1. Humans Have 26 Bitter Taste Receptors, but Platypuses and Echidnas Have Fewer
When you eat something bitter, your body receives the message through bitter taste receptors. The genes that create these sensors vary by species. For humans, there are 26 types. According to the International Monotreme Genome Project, the platypus has 7 types, while the echidna has only 3 [1]. Scientific papers confirm these same numbers [2]. If we divide the platypus number (7) by the human number (26), it is about one-quarter. For the echidna (3), it is about one-ninth. With so few types of sensors, one might wonder if these animals struggle to detect bitter tastes. But what does the research show?
2. One Platypus Receptor Detects About Half of the Bitter Substances Tested
Researchers at Hokkaido University tested these receptors in cultured cells. They checked how the monotreme receptors reacted to 24 different bitter substances [1]. The results showed that the platypus receptors reacted to 18 of the 24 substances. Even more surprisingly, the paper states that a single receptor, TAS2R811, reacted to 12 of the 24 substances. Another receptor, TAS2R813A, reacted to 7 [2]. The number 12 is exactly half of 24. The research team calls these broad-acting receptors "universal type" [1]. It is like a master key that opens many doors, though the receptor is not a key itself, just a way to describe its wide range. The test used 24 substances, not every bitter thing in nature. Despite having few genes, the platypus has receptors that react to a wide variety of bitter substances. How does this help their daily life?
3. How Platypuses May Find Food Underwater Without Eyes, Ears, or Nose
Platypuses live in lakes and rivers, eating small invertebrates underwater [1]. When diving, it becomes difficult to use their eyes, ears, or nose. So, how do they find prey? Their bills have electroreceptors that sense the weak electricity from prey muscles, and mechanoreceptors that feel touch. These two senses allow them to find shrimp and insect larvae even in muddy water [4]. After finding food, they must decide if it is safe to eat. Hokkaido University suggests that the sense of taste, specifically bitterness, is used for this decision [1]. It is currently a guess that the "universal type" receptors are linked to their underwater lifestyle. Their relative, the echidna, lives in a completely different place.
4. Why Echidnas Lost the Receptors That Detect Many Bitter Substances
Echidnas live on land. They mainly eat ants and termites found in soil and dead wood [1]. In the same experiment, echidnas were found to have lost the "universal type" receptors that platypuses have. Their other receptors also detect fewer bitter substances than the platypus's [1]. However, this does not mean they cannot taste bitterness at all; it just means they detect fewer types of substances. It is estimated that platypuses and echidnas split from a common ancestor about 54.6 million years ago [2]. It is thought that the universal receptors were lost in echidnas because their diet of ants and termites may limit the bitter substances they encounter [1]. This is a theory, not a confirmed fact. A genome paper also notes that platypuses have 299 olfactory receptor genes, while echidnas have 693. This difference is linked to the platypus relying on electric senses and the echidna searching for prey underground [3]. When food and lifestyle change, the combination of senses used seems to change too. What do the remaining bitter receptors detect?
5. The Ability to Detect Plant Poisons May Be 200 Million Years Old
Even with fewer receptors, the ability to detect plant poisons seems to remain. According to Hokkaido University, receptors similar to TAS2R16 in humans and other placental mammals are found in both platypuses and echidnas. Some of these react to glycosides (β-glucosides), which are among the poisons found in plants and other sources [1]. Corresponding receptors in marsupials and placental mammals also react similarly. These receptors do not only react to β-glucosides; they also react to other substances. It is estimated that monotremes and live-bearing mammals split about 187 million years ago, roughly 200 million years ago [3]. If the common ancestor before this split already had this ability, then the power to notice plant poisons may have been passed down for nearly 200 million years. The research suggests this possibility. Detecting plant poisons may be one of the oldest roles of bitter taste receptors.
6. How an Animal's Diet and Lifestyle Can Shape Its Senses
We learned that when food and lifestyle change, the combination of senses used seems to change too. For today, you can pick one animal you like and look up its food in a book or at a zoo. Then, write one note about which sense—eyes, nose, or touch—it likely uses to find that food. You might find animals, like the platypus, that do not use their eyes. Official information from Hokkaido University is available in Japanese. It continues the story of the monotreme genome. What combinations of senses do other animals have beyond the 7 and 3 we discussed?