Here is a strange question: does the sight of a peacock’s tail make you sick? Well, it did have this effect on Charles Darwin. The reason was perhaps more cerebral than anything else. With A Taste for the Beautiful: The Evolution of Attraction, professor in animal behaviour Michael J. Ryan gives a superbly readable and accessible account of his and other’s studies that address how sexual beauty comes about, and why we see such a bewildering diversity of traits used in mate choice.
Following the formulation of his theory of natural selection, Darwin struggled to explain why males in some species would prance around with such impractical and cumbersome ornaments as a peacock does. Enter the theory of sexual selection (see also Prum’s recent, rather misleadingly titled The Evolution of Beauty: How Darwin’s Forgotten Theory of Mate Choice Shapes the Animal World – And Us. Misleading because this theory has been anything but forgotten). If traits contribute to making an animal sexually more attractive and thus reproductively more successful, sexual selection will favour them, even if this leads to opposing pressure from natural selection. Darwin was thus able to give us a “why” for the existence of sexual beauty, but not really a how. How does this lead to visual signals in some species (e.g. colours), but auditory in others (e.g. bird song or mating calls in frogs)?
For a long time, a favourite explanation was that choosers (often females) prefer certain courters (often males) based on traits that indicate that the courter is genetically fit and healthy and will pass on this genetic legacy to the couple’s offspring. However, there isn’t all that much empirical support for the “good genes” hypothesis. If this sounds familiar, “good genes are overrated” was indeed the central message of Rosenthal’s Mate Choice: The Evolution of Sexual Decision Making from Microbes to Humans (Ryan mentored Rosenthal as a graduate student, so it is not surprising to see the agreement between their arguments). Ryan has become famous for his idea of sensory exploitation. He argues that beauty is in the brain of the beholder. Brains do a lot more than thinking about sex. Foremost, they busy themselves with survival by finding food, or avoiding becoming someone else’s meal. Depending on the exact ecology of a species, vision might be important for this, or sound. Point is, an organism’s brain will be sensitive to certain queues in the environment, it will have certain biases. Colour vision in many primates for example is very sensitive the colour red, as it helps them detect food in the forest canopy. And that is what sexual selection latches onto. It exploits these already existing sensitivities. Thus, the colour red is used in primate mating displays. But that preference for red was already in place, in-built in the brain for use in other contexts. The preference for certain sexual traits precedes these traits rather than the other way around.
“Brains do a lot more than thinking about sex”
Ryan has spent decades working on túngara treefrogs, a small frog species living in Panama where males gather in groups and produce calls to attract females. They produce a two-part call, a high-pitched whine, followed by low-pitch chucks. But why? Ryan’s neurobiological work over the years has shown that males call at those frequencies that elicit the strongest neural responses in the females.
The middle section of the book looks at other examples of visual, acoustic and olfactory beauty throughout the animal kingdom. It quickly becomes clear that Ryan does not think sensory exploitation is king; there are other explanations for some observations. Birdsong might help a chooser pick a courter of the same species (also important). Smell can be co-opted to prefer a partner who is genetically different enough from you so as to avoid inbreeding.
Ryan effortlessly switches between anecdotes of his research, clear explanations of theoretical ideas (e.g. Weber’s law, fluctuating asymmetry), and remarkable observations. And thus we meet bees that shag flowers that look like bees, on the off chance they miss out on mating with a female. Frogs who walk the fine line between sex and death because their mating calls attract both females and eavesdropping bats that eat them. Or why it is that the girls in a bar are prettier towards the end of the evening (hint: preferences can be fickle, and there is an element of time in here).
“[…] we meet bees that shag flowers and frogs who walk the fine line between sex and death […]”
Ryan has purposefully chosen to exclude weapons and combat, another result of sexual selection, from his book. For that he refers the reader to Emlen’s Animal Weapons: The Evolution of Battle. Similarly, mate choice and sexual selection express themselves in the nuts and bolts of reproduction: genitals, which Ryan leaves out of consideration. For this, Schilthuizen’s Nature’s Nether Regions: What the Sex Lives of Bugs, Birds and Beasts Tell Us About Evolution, Biodiversity and Ourselves is an amusing introduction.
A Taste for the Beautiful is not meant to be an exhaustive overview or a theoretically comprehensive treatise. Don’t expect summaries, diagrams, tables, or formulas. Rather, it is a fun and surprisingly accessible book explaining how certain traits end up being perceived as beautiful and how important the role of the brain is in all of this.
Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.
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