A Matter of Taste

2014 December 27

Let’s say you’re an insect. You’re small, yet nutritious. Lots of things want to eat you, particularly things that out-mass you dramatically like birds and bats and frogs. Life is hard. You really need to avoid getting eaten by these monsters if you are going to have any offspring, so you’ve got a few options:

1. Avoid being seen, by camouflage, or coloration, or burrowing under cover, or being so small as to escape notice (and maybe not being big enough to be worth the effort even if found). This works really well, except for when it suddenly, and without warning, doesn’t. Your predators get really good at pattern recognition or hearing faint sounds that you make in your hiding place.

2. Flee. As long as you are faster and/or more agile than your predator (or, if you are a species that hangs out in large groups, faster and more agile than your companions), you can get away. This works OK, but it takes a lot of energy. It also demands constant vigilance on your part. Your predators get quick, and sneaky. And if you are hanging around in big groups, they develop insatiable appetites and big mouths.

3. Fight. This works OK if you are about the same size as your predator, but not so good when they are so much bigger than you (unless you have some potent weapon, like a stinger filled with Liquid Pain®). Fighting them off is like a person trying to duke it out with Godzilla. Limited utility.

4. Make yourself painful or irritating to eat by growing hairs, spines, or other structures that make eating you be similar to eating a wire brush or broken glass. Pretty effective, but all those projections can make it hard to move very fast, so that the predators that are still interested can catch you easily. And the predatory response is likely to be to find some way around the defensive structures, like skinning you or knocking you around.

5. You can fill yourself with chemicals that will make predators violently ill or maybe even die if they ingest you. Immediate effect: after a few run-ins with members of your species, your predator decides to look for something else to eat. Now we’re getting somewhere!

So overall, making yourself dangerous or unpleasant to eat has a lot to recommend it. The downside, though, is that you need to have a dangerous or unpleasant chemical in your body. Your body needs to synthesize it, or acquire it somewhere. Plus, if it is really toxic, you need to have specific adaptations to keep it from harming you. What to do, what to do . . .

If you are a do-it-yourselfer, you can generate the toxins yourself, and store or secrete them as necessary. But that takes metabolic energy, and if you are expending energy making toxins then you have less available for other things, like moving around and eating and mating and producing eggs. If that’s what you have to do, so be it, but there is often an easier way.

As it turns out, plants have the same problem as insects do, only more so: running and fighting aren’t really even options, and both camouflage and armor are hard to do when you have to maintain large amounts of exposed leaf area for photosynthesis. So, a lot of plants fall back onto chemical warfare, with all sorts of toxins of varying degrees of nastiness. They do have a bit of advantage in that they are so distantly related to the animals trying to eat them. As a result, it is not too difficult for them to evolve toxins specifically to put the harm on the animals, but that are harmless to plants. Also, a lot of plants have evolved a “double circulatory system”, with one set of tubes carrying water and vital nutrients throughout the plant, and a completely distinct second set carrying “latex”, a milky, toxic brew to fend off herbivores[1]. So, a plant-eating insect has, at least in theory, access to all of these wonderful toxic chemicals the plants are producing.

Of course, the insects are exactly the sort of things that the plants were trying to fend off, so this is a bit touchy. Still, insects need to eat, and so if they can either become immune to these toxins, or their digestive systems can separate out the toxins from their food, then they accomplish two things: first, they gain access to a reliable food source that there isn’t a lot of competition for; and second, they either have the noxious toxin spread through their bodies, or have it safely sequestered away somewhere (probably in their skin) so that anything that eats them will get a good dose. Monarch butterflies in particular are well known for extracting toxins from milkweed latex, with good results. See, for example this famous series of photos taken by Prof. Lincoln Brower of young bluejays, introduced to a monarch butterfly for the first time. ACK! In these experiments, the bluejay pretty much never makes that mistake twice.

OK, so far, so good. But the fact remains that you are still dealing with toxins, that are likely to really harm you if you aren’t careful. Is there any way we can manage this so we can get the predator-repelling benefits of toxicity, without actually having to deal with a real toxin?

