Senses as Engineering Compromises

A more significant claim from sensory bias theory is that animal senses have certain features that evolved just because they efficiently solve the information-processing problems of perception, and these features can drive sexual selection. Eyes have to perceive objects in general, and there may be general principles relevant to this task, principles which may influence mate choice;

Consider the area at the very back of the brain called the primary visual cortex, or "V1." This is the conduit for almost all information that passes between the eyes and the rest of the brain. Each V1 brain cell covers a tiny area of the visual world, and fires most actively when the local pattern of light in that area corresponds to the edge of an object. V1 seems to be a set of edge-detectors. Vision researchers believe that this is simply an efficient way to process visual information about the world, since vision is about seeing objects, and objects tend to have edges. This edge-detection principle has been used in most successful robot vision systems designed by humans.

Now consider how a male could grab the attention of a female's V1 system. He has to activate her edge-detectors. He could evolve a body that has many more real edges than average, perhaps a sort of fractal design. But the more real edges he has per unit of body volume, the more fragile his body would be and the more heat he would lose. Better to evolve sexual ornaments that display lots of fake edges. Dots would work, but thin parallel stripes would be even better, displaying more edge information per unit area. Perhaps stripes became popular sexual ornaments across many species because stripes are optimal stimuli for activating the visual cortex.

A similar explanation might account for the popularity of sexual ornaments with bilateral and radial symmetry. Biologist Magnus Enquist suggested that symmetric patterns might be the most exciting way to stimulate animal visual systems. He argued that any visual system capable of recognizing objects when they are rotated will tend to be "wired" in such a way that it is optimally excited by radically symmetric patterns. Enquist and his collaborator Arak did some evolutionary simulations in support of their claim that any neural network capable of recognizing rotated objects would be optimally excited by radially symmetric patterns. Supposedly, this explains the popularity of sexual ornaments that resemble stars, sunbursts, and eyespots.

In addition to fulfilling general engineering principles, the senses of each species must also adapt to its particular habitat and ecological niche. Sensory bias theorists such as John Endler have investigated how different lighting conditions influence the sensitivities of different animal visual systems. This sort of research promises to help biologists predict which animal lineages are more likely to evolve particular kinds of sexual ornament that play upon particular sensitivities. This application of sensory bias theory might help biologists to discern more patterns beneath the apparently chaotic proliferation of sexual ornaments in different species.

Yet a different view of sensory biases may explain why ornamentation evolves so unpredictably. For example, given the same problem of categorizing visual shapes, two different species may evolve two rather different solutions. One may evolve to represent visual shapes as variations on some sort of generalized cylinder, while the other may represent visual shapes as sets of facets and angles. Both ways of mentally representing shape may work perfectly well, but they might respond very differently to a novel sexual ornament that has a particular shape. The ornament might make an aesthetically pleasing generalized cylinder, but a very unappealing set of facets, or vice versa. The ornament may prove a sexual success in one species but not in the other.

One of the deepest insights from sensory bias theory is that there is always some evolutionary contingency in the design of perceptual systems. These contingencies make it impossible to predict all possible responses to all possible stimuli just from knowing what a perceptual system evolved to do. Therefore, if a new sexual ornament evolves that excites a perceptual system in a novel way, it may be favored by sexual selection in a way that could never have been anticipated. For example, biologist Nancy Burley found that female zebra finches just happen to be attracted to males that have tall white plumes glued on top of their heads. Their white-plume preference probably did not evolve as an adaptation, because as far as we know, ancestral finches never had white plumes on their heads. The preference just happened to be a latent possibility in a visual system that evolved for other purposes. I think this idea of evolutionary contingency in perceptual systems is one of the most intriguing ideas to come out of sensory bias theory. It might even work better than runaway sexual selection as a general explanation of why sexual ornaments diversify so unpredictably in different species.

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