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Further reading:
What gives bees their sweet tooth?
Show transcript:
Welcome to Strange Animals Podcast. I’m your host, Kate Shaw.
Right before I left on my trip to Belize a few months ago, my aunt Janice gave me a magazine to read on the plane, the Autumn 2021 copy of LivingBird. It’s about birds and birdwatching. I actually forgot to take it with me and it was in my car the whole time I was gone, but when I got home I took it in to read.
One article caught my eye, titled “Investigating the Sweet Tooth of Songbirds.” Literally the same day that I read that article, I stumbled across another article on ScienceDaily titled “What gives bees their sweet tooth?” And a podcast episode idea was born!
You may have heard that domestic cats can’t taste sweetness, and that’s true. When your pet cat wants to drink the milk in a bowl of sugary cereal, it’s not the sugar they care about because they can’t taste it. Also, milk isn’t good for cats and even if they can’t taste the sugar, it can end up giving them cavities.
The question is, why don’t cats taste sweetness? And what other animals can’t taste it either?
Carnivores like cats don’t need to taste sweet flavors because it’s just not present in meat, which is what carnivores eat. You can test this easily if you put two saucers on the floor for your cat, one with a small amount of unseasoned chicken and a sugar cube in the other. I guarantee you the cat will eat the chicken and play with the sugar cube, which will get sugar all over the floor so maybe don’t do that after all. This is where I share with you, for no reason, that when I was in elementary school I used to eat sugar cubes while pretending I was a horse.
Horses can taste sweet flavors like sugar because they’re herbivores. Herbivores eat plants, and in fact herbivores have a whole lot of taste buds so that they can easily tell what kind of plants they’re eating. Bitter tasting plants might be toxic while sweet ones provide lots of energy. Herbivores are also keenly attuned to the taste of salt since their diet is typically low in salt and they need to seek it out.
Humans are omnivores, and omnivores eat pretty much anything. Like our great ape cousins, we also evolved to eat a lot of fruit. Ripe fruit tastes sweet so we really like our sweet foods. Omnivores like dogs, pigs, and bears also like sweet foods because they’re high in calories and therefore provide a lot of energy.
But how does an animal lose an entire sense of taste? It’s not like all tigers woke up one day and boom, the ability to taste sweetness was gone. It happens gradually as the genes responsible for an animal’s sense of taste mutate over many generations.
Let’s take as our example the bottlenose dolphin. The ancestors of the dolphin and other cetaceans were terrestrial animals related to the ancestors of modern even-toed ungulates like hippos, camels, deer, and pigs, and were probably either herbivores or omnivores. But as the dolphin’s ancestors evolved over millions of years, they shifted to a fully marine lifestyle and a fully carnivorous diet. Over the thousands and thousands of generations, the genes that control the ability to taste sweetness mutated so much that they’re now useless, but since the dolphin doesn’t need to taste sweetness the mutations don’t matter.
In the case of the bottlenose dolphin and other cetaceans, in fact, they also can’t taste bitterness or umami. Umami is what helps you taste the difference between chicken and turkey, steak and pork, tuna and trout. Basically it’s the flavor of meat or savory foods, including cheeses. You can taste the difference between cheddar and Swiss because of the umami receptors in your taste buds, which are determined by genes.
But the dolphin eats nothing but meat! Why would it lose the ability to taste meat? Researchers think it’s because the dolphin swallows fish and other animals whole, without chewing. Cetaceans and other marine carnivores like sea lions that swallow their food whole actually have almost no taste buds at all.
If you’re wondering what happens when an animal that can’t taste sweetness has to adapt to a diet where tasting sweet foods is important, that’s exactly what happened with songbirds. The ancestors of birds lost the ability to taste sweetness millions of years ago when they were dinosaurs. Then, well, you know what happened to the non-avian dinosaurs. Suddenly the ancestors of modern birds had a lot of available ecological niches to take advantage of and they evolved rapidly to fill them. This included small birds who eat berries and nectar.
Genetic studies suggest that the ancestors of songbirds regained the ability to taste sweetness around 30 million years ago in Australia. The same thing happened in hummingbirds at about the same time. In both cases, the genes that control the ability to taste umami evolved to taste sweetness instead—but songbirds and hummingbirds adapted different umami genes. That’s what you call a subtle case of convergent evolution.
Songbirds and hummingbirds adapted to a diet high in sugar because it’s a good source of energy and easily found in flowers. In turn, flowers needed to be pollinated and have their seeds spread around, so they evolved to provide even more sugars in nectar and berries. But birds aren’t the only animals that pollinate flowers and are attracted to nectar. Insects can all detect sweetness. However, bees are exceptionally attuned to sweetness and have two taste neurons instead of one per taste bud.
Insects don’t have taste buds the same way we do, of course. In mammals, reptiles, and birds, taste buds are located on the tongue, in a few parts of the mouth, and at the top of the throat. In insects, taste receptors can be in any number of places. They’re on an insect’s mouthparts but often also on their feet, legs, and antennae.
Some amphibians have taste receptors on the body as well as concentrated in the mouth, and many fish have taste receptors all over their body. Catfish in particular have the most taste buds known, up to 175,000. Humans have about 10,000. Cats only have about 500.
Before you start feeling sorry for your cat for not being able to taste sweet foods and not having a great sense of taste in general, cats have a taste receptor we don’t. It’s the water sense. To us, a nice cold glass of water tastes refreshing but doesn’t really have a flavor. A cat or dog, and many other animals whose diet is mostly meat even if they aren’t specifically carnivores, have the ability to taste water in a way we can’t even imagine. Because meat is high in salt content, having taste buds attuned to water helps the animal drink enough water to process all that salt.
If you gave me the choice, I’d choose sweetness over the ability to taste water. But my cats would probably disagree.
Thanks for your support, and thanks for listening!