Everybody Stinks: Chemical Signaling in the Undergrowth
Most of us know that as far as sense of smell goes, humans are one of the worst mammals around. Compared to many other species our noses are nearly, well, blind. We can usually only detect odors if they are strong, recent, and nearby. Compare that to your dog, who will get a wealth of information and history from sniffing the nearest lamppost. In many animals, and notably mammals, scents are a really good way to communicate anything from readiness to mate to aggression to fear. Humans miss out on a lot of these communications, but we still often smell more than we realize. Numerous studies have shown that we alter our behavior in various ways when exposed to chemical signals produced by others (particular our offspring, parents, or the opposite sex) even though we may not be aware we’ve smelled anything at all.
However, what many people don’t know is that some of the champion smellers of the animal kingdom are arthropods – including insects, centipedes and millipedes, and spiders. They may not all have noses as such but they do have highly specialized receptors that allow them to pick up chemical messages from members of their own species and from other particular species that they have formed some kind evolutionary relationship with. What’s even wilder is that plants do it too. Plants can both send and receive signals from other plants and some animals.
Now, it’s probably obvious why and how a species would be able to communicate with other members of its own species with chemical signals. These signals are called pheromones, a word most of us are familiar with. But technically a pheromone is only a chemical signal that is used between two members of the same species. If it’s a signal between members of different species it’s called an allochemical (even if it’s the exact same signal being used for a different purpose). These words don’t refer to different types or structures of molecules, only to their function. Allochemicals are broken down in to functional groups of allomones, kairomones, and synomones, which I’m not going to get into too much.
But how and why does chemical communication between different species develop in the first place? Well, that has to do with coevolution. Coevolution involves how species who are closely tied together in the food web relate to each other. Take plants and herbivores for example. An herbivore may begin preying on a plant population. Only the plants with thorns will survive the predation, so soon plants with thorns will be the norm. But then, say, only herbivores with extra thick skin or with a talent for stripping off the thorns will be able to browse that plant population, so soon they will dominate the scene.
As this goes on, plant and herbivore often each become more specialized to deal with each other and their relationship grows more entwined. This is a simplification of course, as many coevultionary relationships are very complex and may involve more than just two species. And it happens with carnivores and their prey, with parasites and hosts, even with plants and pollinators. Being able to pick up on another species’ chemical signal is only one part of the “evolutionary arms race” that is constantly going on.
We’re going to mostly talk about arthropods and plants in these examples. Even though when you think of an herbivore you probably think of a deer, or a tiger when you think of a carnivore, the vast majority of herbivores and carnivores in the world are much smaller than your palm. And they have an entire world of interactions of their own, which deeply involves chemical signalling.
Of course the at the base of most solid ecosystems are plants. As Nerdy Christie taught us recently, some plants can release chemical signals when they are being eaten to attract a predator of the herbivore that is attacking them, essentially using the “enemy of my enemy is my friend” logic. This benefits both the plant (defense) and the predator (food). Some plants, like acacias have taken this even further and host ants in their branches and trunks which attack any creature big or small that tries to nom their home.
Chemical signals don’t have to just be scents that waft through the air, though. They can be toxins, which send a very clear signal either because they make the plant taste bad or outright kill or injure anything that tries to eat. We’re all familiar with the culinary herbs such as basil, mint, thyme, cilantro, and sage. We enjoy their distinct flavors in small quantities in our food. But the real purpose of those pungent chemicals is to discourage insect predation. And it works very well (doubly well in the case of herbs, as their chemical make up has convinced us to breed, grow, protect, and propagate many of them in our gardens and farms).
Plants can even product chemicals that sabotage their attackers more subtly. The tobacco hornworm (Manduca sexta) is a moth that will often eat and lay its eggs on the red devil’s claw plant (Proboscidea parviflora). However, when it does choose that species to lay eggs on, it finds that many fewer eggs hatch than normal do when they are laid on other species of host plant. Red devil’s claw produces a chemical that reduces the reproductive success of the moth.
In another example of clever trickery some plants even mimic the alarm pheromones released by herbivores under attack, so that when the first animal takes a bit all the others in the vicinity scatter, which reduces the amount of damage. Plants even eavesdrop on each other and if a plant (of the same or even different species) is sending out chemical signals that is has just been chomped, those around it can ramp up their chemical defenses in the hope of discouraging other herbivores that might be on their way.
Unfortunately, plant signals can backfire too. The chemicals the plant may have intended as a cry for help or a deterrent may end up just alerting other hungry herbivores in range that it is chow time. This is true for nearly every type of signal – any signal you make for your benefit can and probably will be co-opted by another species for their benefit (which more often than not involves your detriment).
