I’ve been thinking a lot about migrations. Right now we have migrating birds, awhile back we had migrating monarch. Monarch Butterflies have an interesting story. Who knew?
As most prey-species do, these butterflies possess several defensive measures:
- Color - bright ‘warning’ colors indicate foul or poisonous taste
- Numbers - they migrate and hibernate in great numbers (up to 10 million butterflies per hectare!)
- Mass Startle Effect - roosting monarchs, when disturbed, fall from their perches all at once, behavior that seems to confuse and disorient predators and allow escape
- Defensive Chemical Compounds - known to be both distasteful and toxic to most predators.
Male Monarch Butterfly
Slightly larger and with
more narrow black veining on wings
The chemical compounds are gleaned from the plants both caterpillars and butterflies feed upon and are toxic to vertebrate predators. As caterpillars, monarchs gain cardiac glycosides (Asclepiadaceae (milkweed.)) from the plants they eat.
Female Monarch Butterfly
Somewhat smaller and with
wider black veining on wings
This toxin is passed on from caterpillar to butterfly, and stored primarily in the cuticle (the outer, non-cellular layer of an insect’s exoskeleton), as well as in the abdomen and thorax. However, the toxin lessens with the age of a butterfly. To help with its toxicity, another compound, pyrrolizidine alkaloid, is gained by the butterfly through the flowers it chooses to feed from (Astraea (Euphorbiaceae family)). As a result, most monarchs face little predation from frogs, lizards, mice, and birds.
Mature Monarch Caterpiller
Most toxic form of the Monarch
While there are a few other predators who occasionally eat a monarch butterfly or two, it is primarily just two birds and one mouse that seem to prey extensively on them. The birds are the Black-headed Grosbeak and the Black-backed Oriole. In smaller colonies, predation levels by birds reached as high as 44%.
Male (L) and female (R) Black-headed Grosbeak
The levels of toxins in adult monarchs reflect the levels in their host plants making some monarchs not so bitter-tasting. Some predators have learned to measure the toxins by taste and reject butterflies with high cardiac glycosides contents, eating only the ones with low cardiac glycosides contents. Some predators are able differentiate these lower-toxic parts and consume only the most palatable ones. None have perfected this so well as has the Black-backed Oriole, which will ‘unzip’ a butterfly and eat only the abdomen and thorax. The Black-headed Grosbeak leaves only the wings.
All photos on this post from Wikipedia