And, this is where the question of taste (and smell) comes in. Let us consider for a moment why animals even have a sense of taste. It’s partly because there are actual nutrients that you want to specifically seek out, like sugars and salts[2]. But, you also want some way of detecting things that you do not want to eat, which is where “bitterness” comes in. The function of the “bitter” receptors is as a toxin warning. It is a sensation that is telling you “this thing you are eating is probably poison. Spit it out now!” And the same with the “bad” smells, which are frequently those smells that are associated with rotting materials that will make you sick if you eat them. It is not supposed to be pleasant, or encourage you to eat more of it. And, since there are a lot of different toxic things in the world, there are a lot of things that trigger the “bitter” tasting and “foul” smelling sensations. Some of which have chemical similarities to poisons, but are not actually toxic themselves.

This presents a great opportunity for things that don’t want to be eaten. They don’t actually have to be toxic, they only need to be percieved as toxic! So if they fill themselves with something that triggers the bad taste/smell receptors in things that want to eat them, but that are not actually toxic, they get all the benefits of chemical defense without the risks of having to have actual poisons in their bodies!

And so we end up with a lot of really bad-tasting and foul-smelling plants and insects. Many of which you could probably safely eat, but your body doesn’t know that they are safe, and so you don’t eat them anyway.

This business of smelling bad without actually being that toxic doesn’t always work, though. Consider this Spined Soldier Bug (a type of stink bug):

While we were handling it, Sandy said that it stank so bad it was smelling up the whole room, and Rosie agreed with her. Sam and I didn’t notice any really significant smell. So what’s going on here?

On further questioning, Sandy said that it smelled like cilantro. As it turns out, cilantro is somewhat notorious for tasting and smelling wildly different to different people, evidently due to differences in the taste/smell receptors that people inherit from their parents. And, the chemicals produced by stink bugs evidently affect the same receptors. And, from previous experience with spices, we already knew that Sandy finds cilantro to be a useful flavoring herb, while as far as I’m concerned it might as well be chopped grass. What this means for the stink bug is that its defenses don’t always work. If its predators don’t actually regularly encounter the real toxin that encouraged the evolution of the receptor that that the stink bug’s stink also triggers, then they might lose the ability to taste or smell it again. And all of a sudden, the predator can happily snack on all those stink bugs that had gotten slow and sassy due to nothing wanting to eat them.

This leads to a point that I don’t think people take nearly seriously enough: the wild variation in human food preferences isn’t just because of early training, or developed prejudices, or sheer cussedness. A lot of foods really do taste completely different to some people than they do to others. Just for a start, there are at least 23 different bitterness receptors that are known in humans[3], but each individual person only has a subset of them. So one person’s bitter, is another person’s flavorless. And, I strongly suspect that the actual sensation of bitterness is probably equally variable[4].

One last thing: you actually can get a pretty good idea of whether an insect tastes awful depending on what it looks like. If it is well-camouflaged, speedy, or aggressive, it is likely to be edible (and maybe even delicious). But if it is brightly-colored (and particularly if it has a lot of orange, red, yellow, and/or a reflective, metallic sheen)[5] and doesn’t seem too concerned about whether you catch it or not, watch it. It is probably specifically colored that way to advertise that if you eat it, it may die, but you’ll be sorry. The obvious visual announcement of its inedibility is a key part of the whole deal. After all, being nasty to eat isn’t going to help it unless you, the predator, have some way of knowing what you’re going to be dealing with. As Dr. Strangelove would put it:

[1] Most of the plants that do this are quite obvious, because if you break off a leaf or stem they “bleed” this white, milky fluid. Milkweed and dandelions are prominent examples, but if you look closely you’ll find a lot of plants that do this. This latex has a lot of horrible compounds in it, including colloidal polymers that make it sticky and clotting. The polymers therefore both trap and smother insects, and seal the wounds that they make in the plant. And in plants that produce a lot of isoprene in their latex, it can be used to make rubber.

