During
the first part of November a wanna-be writer said he wasn't
participating in the annual National Writing challenge because he
couldn't think of anything to write about. One look at this story and
how many different stories pop out in different genres? You better
believe this little gem is going to appear in my writing sometime in
the near future in at least one short story and woven into a novel.
June 21, 2011 12:51
pm I just got back yesterday from the annual meeting of the Society
for the Study of Evolution. It took place in a big hotel on the
outskirts of Norman, Oklahoma, during a windy heat wave that felt
like the Hair Dryer of the Gods. It had been a few years since I had
last been to an SSE meeting, and I was struck by how genomic
everything has gotten. No matter how obscure the species scientists
are studying, they seem to have outrageous heaps of DNA sequence to
analyze. A few years ago, they would have been content with a few
scraps. Fortunately, SSE hasn’t turned its back on good old natural
history. There were lots of fascinating discoveries on offer, about
species that I had assumed had been studied to death. My favorite was
a talk about the rough-skinned newt, the most ridiculously poisonous
animal in America.
The scientific tale of the rough-skinned
newt begins five decades ago, with a story about three dead hunters
in Oregon. Reportedly, the bodies of the hunters were discovered
around a camp fire. They showed no signs of injury, and nothing had
been stolen. The only strange thing about the scene was the coffee
pot. Curled up inside was a newt.
In the 1960s, a biologist named Butch
Brodie got curious about the story. The newt in the coffee pot–known
as the rough-skinned newt–has a dull brown back, but when it is
disturbed, it bends its head backward like a contortionist to reveal
an orange belly as bright as candy corn. Bright colors are common
among poisonous animals. It’s a signal that says, in effect, “If
you know what’s good for you, you’ll leave me alone.” Brodie
wondered if the newts were toxic, too.
Toxic, it turns out, doesn’t
do the newts justice. They are little death machines. The newts
produce a chemical in their skin called tetrodotoxin, or TTX for
short, that’s made by other poisonous animals like pufferfish.
Locking onto sodium channels on the surface of neurons, TTX blocks
signals in the nervous system, leading to a quick death. In fact, TTX
is 10,000 times deadlier than cyanide. While we may never know for
sure what killed those three Oregon hunters, we do know that a single
rough-skinned newt could have easily produced enough TTX to kill
them, and have plenty of poison left over to kill dozens more.
Now, if the whole idea of evolution
makes you uneasy, you might react by saying, “That couldn’t
possibly have evolved.” Experience has shown that this is
not a wise thing to say. Brodie said something different: the most
plausible explanation for a ridiculously poisonous animal is that it
is locked in a coevolutionary arms race with a ridiculously
well-defended predator. Another biologist mentioned to him that he’d
seen garter snakes dining on rough-skinned newts, and so Brodie
investigated. He discovered that garter snakes in rough-skinned newt
territory have evolved peculiar shape to the receptors on their
neurons that TTX would normally grab.
The co-evolution of newts and snakes
became a family business. Brodie’s son, Edmund, grew up catching
newts, and today he’s a biologist at the University of Virginia.
Father and son and colleagues have discovered that snakes have
independently evolved the same mutations to their receptors in some
populations, while evolving other mutations with the same effect in
other populations. They’ve also found that both newts and snakes
pay a cost for their weaponry. The newts put in a lot of energy into
making TTX that could be directed to growing and making baby newts.
The evolved receptors in garter snakes don’t just protect them from
TTX; they also leave the snakes slower than vulnerable snakes.
They’ve studied newts and snakes up and down the west coast of
North America and found a huge range of TTX potency and resistance.
That’s what you’d expect from a coevolutionary process in which
local populations are adapting to each other in different
environments, with different costs and benefits to escalating the
fight.
This story is so irresistible that I’ve
written about it twice: first, ten years ago in Evolution:
The Triumph of an Idea,, and then in updated form last year
in The
Tangled Bank. I figured that the Brodies et. al. had pretty
much discovered all there was to know about these creatures. But in
Oklahoma, I discovered that they had missed what is arguably the
coolest part of the whole story.
Think about it: you’re a female newt,
you’ve fended off attackers with a staggering amounts of poison in
your skin, and now you want to pass on your genes to your
descendants. You lay a heap of eggs in a pond, and what happens? A
bunch of pond creatures come rushing in and have a feast of amphibian
caviar.
What could you possibly do to ensure at
least some of your offspring survived? Well, you have an awful lot of
TTX in your system. You have enough of the stuff to give your eggs a
parting gift to help them out there in the cruel, predator-infested
world. Make your eggs poisonous.
That is exactly what female newts do. In
fact, they load their eggs with TTX. To figure out if this poison
provided a defense against predators, the Brodies and their students
traveled to a group of ponds in central Oregon that are home to
thousands of rough-skinned newts apiece. They collected dragonflies
and other aquatic predators from the ponds and put them in buckets
filled with newt eggs, along with muck from the pond bottoms. The
scientists found that almost none of the predators would touch the
newt eggs. Since these predators eat plenty of eggs of other species,
this result shows that TTX does indeed help the newt eggs survive.
But there was one exception. Caddisfly
larvae turned out to relish the newt eggs. In fact, the caddisflies
actually grew bigger if they were supplied with newt eggs and pond
muck than with pond muck alone. And yet the Brodies and their
students estimate that there’s enough TTX in one newt egg to kill
somewhere between 500 and 3700 caddisflies.
You know where this is going. At the
evolution meeting, one of their students, Brian Gall, described
feeding newt skin to caddisflies both from the central Oregon ponds
and from ponds elsewhere without newts. The newt-free caddisflies
would happily munch on newt skin from which all the TTX was removed.
But if there was more than a trace TTX in the skin, they refused to
eat. The caddisflies that fed on newt eggs, on the other hand, would
eat the most toxic skin Gall could provide.
It appears that the caddisflies have
evolved much like the garter snakes. In ponds where rough-skinned
newts lived, the caddisflies have evolved defenses against TTX. In
fact, Gall reported, the caddisflies appear to put the snakes to
shame. Evolved snakes are 34 times more resistant to TTX than
vulnerable ones. The caddisflies have increased their resistance 175
times.
It’s not clear whether the caddisflies
and the newts are truly co-evolving, however. The Brodies will have
to find out whether adding extra TTX to eggs increases their survival
in the presence of caddisflies. Another intriguing possibility arises
from their discovery that the caddisflies actually harbor some of the
TTX they eat in their tissues for weeks after eating the eggs.
Perhaps the caddisflies are stealing the poison to protect
themselves, as happens in monarch butterflies eating toxic milkweed.
In other words, this wonderfully deadly
story isn’t over yet.
Have
a tremendously productive new year and Happy Writing.
--Sean
