How often have we heard the saying that "We are what we eat"? This isn't perhaps quite as true as we've been led to believe. After all, most things we eat are disassembled and the parts used to manufacture our own versions of proteins, fats, and carbohydrates rather than simply incorporating the food as is. There are times, though, when some animals need the unadulterated real thing. One such animal is the Monarch Butterfly, an occasional visitor to our Chihuahuan Desert.
The Monarch is known far and wide by predators as something to be
avoided. Why? Simple. They taste horrible. This taste is not something innate,
something genetic. Instead, it is a result of ingesting alkaloids, a class of chemicals
that includes poisons and hallucinogens. The butterfly caterpillar feeds on milkweeds,
plants that are themselves protected from most herbivores by the presence of these
alkaloids--but not protected from the Monarch, who adds insult to injury by not only
munching happily on the milkweed, but also by stealing its protective juices for its
own defense. 
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.