One of the most famous examples of this is the Monarch butterfly (Danaus plexippus) and milkweed plants (Asclepias spp.) Milkweed is a highly defended plant employing physical barriers (dense hairy leaves and indigestible latex sap) as well as chemical signals (toxins and high acidity) to discourage insects, particularly caterpillars from eating it. But the Monarch butterfly larva cheerfully chomp on the leaves and instead of being injured by the toxins, they sequester them in their own body to make them just as undesirable to predators as the milkweed is (many insects synthesize toxins of their own, but for those species that can co-opt a plant toxin the energetic savings is huge). The Monarch is not the only species to have made its way past milkweed defenses, and as more species are able to use it for food, more recently evolved milkweed species are putting less energy into chemical defenses and more energy into growing back lost growth quickly.
If the array of ways plants can use chemicals to communicate is dazzling, they have nothing on their arthropod neighbors. There are so many different ways and scenarios that a single post will hardly be sufficient to cover them. I’ll just try to hit the major and most common types of chemical signals, and if it really piques your curiosity I encourage you to check out the literature – it’s a very well studied field, although it has an unfortunate emphasis on agriculturally relevant species (that’s where the money is, after all) to the neglect of many wild systems.
A nearly universal type of chemical signal is, of course, the sex pheromone. Herbivore, carnivore, parasite, or detritivore, you gotta find the opposite sex and let them know you are ready to go. Sex pheromones are particularly important for small creatures who may need to cover huge (to them) distances to find each other. However, as they are so obvious and essential they are also very commonly used by predators to locate their prey.
It’s a method that pays off well – you know the prey is there, that there’s more than one of them, and that they may not be paying attention or be in a position to leave in a hurry. And there’s not much they can do to avoid it. Suddenly altering the type of pheromone they put out means that members of their own species they want to mate with won’t get the message either. All they can really do is learn to mate fast and get out. Not surprisingly some predators (such as bola spiders) also have learned to mimic their prey’s sex pheromones and draw the hapless, horny insects right to them.
Aggregation pheromones are chemical signals that animals use to call a group together for some joint venture. These may be used by species who live in colonies like bees and ants (eusocial animals) or by herbivores who need large numbers in order to successfully overwhelm plant defenses. Many species of beetle, who may not live together in a family-based social group, still will release an aggregation pheromone that will call other beetles to help them attack a tree or other plant with good chemical defenses. The tree won’t be able to produce enough defensive compounds to drive away all the beetles, and then they all get a meal. Unfortunately, aggregation hormones also alert predators that there are large numbers of distracted prey in one area and the predator may get a meal too. However, in many cases there are enough beetles and enough resources at stake that only a small percentage will be predated each time and the other will receive a large benefit, making the strategy of aggregating worth it more often than not for any individidual.
We’ve already mentioned alarm pheromones, which may be released when a prey animal notices a predator (or even a rustle in the bushes that might be a predator) or is attacked. These tell nearby members of the species to scatter, hide, or prepare to defend themselves. But predators can use even these pheromones to their own advantage. They may sense the pheromones caused by another predator, which can help them figure out that prey may be running their way to escape. Some spiders even use ant alarm pheromones to locate battles between colonies because they know there will be weak and injured ants to prey on when it’s over. Others mimic prey species’ alarm pheramones, in order to fool that species into thinking the predator is one of them, allowing them to steal their food or attack them by surprise.
Chemical signals are also used for marking: to delineate territory, tell other members of a colony which way to go, or to mark a food source to go back to. These are also easily co-opted, sometimes by members of the same species who use it to steal food and sometimes by predators who use it to find prey returning to the spot that they marked.
All this can make it sound that predators always win, but as much as the predators use prey signals to find them, prey may use predator signals to avoid them and predators may use each other’s signals to steal food or mates. Prey sometimes even turn the tables on the predators using their own chemical signals. A Japanese hornet will mark the nest of the Japanese honeybee and then return later with its nestmates to destroy the honey bee colony. But thanks to the marking scent it left behind they hornets will find the bees massed and ready to ambush the hornets and overwhelm them with sheer numbers.
It is important to remember that chemical signalling (like any other trait) is a dynamic, ever-evolving system, as each individual continually tries to get the edge over its competitors and predators. As soon as one species develops an innovation that will give them a benefit in communication, feeding, or escape it won’t be too long before another manages to hijack it for it’s own benefit.
Greenstone, M. H. and J. C. Dickens. 2005. The production and appropriation of chemical signals among plants, herbivores, and predators. Ecology of Predator-Prey Interactions, pgs 139-156. Oxford University Press, Inc.