Re: Plants, Herbivores and toxins

From: r norman (rsn__at__comcast.net)
Date: 03/03/05 

Date: Thu, 3 Mar 2005 16:35:22 -0500 (EST)

On Thu, 3 Mar 2005 13:07:17 -0500 (EST), Binesh Bannerjee
<binesh-dated-1110441927.34321e@hex21.com> wrote:

>Hi.
> (Not a specialist by any means here, just a lowly programmer
>curious about a question)... So, here goes...
>
> I read the Blank Slate, and How the Mind Works by
>Steven Pinker, and several other references, where it's said
>that plants, not wanting to be predated on by herbivores, have
>evolved toxins in their bodies to prevent the predation. The
>evolutionary arms race that follows leads to livers in animals,
>etc. I think I understand this.
>
> I'm curious if any similar phenomena happens with
>herbivores and carnivores. Do herbivores engage in any sort
>of chemical warfare with _their_ predators? Would it be correct
>to assume that the normal lesson associated with butterflies
>mimicing the _appearance_ of bad tasting butterflies implies
>a hidden lesson that the bad tasting butterflies acquired
>that taste, because all the ones that tasted better to their
>predators were eaten?
>
> Are there examples of animals that change their taste,
>as a measure against predators eating them? Is it a myth that
>fear alters the taste of meat is another question I'm curious
>about.
>

You are on the right track, but are a bit off in the way you express
things. It is not that plant don't "want" to be eaten. Wanting has
nothing to do with it. It is simply that plants that produce bad
tasting compounds are eaten less and therefore survive and reproduce
better than plants that don't. However, there definitely is an
evolutionary "arms race" between plants and their herbivores.

You are also on the right track in saying that animals have developed
a series of complex detoxification systems to defend against the plant
toxins. The liver is the site of many of these in the case of
vertebrate animals. However the liver, itself, is not really the
"result" of that arms race. The liver runs a tremendous number of
biochemical processes far beyond detoxification. And most plant
herbivores (the insects) don't have an organ called the liver at all.

It is interesting that humans (and probably many other animals) have
learned that eating just a small quantity of the "noxious" or
"bad-tasting" or even toxic plants produces a sub-lethal effect that
some people think is desirable. Hence we go to great lengths to burn
plants and inhale the smoke, to boil plants in water and ingest
whatever dissolves out of them, or to chemically extract the chemicals
and inject them into our bodies. Hence the tobacco industry, the
coffee and tea industry, the chocolate and cola industries, and the
diverse industry in drugs of abuse. Many of our medicines are derived
from compounds initially discovered in this way. In many other cases,
we travel great distances to the far corners of the earth to find
these plants, dry them, and add small quantities to our food. Or we
add the fresh leaves to our food. In either case, these spices and
herbs add to our pleasure in eating.

Herbivores and carnivores have a similar arms race. Bad taste is one
defense, as is the development of toxins. Many amphibians secrete a
bad tasting or toxic mucus that most predators avoid. There are a few
predators, however, that have managed to become immune or insensitive
to these compounds and prey on the otherwise noxious or toxic frogs
and toads. A number animals squirt a bad smelling or bad tasting
chemical on potential prey. Not just skunks, but also the garter
snake and the stink bug are common examples. We in Michigan have
recently been infested by an Asian ladybug (more correctly, Lady Bird
beetle) that does this.

However, animals have a lot more tricks up their sleeve than plants.
A plant can build protective structures: a thick covering or spines
and thorns, but it can't run and it can't hide. However it can do
biochemistry very well. Animals usually move, and so the more common
defenses against predators involve behavior rather than chemistry:
running and hiding are two major defenses.

Your example of bad-tasting butterflies is a classic textbook example
of different kinds of mimicry. However, it is now realized that many
examples of "benign" insects that mimic bad-tasting one may be
incorrect. The mimics may be equally bad-tasting!.

I don't know examples of animals that can readily change their taste.
As I indicated, animals are not very good at changing their chemistry.
I would guess that some hormones that may be involved in "fright"
might alter the taste of the food. I know that reproductive hormones
alter the taste of commercial pork, for example. However, it is hard
to imagine that any prey animal that is finally run down, caught, and
killed by a large predator -- whether giant cat or hawk or whatever --
in not already in a state of great excitement (whether it is
"frightened" is a very different matter -- that is an emotion we
humans experience). However you can imagine the adrenaline pumping at
least in the case of vertebrate prey. And certainly that doesn't deter
the predator from eating its catch!

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