Sabores

OUTLOOK TASTE
trio wrote1 in a letter to Nature. More than 70 years later, biologists are still trying to figure it out. The availability of different animal genomes has given scientists more insight, culminating in the startling discovery that, for many creatures, some tastes have no evolutionary benefit at all. Kurt Schwenk, a biologist at the University of Connecticut in Storrs who studieschemical sensing in lizards and snakes, says: “The whole story of the evolution of taste is really the evolution of loss of taste.” The most obvious explanation for the changes is lifestyle. At some point in evolutionary history, a shift in diet removed the need to sense certain chemicals in food. Evolution is a game of ‘use it or lose it’, and genes that do not aid an animal’s survival orreproduction are liable to build up random mutations that destroy their ability to make a working protein. Gary Beauchamp, a geneticist at the Monell Chemical Senses Center in Philadelphia, Pennsylvania, likens the situation to that of sight in cave-dwelling fish. A life of darkness eliminated the usefulness of vision, so the fish collected mutations in genes involved in eye development and eventually losttheir sight altogether. Another example of taste loss lies closer to home: cats cannot taste sweet substances. Beauchamp noticed this quirk in the 1970s, and in 2005 his team finally found out why2. All cats share a mutation that disables one of the two genes that build a working sweet receptor, whether for the tongue, intestine or any other part of the body. Because all felines — from domesticcats to lions — have an identical mutation, it is likely that the sweet-receptor gene became inactive in their common evolutionary ancestor. Beauchamp speculates that this ancestral animal moved to a diet composed of protein-rich meat, devoid of sugary plants, negating the need for a sweet receptor. Beauchamp’s team recently discovered that the inability to taste sweetness is more widespread. Theyanalysed 12 non-feline species belonging to the order Carnivora, including sea-lions, otters and hyenas, and identified crippling mutations in the sweet-receptor gene in 7 of them3. What’s more, six species carried unique mutations, suggesting that the ability to taste sweetness had been lost repeatedly over the course of evolution. Presumably the mutations appeared after each species or itsancestors switched to a meat-only diet, Beauchamp says. This interpretation is supported by the team’s finding that an omnivorous member of Carnivora, the spectacled bear, still has a working sweet receptor. There may be other reasons why tastes are so dispensable. As well as lacking sweet receptors, dolphins also lack the ability to build working umami (which sense amino acids in proteins) and bittertaste receptors. One theory posits that because dolphins swallow their food whole, moderating their intake is irrelevant. However, this theory applies only to
SOPHIE CASSON /THREE IN A BOX

The lost appetites
Many vertebrates can detect the same five basic tastes that humans can, but there are exceptions. Are the differences caused by a change in diet?
B Y E W E N C A L L A W AY

E VOLUTIONARY B IO LO GY

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onald Fisher must have been relieved when a chimpanzee at Edinburgh Zoo took a sip of water, looked him in the eye and spat at him. It was August 1939, and Fisher was testing whether chimps could taste water laced with a chemical called phenylthiocarbamide (PTC) that some humans find nauseatingly bitter and others can’t taste. Fisher and his colleagues Edmund Ford andJulian Huxley
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had been worried that the apes wouldn’t make their preferences known, rendering the experiment pointless. Instead, about three-quarters of the chimps they tested expressed their displeasure with PTC. Fisher’s team speculated that the variation in sensitivity to bitter tastes was caused by a genetic mutation shared by humans and...