In my class yesterday I reviewed a paper selected by my students that explored the sweet tasting abilities of bats. We learned that most, but not all, bats can taste sugar like other mammals. I did not know much about mammalian taste receptors (that is one problems with letting students pick the topics!) and I so I needed to do some background research before our discussion. I found the taste receptor story among all mammals quite interesting and I thought I would share here.
What allows us to taste sugar?
I’m sure that most of you learned at some point about taste buds on the tongue and how there are at least five different primary tastes that all people share (sweet, sour, bitter, salty and the lesser known umami). Sweet and Umami taste is the result of the interaction of molecules in the food we eat with protein receptors on the surface of cells that form the taste buds. If you don’t have receptor protein that can interact with the molecules of a substance you put in your mouth then you won’t be able to taste it. For example, paper is made of cellulose which is made of long strings of sugar molecules but because the particular configuration of those sugar molecules they don’t interact with the sweet taste receptor and so paper doesn’t taste like much and certainly doesn’t taste like sugar.
Specifically, there are three genes called Tas1r1, Tas1r2 and Tas1r3 that are responsible for umami and sweet taste in animals. In taste buds the Tas1r3 makes a protein that either combines with the protein product of gene Tas1r1 or Tas1r2 to form a receptor complex. The Tas1r2/Tas1r3 combination is the one that can sense the sweet taste of many types of carbohydrates (sugars).
Do all mammals have a sweet tooth?
No, there are some animals that show no evidence that they prefer or can even taste sweet. The best known mammals that lack a sweet tooth are the members of the cat family. Your feline friend has no interest in sweet tasting food. This doesn’t’ mean that they won’t eat something that has sugar added but experiments have definitively shown that they are oblivious to the sweet taste that you and I get from the same foods. But what about other animals? Until recently not much was known about other mammals’ ability to taste sweet but multiple studies in the past three years have provided some sweet results. It seems that there are multiple other animals that can’t taste sweet. A feature that some of these deprived animals share is that there are carnivores. This might not be surprising to you since strict carnivores, like cats, are not likely to encounter sugary food sources and thus do not need to be able to taste sweet to tell them that they are eating something good for them. In a recent paper, Jiang et al. (2012) have demonstrated some otters (but not all!), all tested species of seals, spotted hyena and fossa all of which are carnivores are not likely to be able to taste sweets. In addition, a dolphin tested was also found to lack sweet taste in addition to likely being unable to taste bitter or umami.
The paper that I am discussing with my class reviews the molecular genetic evidence for the lack of ability to taste sweet in one group of bats. That group consists of all tested species of vampire bats while all other bats tested have the ability to taste sweet. Of course it makes sense that fruit bats would be able to taste sweet since fruit is a food source for them. Another recent paper by same authors (Zhao et al. 2011) describes evidence that all bats apparently lack the ability to taste umami. Here again, this might not be surprising since fruits and insects generally lack in chemicals that would trigger the umami (savory) taste response any way and so the bats aren’t missing much by not having functional taste receptors proteins.
Why can’t some mammals taste sugars?
I suggested that some mammals don’t need to taste sugar. Dolphins swallow fish whole and other carnivores wouldn’t seem likely to care about sweet flavors so there is a reason we might not expect them to taste sweet. But a more fundamental question can be asked: Do they not tastes sweet because they have never tasted sweet and were not “designed” to taste sweet, or could them taste sweet in the past but have lost the ability to taste sweet?
The answer appears to be the latter. The bats paper and the others that I reference below demonstrate that the mammals mentioned above ALL have a copy of the Tas1r2 gene in their genome that is responsible for making the sweet taste receptor. The problem in each case is that the Tas1r2 has errors in the gene that cause it to be non-functional. In some cases there are missing pieces of the sequence causing it to produce a misshapen protein that doesn’t work. In other cases there are single base mutations that result in the gene not being able to be produced at all. It is this genetic data that has been used to predict which animals are not able to taste sweet after which many of the animals ability has been tested in the lab to confirm their lack of sweet taste ability.
So how did these animals come to have bad versions of their sugar receptor proteins? The presence of this gene in all mammals suggests that all mammals have used a sweet receptor at some point in their history. I say this because it wouldn’t make much sense to have the gene for sensing sweet and yet never to have used it. Both the evolutionist and the creationist would probably agree that the ancestors of the organisms alive today had the ability to taste sugar. However, what this ancestor was, when it lived and why it lost the ability to taste sugar are questions that would be answered very differently. Evolutionary theory predicts that the presence of genes for sensing sweets in all mammals is the result of the common ancestor of all mammals having this ability and then sharing that gene with all its ancestors. Take the dolphin or seals for example. Evolutionary biologists have predicted that dolphins (cetaceans) and seals all had a land animal ancestor. If this were correct it would be expected that this land mammal ancestor would have been able to taste sugar and thus have a functional Tas1r2 gene in their genomes. Once evolved to an aquatic habit it would be further predicted that this gene would not be as useful as it once was on land (much like the genes for smell are not as useful in water as they are on land) and so these animals may have lost these functions and possibly the genes for those functions altogether. As I pointed out above, so far all seals and dolphins examined to date still retain a copy of the gene for tasting sweet but in each case there are fatal mutations to that gene rendering them non-functional. In the case of the dolphin, it isn’t just sweet taste but the genes for tasting umami and bitter are also present but mangled by mutations rendering them functionally useless.
