Beauty in the eye, or brain, of the beholder

Review of: Ryan, Michael J. 2018. A taste for the beautiful: The evolution of attraction. Princeton Univ. Press, Princeton. 224 pp. ISBN: 9781400889150. $27.95 HB Beauty exists entirely in the eye, or rather the brain, of the beholder. This is the central theme of Michael Ryan’s, “A Taste for the Beautiful,” which focuses specifically on perception of sexual beauty. Sexual signals, often displayed by males, are among the most beautiful and extravagant traits found in nature and have fascinated biologists for centuries. In his book, Ryan tackles the question that has long gripped evolutionary biologists: why would females evolve preferences for such costly traits? Classic hypotheses for explaining female sexual preferences focus on fitness consequences. These traditionally suggest that females prefer elaborate male traits because such traits confer indirect benefits to offspring through good genes, render offspring more sexually attractive, or indicate a male’s ability to provide direct benefits such as food or high-quality nesting sites (reviewed in Andersson 1994). Another explanation, sensory bias, posits that females prefer particular sights, sounds, and smells of male sexual signals because the associated neural processing centers became highly developed for reasons other than mate choice, such as finding food or avoiding predators (Fuller et al. 2005). As one of the primary architects for the concept of sensory bias and exploitation, Ryan falls firmly into the latter camp (Ryan and Rand 1990). He argues that sexual traits evolve by exploiting preexisting sensory biases resulting from selection for other unrelated ecological tasks. Therefore, to see the full picture of the evolution of sexual preferences, we must consider the underpinning neurological mechanisms. While many evolutionary biologists are already familiar with the idea of sensory exploitation, this book takes a more general, engaging tack intended for nonbiologists. One of the book’s main themes is that what is deemed beautiful or attractive in one species may be repulsive, or not even perceived, in another—beauty is entirely subjective. As Ryan puts it, “I find the Mona Lisa beautiful, and perhaps you don’t. We both see the same arrangements of colors within the frame; we just process them differently” (p. 19). Each individual is a unique combination of neural circuitry and behavioral traits that ultimately decides what is beautiful. Using vivid imagery, Ryan makes this point by showcasing taxonomically diverse examples of animal communication across a number of sensory modalities. Readers are invited to marvel at the majesty of a Bowerbird’s lair, listen to a cricket’s charming song, and smell the perfume of an orchid bee, while simultaneously learning how each species’ brain and sense organs process this information. The examples are enhanced by a series of stunning plate photographs. Ryan’s discussion of the evolution and perception of sexual beauty also masterfully cuts across the organizational scale of living things, jumping between genes, cells, sensory systems, and organisms. This is best evidenced by his engaging description of the sexual preferences of the Túngara frog, which ranges from describing the role of genes to sensory organs to behavior as they relate to the frogs’ whine-chuck system. Because this is Ryan’s primary study species, he also includes interesting “behind the scenes” details to explain how the research unfolded. Ryan tackles his argument in threefold, providing examples of visual, acoustic, and olfactory signaling systems and discussing the brain’s importance in processing these signals. However, while he makes a clear case for the need to include the brain’s role in the story of sexual signal evolution, only occasionally are these neurological mechanisms discussed in detail. More often than not, he simply describes examples of different animal signaling systems while the underlying reason for the sensory bias is not revealed. Certainly though, these tales of different signaling systems and female mating behaviors are fascinating and provide the reader with a deeper appreciation for the diversity of sexual beauty. Ryan also emphasizes that existing biases could lead to future exploitation and explains how there are many potentially untapped ways