Coevolution is a special kind of evolution. And monarchs and milkweeds exemplify this special process. In particular, what makes coevolution special is reciprocity. In other words, coevolution is one species that evolves in response to the other, and the other species evolves in response to the first. Thus, it is a back-and-forth that has the potential to spiral out of control. In some arms races, the two organisms both benefit, such as that between some pollinators and flowering plants. But coevolution is more common among antagonists, like predators and their prey.
When biologists first described coevolution, they likened it to an arms race. An arms race, such as that between political entities, occurs when two nations reciprocally increase their armament in response to each other. So how does an arms race between monarchs and milkweeds, or between cats and mice, or between lions and wildebeest, or between plants and their pathogenic fungi, proceed? When coevolution occurs, it proceeds with “defense” and “counter defense.” And one of the few rules of coevolution is that for every defense that a plant or prey mounts, the predator mounts a counter defense, or an exploitative strategy to overcome the defense.Once a monarch butterfly lays an egg on a milkweed plant, the natural history of coevolution unfolds. For every defense that the plant mounts, milkweed mounts a counter defense. Once the caterpillar hatches, it must contend with a bed of dense hairs that are a barrier to consumption of the leaf. But monarchs are patient, and have coevolved with the milkweed. So their first strategy is to shave that bed of hairs such that the caterpillar has access to the leaves that lie beneath.
For every defense there’s a counter defense. But next, when the monarch caterpillar sinks its mandibles into the milkweed leaf, it encounters a sticky, poisonous liquid called latex. In this video we will see how the monarch caterpillar deactivates the latex bomb that the milkweed puts forward.
And so the arms race continues, with reciprocal natural selection resulting in coevolution between monarchs and milkweeds. In my book, Monarchs and Milkweed, I outline the third level of defense and counter defense between these two enemies. Milkweed next mounts a remarkable and highly toxic defense chemical called a cardiac glycoside. But, yes, again the Monarch has evolved the means to not only not be poisoned by the cardiac glycoside, but to sequester it away and put it to work in defense of the Monarch itself from its enemies, such as predatory birds. For more on the Monarch – Milkweed arms race see this video, filmed in Ithaca, New York outside of Cornell University where we conduct our research.
Are the latex toxins in the milkweed what make the monarch toxic for birds to eat?
It is the cardiac glycosides (aka cardenolides) which are often concentrated in the latex. Thanks for your question!
Nothing here was shown to have evolved “in response to the other”. All that has been demonstrated is how certain life cycles of the Monarch butterfly interact with the milkweed plant.
Eggs and larvae are two parts of the Monarch’s life cycle. and the Monarch is one species: no evolution at all is occurring between those life cycles.
How long would the Monarch population last getting sick from eating milkweed and being devoured by birds before this perfect balance “evolves”?
I am not exactly sure your meaning… the plant is certainly responsible for the death of perhaps 50% of all monarchs (!) as they drown in latex… and the monarchs has certainly reduced milkweed fitness over time through its herbivory. What I have outlined here is the natural history of coevolution. In my book I spend more time developing the evidence for coevolution per se. I hope you enjoy it!
How do you think monarchs developed immunity to the toxins of milkweed?
Lots about this in my book, you can read the first chapter here: https://live-agrawal.pantheonsite.io/wp-content/uploads/2018/12/MM-Agrawal-Chapter1.pdf
https://www.amazon.com/Monarchs-Milkweed-Migrating-Remarkable-Coevolution/dp/0691166358/ref=sr_1_1?ie=UTF8&qid=1479211622&sr=8-1&keywords=monarchs+and+milkweed+agrawal
Thank you for this interesting look at coevolution. I see another way of looking at it, a relationship that is more commensurable. What does the plant want but to rise up, extend itself up and out. It has some firm constraints to that urge for freedom; genetic constraints, environmental constraints. Even it’s seed dispersal us for up an out, flying. How much more of its “desire” to fly can it achieve if linked to a butterfly? Linkage requires much control, maintaining a dynamic (and evolving) balance between insect and plant. The plant give food to the insect. It manufactures chemicals that function to make the butterfly poisonous. Hungry caterpillars are voracious and so they must be regulated. Mechanisms to slow them down. If not slowed down there would soon be no monarchs and no milkweed. Biologists will understandably bristle at the idea that the milkweed has a “desire” to defy gravity, to fly. Most plants display tropisms though so what if we just call it an up-and-out tropism. Whether a plant “wants” or not we can see what it does. The butterfly gives flight to the plant, extends the ways in which it is already moving up and out, in growing and in gliding on the wind. To stop seeing these as separate lives is not easy. Start maybe with Turner’s Extended Physiology. The plant and the insect are entangled, embedded in each other.