A young earth creationists likely interpretation of taste genes
How does the creationists account for the presence of defective sweet sensors in animals? Generally speaking they would claim this as just a loss of information and say that this is what is to be expected in a world corrupted by man’s sin. Here is just one example of just such a response made directly to the paper I have been talking about:
These results are exactly what you would expect from the Biblical history of the world, i.e. created perfection followed by degeneration. In the beginning all animals ate plants and had fully functioning taste genes. Following the Fall of Man and Noah’s Flood many animals have lost functional genes due to degenerate mutations as their genomes gradually collapse over time.
Applying the logic of this response to the origin of cats and their taste receptors, most creationists already accept that all cats (lions, tigers, leopards, domestic cats, etc…) likely had a single common ancestor (the pair on Noah’s Ark) and so they might say that common ancestor originally had a working version of this gene but that “degenerate mutations” in the sweet taste genes of these first cats was passed on to all other living cats today causing them to lose their taste for sugar. Superficially this would also seem to fit with the belief that scripture requires that all animals ate plants prior to man’s sin and thus cats did not have a carnivorous diet.
But what about the dolphin or the seals that lack sweet taste? Young earth creationists believe the dolphin was created as a separate kind and thus was created to live in the water where it would not have the need for taste receptors that land animals have. Likewise, seals are always seen as perfectly created with adaptations to life in water. If these animals had no land animal ancestors but were created ex nihlo for their watery habitats, one has to wonder why they have the genes in their genome for tasting sweet, umami and bitter like land mammals? Their presence certainly isn’t expected or predicted by creation science, unlike evolutionary theory which predicts that such taste genes very well could be present in their genomes even if they are not needed. There is little need for these animals to taste their food as seals and dolphins swallow their food whole. The author of the response quoted above uses the same universal terms that the Bible does in describing the Flood but does he really mean that all animals ate plants when those animals include seals, dolphins and whales? The Bible clearly is referring to the green plants of the field and not to plankton in the sea when it says that plants are given to all the animals live and crawl on the surface of the land or that fly above the land. The Bible does not say anything about the provision of food for fish and aquatic mammals.
The same question about dolphins could be asked for the sense of smell. Cetaceans (dolphins and whales) barely use their noses and in some cases their nose is not even really connected to their brains. And yet, cetaceans have huge numbers of smell genes in common with land animals except that nearly every one of them is a broken version of the land-mammal gene. If whales and dolphins were created solely for life in water most of these smell genes for specific plant and animal odors would be useless to a whale and yet they are carrying around these genes in their genome. An evolutionary model readily explains why such genes would exist in cetaceans and yet be non-functional. Had the cetaceans no land-mammal ancestor one would certainly not predict a priori that they would have all of the genes for taste and smell that land mammals have.
I’m not sure how to end this post. In recent posts I have been critiquing the evolution-like views of creationists with respect to the origins of groups of species that they believe are derived from common ancestors. I don’t think that creationist have really come to grips with the type of data that they need to incorporate into their genetic models of fast-evolution. The quote above shows that this author is trying to explain away much evidence of speciation as genetic sorting and loss of information but the devil, as they say, is in the details. It is one thing to throw out general theories that sound plausible; it is another to produce explanations that stand up to close inspection. It would be difficult for me to say that given the data about sweet taste receptors, the evolutionary model of loss of sweet taste does not sound plausible. Rather it accounts very well for the data. For those that believe that the Bible requires that dolphins must have been specially created as aquatic mammals then the burden would seem to be on my brethren to explain why the more scientists look at the genomes of organisms like dolphins and whales that those genomes keep suggesting to us that they were derived from land animals? To be fair, this is a challenge to my 6-day creationist friends but I am not unaware that this same data directs a challenge my way as well. That is, if this evidence is to believed regarding common ancestry then how shall should the origin of these whales and dolphins be explained within the confines of Biblical theology? I will explore that question in future posts.
References:
Zhao et al. 2010. Evolution of the Sweet Taste Receptor gene Tas1r2 in Bats. Molecular Biology and Evoltuion. 27(11): 2642-2650.
Jiang et al. 2012. Major Taste Loss in Carnivorous Mammals. Proceedings National Academy of Science 109(13): 4956-4961.
Zhao et a. 2011. Genomic and Genetic Evidence for the Loss of Umami Taste in Bats. Genome Biology and Evolution.
Naturalis Historia 