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Male monarch butterfly
(Danaus plexippus)
© Beatriz Moisset
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People fall in love with monarch
butterflies because of their bright black and orange pattern. Many
monarch enthusiasts are busily raising these butterflies in their
gardens and homes. Some have become real experts on these insects and
their life cycle. They are profoundly distressed when something goes
wrong and one of them fails to make it to adulthood and freedom.
Some particularly despise butterfly
enemies. If they catch a predatory bug sucking the juices out of an
unfortunate caterpillar, they rage against the predator. Parasitic
flies also generate a violent reaction. It is hard
to believe, but monarchs
owe their
beautiful colors to their enemies. Here is the whole story.
Monarchs feed on
milkweeds. They are dependent on these plants and cannot digest
others. Milkweeds, like many plants produce powerful toxins as a
defense against herbivores. These toxins go by the name of cardiac
glycosides because they cause heart paralysis. As an additional
defense they produce a sticky milky-looking substance that gives them
their name. The milk is present in most tissues of the plant and
bleed easily, gumming the yaws of a hungry attacker that tries to eat the milkweed plant.
This
is enough to deter most plant eaters, but
monarch butterfly caterpillars and more than a dozen other creatures
have learned to overcome such defenses.
Earlier milkweeds, millions of years ago, had milder forms of the
toxins. That
was all they needed, but some early
insects learned to tolerate them and proceeded undeterred to feed
from these plants. Thus, milkweeds were forced to create stronger and
stronger glycosides and, in turn their feeders found ways to deal
with the more powerful toxins. Arms races of this type abound in the
natural world.
The monarch
butterfly adapted itself to these plants by
several means: it avoided the most toxic
plants or their most toxic parts; it developed enzymes that could
deal with the toxins, or it stored them in parts of its body where
they could do no harm. In doing all this, it became dependent on
milkweeds. This dependence added a bonus to the monarch's survival,
its body is loaded with bad tasting, toxic glycosides, which constitute a powerful defense against its enemies. Most predators
avoid the toxic butterfly. However a handful of these predators
developed ways to handle the monarch's toxins by eating only the
parts with less glycosides, or by evolving enzymes that neutralize
these toxic substances. This is another case of the arms race at
work.
This is not all. The
monarch advertises its toxicity and horrid taste to possible
predators. Birds who never saw a monarch butterfly before eagerly
take a bite of one. The immediate reaction is that of disgust,
spitting up the morsel and shaking their heads or rubbing their beaks
in an effort to remove the unpleasantness. They have no trouble
remembering the strikingly
colored creature and its bad taste. They are not likely to repeat
such experience.
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Large milkweed bugs
(Oncopeltus fasciatus)
on milkweed seed pod.
Adults and nymphs
© Beatriz Moisset
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Milkweed beetle (Tetraopes
tetrophthalmus)
© Beatriz Moisset
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Dendrobatid frog, Peru
© Tim Ross. Wikicommons
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Polished Lady Beetle
(Cycloneda munda)
Ladybeetles are another
example of
brightly colored bad
tasting insects
© Beatriz Moisset
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So, as I said at the
start, a monarch's lovely colors are due to a constant battle with
their enemies. In a perfect world (perfect for monarchs, that is)
these butterflies wouldn't need to be loaded with toxins, nor would
they need to tell their enemies to keep away. In such a perfect
world, monarchs would have plain colors. Is this what we want?
Further readings:
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The monarch caterpillar is
also toxic
and also has bright
warning colors
© Beatriz Moisset
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Further readings:
