Defense Mechanisms in Monarch Butterflies
One of the most effective defense mechanisms used by monarch butterflies to protect themselves from their predators is the use of milkweeds. During the 1960s researchers discovered that cardenolides are chemicals in milkweed that made monarchs toxic and bittertasting. The graph below shows the range of cardenolide concentrations found in one species of milkweed, A. Viridis, and adult monarchs that fed on that milkweed as larvae. Since many adult monarchs do not eat milkweed, the figure shows that they have stored the cardenolides that they ingested as larvae.
The cardenolides obtained from milkweed make monarch butterflies toxic to
many vertebrate predators. It has been shown that certain animals exhibit
deathly signs after coming into contact with monarch butterflies. One such
species includes the captive bluejays. Captive blue jays that fed on monarchs
containing cardenolides threw up after eating the monarchs. The chances of
throwing up was proportional to the cardenolide concentration in the monarch butterfly. Wild monarch butterflies consisting of high levels of cardenolide concentrations are also less susceptible to natural predation by both birds and mice.
The bright colors of monarch butterflies is another major defense mechanism used by monarch butterflies since these colors are associated with bad tastes, thus their predators are less likely to eat them. Monarchs may also release fluids from their mouth to protect themselves from predators that eat them. Below is a figure of an ant trying to eat a monarch larvae.
Figure 2: Monarch releasing fluids to protect them from predators
Researches still have not developed an exact correlation between the amount of cardenolides in plants and the monarchs that consume them. The figure below (Figure 3) shows the concentrations of cardenolides in various milkweed species, and the concentrations in adult monarchs that were fed these milkweeds as larvae. Monarchs that fed on A. Viridis (from Florida) had the highest cardenolide concentrations. It was also found that monarchs that fed on plants with high cardenolide concentrations do not concentrate the toxins as those that fed on low or intermediate concentrations in plants.
Figure 3: Mean cardenolide concentrations in 12 milkweed species, and monarchs fed these plants as larvae.
A certain plant species, known as A. Syriaca, has cardenolide concentrations
ranging from 0 to 792 micrograms/0.1 g dry weight. Thus, there is a large
variation in cardenolide distributions among the same plant. Current research has shown that plants produced more cardenolides within 24 hours after being damaged. Cardenolides are an example of inducible plant defenses, thus the defense mechanism is used only when needed. Recent evidence has also shown that cardenolide concentration decreases, as the monarch butterfly gets older. During a 30 day period, the concentration of cardenolides in the wings and abdomens of monarchs kept outdoors decreased by 600 microgram/0.1 g dry weight.
According to Dussourd, žmilkweed leaves contain a ramifying network of latex canals pressurized with a lethal brew of toxic cardenolides in a quick-setting glue,Ó thus monarchs as well their predators suffer from feeding on the milkweed. Larvae use a žtrenching behaviorÓ in which they chew a small circle through the surface of the leave, creating a circular area to which latex does not flow (such as the first instar monarch Ů Figure 4). Larger larvae usually cut through the mid-vein of the leaf, cutting off latex flow to the entire leaf (Figure Ů 5). Refer to the figures below:
Figure 4: (A first instar monarch larvae)
Figure 5: (A fifth instar monarch larvae)
Another common defense mechanism used by monarch butterflies is their camouflaged coloration and bright colored spots to confuse predators. Bright colors act as signals warning other animals that they are poisonous. The patterns are called žaposematicÓ. Thus, when an animal attacks or eats such animals, the warning colors are associated with a bad experience. Refer to Figure 6 to see an example of the bright coloration
Figure 6: Adult Larvae and Monarch displaying bright coloration
Certain predators have a high tolerance level to the chemical defenses produces by monarch butterflies. Species such as the black-beaked orioles and the black headed grosbeaks are common predators that can tolerate an increased level of cardenolides. Attack by these predators accounts for over 60% of monarch butterfly mortalities. Refer to Figure 7 to see a black beaked oriole overwintering in Mexico and the dead monarch on the forest floor after their massive attacks in Mexico.
The above information was taken from the Monarch Lab: Exploring Monarch Butterfly BiologyWebsite constructed under the University of Minnesota St. Paul. The information is based on current monarch research that is being conducted at the University. Here are some other Links if you are interested in learning more about Monarch Butterflies.