RE: monarch butterflies

	There was a fascinating line of research by Lincoln Brouwer and his
associates a few years ago untangling the milkweed/insect web.  Basically,
their results go like this: the amount of milkweed toxins ["cardiac
glucosides"= CG] in a monarch's body is directly related to the
concentration of these toxins in the larval food plant.  And this amount
varies widely between species of mikweeds, between populations of the same
species of milkweed, and between individual plants in a population.
Although this thread wasn't followed out in the literature that I read, it
would appear that something like mimicry is going on at the plant end:
that some milkweeds free-load [by not producing toxins] on their neigbors.

	The Brouwers used bluejays as their test predator.  And as you
say, a "naive" jay happily eats monarchs, and anything that looks like a
monarch.  Up to the point that the butterflies have a certain level of the
cardiac glucosides.  And that point is not lethal [though these toxins
could kill a vertebrate]. Nor is it just tasting yukky: birds, unlike
mammals don't seem to depend much on their sense of taste.
	Rather, it seems that many birds and mammals are protected from
ever poisoning themselves by this class of toxins by an automatic emetic
response.
	The Brouwers described a "bluejay emetic unit" [BEU] equivalent to
a
specified amount of the milkweed toxins.  Less than a BEU, and bluejays
munched away.  More than a BEU and the jays lost their lunch, and then and
only then began to avoid the butterflies, and anything that looked like
them.  Presumably the African mikweeds you describe have fairly low levels
of CGs, and the jays have learned that monarchs [in that locale at least]
are pretty good food.
	Another thing to point out is that the monarchs, significantly,
don't absolutely rely on their chemical protection -- as would be expected
for a uniformly toxic insect.  They have bright and distinctive colors,
but they retain active defense -- they are fast, erratic flyers, etc.
Really, the monarch is an amazing species, and much remains to be learned
from it.
	The work I'm describing was done in the 1970s, and there were some
Scientific American articles on it. Brouwer published popular book, too,
if I remember correctly, though I've never read it.

loren russell, corvallis oregon

On Tue, 25 Jan 2000, William Bade wrote:

> There are milkweeds native to Africa planted at UC Botanical Garden, in
> Berkeley, California, and migrating monarchs laid eggs on them one or two
> years. The eggs hatched, the caterpillars chewed and went on to pupate,
> but the Stellar and Scrub jays had a field day eating, picking them off
> where they were attached to buildings. It seems that some milkweeds do
> not have toxicity to protect the butterflies. I later learned that African
> butterflies don't have mimic spieces as the monarchs of the western
> hemisphere do.
> Elly Bade
>
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