AnimalsPhysiology

Tasteless animals

Whale-watching is big business, and for good reason: Many humans are fascinated by the size and social behavior of whales. From their complex communication systems to their feeding strategies, whales are intelligent, acrobatic, and fearsome.

 

Whales belong to a group of mammals called the cetaceans. Biologists divide the 86 or so species of cetaceans into two groups: baleen and toothed whales. The baleen whales, including the enormous blue whale, produce giant oral baleen plates. Huge gulps of ocean water are forced through the baleen, trapping krill and other small prey for the whale to eat. The toothed whales, which include orcas, dolphins, and porpoises, use their teeth to grab fish, seals, and other prey.

Researchers have known for some time that some whales have a limited sense of taste. For example, behavioral research suggests that bottlenose dolphins have a reduced perception of sweet, umami, and bitter tastes when compared to other mammals. This result is not unique, as some other mammals have their own food-related idiosyncrasies; cats and some otters are indifferent to sweets, and umami sensations are reduced or absent in giant pandas.

 

DNA sequencing technology has enabled researchers to study flavor perception in unprecedented detail. For example, we now understand that embedded in the cell membrane of each taste receptor cell is a protein or combination of proteins. These proteins are encoded by genes: Combinations of T1R (taste receptor type 1) genes encode umami and sweet taste receptors, and T2R (taste receptor type 2) genes encode bitter taste receptors. Taste molecules bind to T1R and T2R proteins, producing sensations of umami, sweet, or bitter. (The proteins of sour and salty taste receptor cells are ion channels, and they are encoded by genes other than T1R and T2R.)

 

A mutation in any taste-related gene may render a receptor protein nonfunctional, but natural selection often weeds out mutations that reduce the sense of taste. After all, flavors allow animals to detect both nutrients and potentially noxious chemicals in food. In some cases, an animal’s feeding behavior may help explain the loss of a taste sensation. Cats are carnivores, so a taste for sweet foods might not help or harm them. Giant pandas eat bamboo, which lacks umami flavor components. Ancient mutations in the panda T1R gene would have made little difference in survival.

 

And what of the cetaceans?

A partial dolphin genome released in 2012 revealed that these marine mammals lack intact T1R and T2R genes. Are these gene mutations unique to dolphins, or do related whale species also lack functional taste receptor genes?

 

Researchers Ping Feng and Huabin Zhao of Wuhan University in China teamed up with other scientists in China and England to learn more about what flavors whales can taste. The team extracted DNA from preserved tissue samples of seven species of toothed whales (including bottlenose dolphins) and five species of baleen whales. They already knew the sequences of the 3 T1R and 10 T2R genes of the bottlenose dolphin, based on the partial genome. Using these DNA sequences, the team used the polymerase chain reaction (PCR) to identify and amplify taste receptor genes from the other whale species. Finally, they sequenced each gene they found and searched for disruptive mutations, which either change the reading frame of the gene or cause premature stop codons.

 

Figure summarizes the results of their analysis. All species of toothed and baleen whales in the study had disruptive mutations in T1R genes, indicating that the marine mammals likely cannot taste umami or sweet. Moreover, all T2R (bitter) genes were nonfunctional in all seven toothed whale species. One T2R gene appears to be intact in three species of baleen whales; however, the researchers suggest that this one bitter taste receptor gene does not substantially confer bitter taste.

 

The researchers also searched for one sour taste gene in three whale species. In each case, stop codons preceded the end of the gene, strongly indicating that the whales cannot taste sour foods. Using the same method, the team searched for the three genes encoding salty taste receptors. These genes were nearly intact in each whale species, suggesting that the whales can likely taste salt.

 

Overall, the results indicate that whales have lost four of the five flavor sensations that most other mammals experience; salt is the lone exception. What makes these marine mammals different from their relatives on land? One possible explanation relates to the unique feeding behavior of whales: They typically swallow their food whole. This behavior may have selected against the maintenance of taste receptor genes; over time, the genes have accumulated so many mutations that they now encode nonfunctional receptor proteins. This research also raises new questions:

How do whales avoid ingesting toxic or noxious foods without functional bitter taste receptors? Do other whale senses compensate for the loss of taste? Future research may reveal more about sensation in these “tasteless” animals.

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