millions of sperm in each ejaculate.

(Given most of the sperm counts were within the normal range for functional fertility, there’s also a chance the relationship may not be telling us much at all.)

Deep voices are sexy, but why?

So you might ask: if men with deep voices have lower sperm counts, why is a deep voice considered attractive by women?

Well, men with deep voices are perceived as being stronger, larger, better fighters and providers, and more dominant. Perhaps fertility is not the important character that is being assessed. More important may be genetic quality and dominance, with its associated abilities to provide for offspring.

In fact, most exaggerated, sexually-selected characteristics are explained as “honest” signals because they are difficult to fake. It may take a lot of energy and good health for a bird to develop and maintain bright plumage, making it a reliable indicator of the individual’s underlying genetic quality. Similarly, a deep voice, requiring a large, well developed larynx, may be an indicator of good genes.

This new study asks an interesting question about human mating behaviour and will hopefully inspire further studies to uncover the deeper meaning of the attractiveness of a low-pitched male voice.

Until then, may the force be with you.