Vampire bats care little for sweet blood

This is the first blogpost in a continuing series on “sensible evolution‘: how our senses evolved and shape the way we see the world. We perceive everything that we can see and feel as ‘real’, but we know that our human senses only capture a tiny part of the natural world. There are other realities out there. At least, we seem to have a richer taste palette than some of our animal cousins, including the mythical vampire bat..

Tasteful mechanisms
Having ‘good taste’ is not only necessary to gain the approval of ‘high society’ – it also vital for survival in many animal species. Bitter or sour tastes can warn you that a certain food source contains toxins or is otherwise harmful, while a salty taste indicates the food is rich in minerals. Sweet and umami tastes hint at the presence of carbohydrates and proteins – telling you this piece of food is likely nutritious. Special taste receptor cells on the tongue are responsible for detecting the tastes, but they don’t recognize each taste equally. Sour and salty tastes are detected via ion channels, whereas bitter, sweet and umami are recognized by GPCRs.

Animals display a huge variation in bitter taste receptors, since what is toxic for some animals, can be nutritional for others. Rats have twice as much different bitter taste receptors (42) as dogs (21), for example. There’s not that many ways to recognize sweet and umami though. The Tas1r1 (umami) and Tas1r2 (sweet) proteins both team up with Tas1r3 as heterodimers to form the functional taste receptors, as in the figure below.

Tas1r3 heterodimerizes with Tas1r1 or Tas1r2 to form sweet or umami taste receptors. Tas2rs are the bitter receptors and don't dimerize.

Sweet evolution
Some species seem to get by without tasting sweetness or umami at all. The Giant Panda has an inactive umami Tas1r1 and cats have their sweet Tas1r2 pseudogenized, for example. These losses might be driven by changes in diet (with the panda being an extreme example – going from being a carnivore to living off bamboo), but without closely related relatives with different dietary lifestyles to compare them to this is difficult to say.

Studying bats, which have a more recent history of diversification and lifestyle changes, might be more fruitful. Most bats either are insectivores or fruitivores. However, three bat species chose the more sinister sanguivorous way of life. They are the only mammals that exclusively feed on the blood of other animals. Thirty years ago, researchers already discovered vampire bats have little interest in sugar. Now, Zhao and colleagues sequenced the Tas1r2 gene in 40 different bat species to find out more about the evolution of sweet taste perception.

Vampire bats, cool like that. Source: http://www.casadosmorcegos.org

They reconstructed the gene tree of Tas1r2 with maximum likelhood methods and found that Tas1r2 was under strong purifying selection in both insectivores and fruitivores, meaning that for these species there’s a strong evolutionary pressure to keep the sweet taste receptor intact and functional. However, in all three vampire bats the Tas1r2 gene was disrupted in different ways. The damage included nonsense mutations, insertions and deletions and make the entire sweet taste receptor non-functional. Such a non-functional gene will not get eliminated from the genome immediately. Instead the sequence of such a pseudogene will slowly deteriorate over time, until it lost all resemblance to the gene of the protein it once coded for. The researchers show that the selection pressure on Tas1r2 has likely been lifted in the common ancestor of vampire bats, and has been evolving neutrally ever since.

Since vampire bats find their prey by smell and infrared vision and exclusively feed on blood, they seem to have little need for refined tastes. Any myths about vampires preferring sweet blood can thus be dispelled: these poor suckers (literally) will not enjoy glucose rich blood more than blood-light products!

The Tas1r2 gene is also inactivated or deleted in chickens, zebra finches, horses, the western claw frog and cats. There’s not a clear relationship between diet and selection/retention of Tas1r2, since all these animals prefer different food sources and not all of them had dramatic life style switches like the vampire bat. The chicken and zebra finch case is interesting because they are only distant cousins, implying that sweet taste perception either was lost in the common ancestor of birds, or that it was lost multiple times.

I guess in this respect, we primates are just lucky! I wouldn’t even want to know how stroopwafels would taste like without my beloved Tas1r2 gene!

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On the Origin of Animals