[2] Our ancestors really did need to specifically seek out sugar-rich foods as energy sources, and salty foods to maintain their electrolyte balance in their blood, because these things are both very necessary to us, and fairly scarce in nature. But then, because we liked the taste of sugar so much, we learned how to cultivate plants and extract pure sugar from them at a cost of pennies per pound. And for the same reason, we learned to mine and purify salt by the millions of tons. So now even we can easily and cheaply get as much sugar and salt as we want. Which unfortunately often turns out to be a lot more than our bodies actually need. So now, foods that used to be highly desirable (because they were both necessary and hard to find), are now demonized as something to avoid.

[3] There doesn’t seem to be as much variation in other taste receptors like “sweet”, “sour”, and “salty”. Back in the old days, since everybody had equal need for the sugars and salts, everybody had generally similar ability to taste them. The “Sour” sensation seems to be similar, in that sour tastes are often associated with sugars, and so foods that are sour are also often good sources of sugars, and so we’ve evolved to be attracted to mild sourness as well[3]. We also generally like “meaty” things for their protein, leading to the receptors for “umami”. And then on top of the tastes, there are all the various smells. Again, the things that smell “good” tend to be calorie-rich or otherwise nutritious, and our brains tend to roll the sensations from our taste receptors together with those from our smell receptors to make the overall “Taste”. And, the list of things that our body actually needs is a lot shorter than the list of possible toxins, so receptors for things that we need can be much more focused, leading to less variation from person to person.

[3] And also, our “sour” receptors have a lot of similarities to our “sweet” receptors, to the point that it is evidently possible to trick one into registering another. For example, there is this berry called “miracle fruit”, that contains a compound that has an odd effect on one’s sense of taste. If you swish the juice from this fruit in your mouth, it evidently binds to and blocks your “sour” receptors from registering, while still leaving the “sweet” receptors active. And, it turns out that the “sweet” receptors are activated by acid, like the “sour” receptors are, and will cause acids to register as sweet in the absence of the “sour” receptors telling our brains otherwise. So then, for a period of time that can be up to a couple of hours, you can do things like drink straight lemon juice, and it will taste not just sweet, but extremely sweet, to the point where you can chug the stuff like lemonade. You don’t have to take my word for it, you can get boxes of “miracle frooties” that are made of compressed pulp from these berries, and try it yourself. Amaze your friends!

[4] In my case, the “bitter” sensation is excruciatingly unpleasant, and I can’t think of a single case where having some bitterness in a food or drink makes me like it better. At best, I find a small amount of bitterness tolerable, but I always consider removal of the bitterness to be an improvement. And yet, there are a lot of definitely bitter foods (some so bitter that I find them inedible), and a lot of people like them. Sometimes they like them a lot. Like beer, where people actually intentionally add things like hops to make them even more bitter. Clearly, what they are tasting is not what I am tasting. Unless they actually enjoy torturing themselves, which I doubt.

That’s OK, except that I have to take what people say about things like dark chocolate, beer, tea, wine, caffeinated drinks, many kinds of cheese, a lot of varieties of salad greens and herbs, marmalade, etc. with a certain amount of caution. It’s fine with me if other people like these things, but I’m not going to eat or drink them.

[5] Locally, orange and black are popular warning colors. Here are a few examples:

4 Responses
  1. December 27, 2014

    Great post! Visual mimicry is mentioned quite frequently in science classes/biology shows, but chemical mimicry doesn’t get nearly enough attention.

  2. Carole permalink
    December 27, 2014

    Very good piece. Hope you have a great, big, buggy, new year.

  3. December 28, 2014

    Love this, Tim. The individual bug profiles are great but it’s nice to have a topic overview now and then. And to get Peter Sellers do a cameo for you — how fun is that!

  4. January 6, 2015

    Thanks, everybody! I still haven’t decided on what the next big topic will be, maybe the diversity of arthropod eyes? Or possibly digestive systems? Or varieties and uses of venom?

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