Mexican Butterfly Weed from our backyard
The first of evolution's ironies is that humans through the use of their ingenuity have managed to use the poisons to SAVE lives. Digoxin, another cardiac glycoside from the foxglove, Digitalis lanata, is used to treat congestive heart failure. In congestive heart failure the heart does not contract forcefully enough. The heart is a muscle, and muscles need calcium ions (Ca++) to contract. It turns out that in heart muscle there is another protein pump (Na/Ca exchanger) that exchanges Ca++ with Na+. By blocking the Na/K pump, sodium builds up on the inside. Some of this extra sodium can be removed by the Na/Ca exchanger. The consequence of that is that there is more Ca++ inside the cell. More Ca++ inside the cell means the heart is able to contract more forcefully.
The second of evolution's ironies is what gives the plant its common name, Butterfly Weed. One of the biggest predators on plants is insects. Cardiac glycosides do repel many insects and it is almost certainly the case that insect predation was the selective factor most responsible for the development of the genetic machinery within the plant to produce the poisons. However, milkweed plants like the Butterfly Weed are the sole food of Monarch larva.
Certainly if you were to look at our plants you would be hard pressed to show evidence that the cardiac glycosides were effective against insect predation. Indeed, they are not. It is BECAUSE the plant produces those toxins that the Monarch larva (aka catepillars) eat it.
Monarch catepillar on our Butterfly Weed
Monarch egg
Monarch catepillar
Monarch chrysalis
Monarch chrysalis on day 9
In the above picture you can clearly see the adult Monarch developing. Here it is shortly after eclosion ... the emergence of an adult butterfly from its pupal casing.
Adult Monarch shortly after eclosion
It is well known that Monarchs migrate. One could say that they start off overwintering in the south ... California, for the ones west of the Rocky Mountains - Mexico for the ones east of the Rocky Mountains. They then range as far north as Canada before returning back to their overwintering grounds. However, it is not the original butterfly that returns to the overwintering grounds. It is the great-great grandchildren of the original butterflies, or the fourth generation that return.
The first generation comes from eggs laid by parents that overwintered in the south. They hatch in March or April. After eclosion they begin their flight north feeding off of flowers and trying to find mates. They live only 2 to 6 weeks. The second generation butterflies are born further north during May or June. The third generation is born in July or August. Like the first generation, the second and the third live 2 to 6 weeks. The fourth generation is born in September and October. These are the ones that make it back to the overwintering grounds. They are also blessed with a much longer life, 6 to 8 months (although much of that extra time is taken up by hibernation). The ones I took the pictures of above are all fourth generation Monarchs.
One question that must arise is, "Why do Monarchs migrate?" The answer seems to be that they are following the milkweed. The milkweed does not grow year around in their overwintering homes. It blooms later in the year further north. Note that the adults do not feed on milkweed, it is only the larval catepillars. But the effectiveness of the cardiac glycosides lasts for a lifetime. In fact, it is so good that it even protects other butterflies. As birds learn that Monarchs are toxic then any other species of butterfly that looks like a Monarch will also benefit. This is called mimicry.
Viceroy Butterfly photographed at Brazos Bend State Park
The above picture is not a Monarch butterfly. It is instead a Viceroy (Limenitis archippus). Viceroys are slightly smaller than a Monarch, but in the field unless the two butterflies are side-by-side, that size difference is unlikely to distinguish them. The way I distinguish them is by the extra black line on the hindwing of the Viceroy.
The Monarchs belong to the milkweed butterflies (Subfamily Danainae). That group not only includes the Monarch, but also the Queen (D. gilippus) and the Soldier (D. eresimus). Below is a picture of a Queen; unfortunately, I do not have a good picture of a Soldier.
Queen
As you can see the Queen is on a Mexican Butterfly Weed. It (and the Soldier) also lays its eggs on milkweed plants. Their eggs, catepillars, and pupae look very similar to that of the Monarch. They too are toxic and gain their toxicity the same way as does the Monarch.
However, Viceroys are closely related to the Admirals (sorry no pictures). They lay their eggs on willow trees not milkweed. They do ingest cardiac glycosides to become toxic. Neither their eggs, catepillars, or pupae look anything like the catepillar and pupa of Monarchs. Yet while they all look somewhat similar it is the adult Viceroy looks more like the adult Monarch than do either the Queen or the Soldier.
Biologists recognize two forms of mimicry: Batesian and Muellerian mimicry. In Batesian mimicry a non-toxic species gains an advantage by mimicking a toxic species. In Muellerian mimicry similar toxic species benefit from shared appearance.
Thus, the similarities between the closely-related Monarchs, Queens, and Soldiers could be ascribed to Muellerian mimicry. But what about the mimicry between the Viceroy and the Monarch? The answer to that question is not fully clear. For a long time it was considered the textbook example of Batesian mimicry, but the evidence now points more toward Muellerian rather than Batesian mimicry.
In the 1950's biologists took young nestling birds. These birds would gladly eat Viceroys when offered. However, when given a Monarch, they quit eating Viceroys as well. This strongly argued for Batesian mimicry.
But in science no answer is immune from reinvestigation as new data arrives. As mentioned before Viceroys lay their eggs on willow trees. Willow trees are members of the Salicaceae family, so named because they produce salicylic acid. Aspirin is a salicylic acid. If you have ever munched down on an aspirin then you know how bitter it tastes. Chemical analysis revealed that there was a lot of salicylic acid in the body of Viceroy butterflies. If so, then why aren't they unpalatable as well?
In 1991, biologists decided to reinvestigate the question of Viceroy palatability. This time it showed that birds rejected Monarchs and Viceroys equally. Indeed Viceroys were judged to be more unpalatable than Queens.
Thus, we have conflicting data. My guess is that the experiments performed in the 1950s used Viceroys that had not absorbed very much salicylic acid as larva. I suspect that some years weather conditions and the presence or absence of local insect pests combine to make the production of salicylic acid in willows (and cardiac glycosides in milkweed for that matter) better or worse than normal. Butterflies produced during those years will be more or less toxic than normal. If you have a less toxic Viceroy (or Monarch) then birds might be more than happy to eat it. And I suspect the scientists in each case got the results they expected so in their minds there was no need to check them again.
