Why whales can only taste salt

Taste, as people are generally familiar, is dependent upon taste buds located on the tongue. These taste buds consist of cells with taste proteins embedded on them to directly interact with chemicals that the tongue encounters. These proteins detect at least five basic tastes in mammals: sweet, umami/savory, bitter, sour and salty.

We’d generally expect that the taste receptors present in a particular animal would correspond to things in an animal’s diet. Humans are omnivorous (i.e., we eat plants and animals) and therefore benefit from experiencing all of these types of tastes. But what if an animal adapted to an extreme diet does not require the ability to detect all sorts of tastes? An evolutionary biologist would expect that they would lose at least some of their taste capabilities, and this change in taste would be reflected by the presence of taste receptor pseudogenes.

Whales potentially fit this criterion, as they live in a marine or freshwater environment and feed on fishes, krill and related animals exclusively. When was the last time you saw a whale eat a sweet apple or a sour citrus? Another thing to consider: if you’ve ever been in the ocean, do you think if you tried eating fish with your bare teeth you could taste anything but salt? Perhaps this extreme diet and salty medium have induced whale tastes to change through time.

McGowen et al. [1] highlight the fact that taste buds in whales are rare or completely absent, and a recent paper by Feng et al. [2] demonstrated that this taste bud reduction/loss is mirrored in the genes that code for taste proteins. Specifically what they discovered is that most whales appear to have lost the ability to taste anything but salt.

orca_tongue_by_remedy_kiua-d50mn10

This means Shamu here could at least appreciate some french fries.

The genes T1R1 and T1R3 encode the umami/savory receptor, whereas T1R2 and T1R3 encode the sweet receptor. This means that the protein encoded by T1R3 is crucial for both of these tastes. Feng et al. [2] acquired DNA sequences for the T1R3 gene in two baleen and five toothed whales and discovered that they all have the same shared inactivating mutation (pseudogene), suggesting that the ancestor to modern whales had lost both umami and sweet tastes. Additionally, the T1R1 gene has a mutation shared by three baleen and six toothed whales, suggesting this too became a pseudogene in the ancestor to modern whales, while T1R2 appears to have been lost independently in the ancestor to baleen whales and toothed whales respectively.

The bitter taste is encoded by many genes (T2Rs), with at least 10 discovered in whales. Eight of these had inactivating mutations shared by all baleen and toothed whales looked at in the study, while the other two genes were apparently inactivated in different groups of whales independently. The exception is that the T2R16 gene, while inactivated in two baleen whales, appears to be intact in three others, which would seem to indicate that these whales (fin, minke and bowhead) still have some bitter taste capacity.

The sour taste is encoded by the genes Pkd1l3 and Pkd2l1. While the authors could not find the former gene, they did find pseudogenic versions of Pkd2l1 in one baleen and two toothed whales.

The salty taste receptor, encoded by Scnn1a, Scnn1b and Scnn1g is, however, intact in the one baleen and two toothed whales that were examined. Whether or not these are actually used for taste however is unclear. These genes are also associated with other aspects of physiology, such as blood pressure, meaning that these genes may be maintained strictly for other purposes. Nonetheless, it does suggest that there is a possibility that whales can still taste salt.

Questions for Creationists

Why would God create whales with taste genes that are nonfunctional? Additionally, why would He create them so that both baleen and toothed whales share the same mutations? Would it not have been more straightforward to create whales that completely lack the genes entirely rather than having nonfunctional remnants?

References

1. McGowen, M. R., Gatesy, J., & Wildman, D. E. (2014). Molecular evolution tracks macroevolutionary transitions in Cetacea. Trends in ecology & evolution29(6), 336-346.

2. Feng, P., Zheng, J., Rossiter, S. J., Wang, D., & Zhao, H. (2014). Massive losses of taste receptor genes in toothed and baleen whales. Genome biology and evolution6(6), 1254-1265.

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2 thoughts on “Why whales can only taste salt

  1. Pingback: Pseudogenes: Why giant pandas probably can’t appreciate a filet mignon – Evolution For Skeptics

  2. Pingback: Pseudogenes: Why cats, hyenas and seals don’t love sugar – Evolution For Skeptics

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