What the Nose Knows - Avery Gilbert [31]
Scientists are taking a fresh look at the conventional wisdom regarding the sense of smell in animals. The anatomists Timothy Smith and Kunwar Bhatnagar, for example, are questioning the textbook distinction between macrosmatic and microsmatic animals, i.e., those with good and poor olfactory abilities. The long-standing assumption is that what separates macrosmatic and microsmatic species is the amount of surface area inside the nose. This turns out to be a bad assumption; internal surface area is more about air conditioning—warming and filtering incoming air—than about odor perception. Of more relevance is the amount of sensory tissue in the nose. But Smith and Bhatnagar find that the amount of sensory tissue varies from species to species independently of total surface area. Further muddying the waters, the number of olfactory nerve cells per square inch varies among species. All in all, surface area is a dubious proxy for smell ability. Smith and Bhatnagar suggest that the traditional macrosmatic/microsmatic distinction has outlived its usefulness. Size isn’t everything.
The Yale University neurobiologist Gordon Shepherd agrees that counting nerve cells is a poor way to estimate sensory talent. In his view, the number of cells available for odor detection is less important that what the brain does with the information those cells provide. He makes the analogy to hearing: humans have about the same number of auditory nerve fibers as cats and rats, yet we have far superior speech abilities. It’s the brain areas that analyze and interpret speech sounds that provide the advantage, not the number of cells in the ear.
The German sensory physiologist Mathias Laska cuts right to the chase by measuring odor perception in different animal species. He has used reward-based conditioning techniques to find odor detection thresholds in spider monkeys, squirrel monkeys, and pigtail macaques. According to conventional wisdom, these primates are less sensitive than dogs and rabbits, but Laska finds they perform quite well—monkey thresholds are comparable to those of dogs and rabbits across a variety of odors. And contrary to Darwin’s gloomy belief, Laska finds that humans have odor sensitivity similar to that of apes and monkeys.
New evidence suggests that humans and animals may be more similar in odor perception than we thought. In 1991, Linda Buck and Richard Axel discovered a large family of mammalian olfactory receptor genes, work for which they eventually received the Nobel Prize. Each gene produces a different receptor. In general, more receptors means more detectable odors, and therefore greater smell ability. Rats have about 1,500 functional receptors, followed by dogs with about 1,000, mice with about 900, and chimpanzees with about 350. Humans have somewhere between 340 and 380. Dolphins have zero.
Does this mean rats are five times better smellers than we are? Not really. We can use DNA sequence similarity to arrange odor receptors into families and subfamilies. In theory, similar receptors detect similar odor molecules, so a receptor subfamily detects a class of related odors. When we compare odor receptor subfamilies, the human-animal gap doesn’t look too large. Humans and dogs have about 300 subfamilies, rats have 282, and the mouse 241. The overlap between species is substantial. About 87 percent of human receptor subfamilies have counterparts in the mouse genome, while 65 percent of mouse subfamilies are shared by humans. This suggests to Linda Buck and her colleagues that “the majority of odorant features [i.e., smells] detectable by one species may also be recognized by the other.” Perhaps a mouse can smell more of our world than we can smell of his. (Unlike us, he may have a whole subfamily of receptors devoted to cat urine.) For man and mouse the differences are not as big as the similarities. For man and chimp this is