My main worry about sensory bias theory is that stimulating a sensory system is only the first step in influencing a mate choice decision. Grabbing a potential mate's attention is a long way from winning his or her heart. Granted, for animals that live widely separated from one another, it may take a lot of effort to find anyone of the opposite sex during the mating season. Under these conditions, making a strong sensory impression would give an animal a reproductive advantage. A whale song audible from hundreds of miles away can help two lonely whales to find each other. For many species, locating a mate—any mate—is a big problem. The sensitivity of their senses may be crucial to finding a mate, so may have a significant impact on sexual selection.
For highly social animals like most primates, finding potential mates is not the problem. Many primates already live in large groups, and interact regularly with other groups. They are spoiled for choice. When mate choice depends more on comparing mates than locating mates, the sensory engineering argument seems weaker. Why should an individual be perceived as a more attractive sexual partner just because its ornamentation happens to excite some brain cells in the lowest level of one's sensory systems? If it were that easy to make animals come running, predators would more often evolve lures to dupe prey into approaching them.
Our intuition may tell us that strong sensory effects are sexually attractive, but I doubt this attractiveness is explained entirely by sensory bias arguments. There are good adaptive reasons why ornaments that produce strong sensory effects make good fitness indicators. Consider the list of sensory bias effects that Michael Ryan and A. Keddy-Hector compiled in an important review paper of 1992. They noted that animals usually respond more strongly to visual ornaments that are large, brightly colored, and symmetrical, and to auditory ornaments (e.g. songs) that are loud, low in pitch, frequently repeated, and sampled from a large repertoire. These responses could be attributed to sensory engineering effects. But that begs the question of whether the sensory engineering evolved to help animals choose good sexual partners. Large, healthy, well-fed, intelligent animals can produce larger, brighter, and more symmetric visual ornaments, and louder, deeper, more frequent, and more varied songs. As far as I know, there is no example of a sensory bias that leads animals to favor sexual partners that are smaller, less healthy, less energetic, and less intelligent than average. Most sensory biases are consistent with what we would expect from adaptive decision-making machinery that evolved for mate choice. It may not have evolved specifically for mate choice, but it might as well have.
Many sexual ornaments may look as if they are merely playing on the senses. They may appear to be nothing but fireworks, sweet talk, eye candy, special effects, and manipulative advertising. But maybe we should give the viewers more credit. What look like sensory biases to outsiders may have a hidden adaptive logic for the animal with the senses.
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