Biology concepts – venom, toxin, poison, fangs, evolution,
toxicofera hypothesis
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The
California Red Sided Garter Snake is one beautiful
reptile. It has round pupils, so it might be
non-venomous,
but
it has a highly patterned and bright colored body, so
it
could be poisonous. It has a triangular head, but not to
broad,
so it could be either. For many years we thought
it
was non-venomous, but recent work says it has toxic
saliva.
It’s probably not too harmful to humans, but it
goes
to show that you can’t make assumptions.
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However, many people will tell you that the eyes of a snake
will give them away –round pupils means non-venomous, while slit pupils (like
cats) means venomous. Or that venomous snakes have patterned bodies while non-venomous
snakes wear solid colors. Lastly, some people will tell you that venomous
snakes have triangular heads, while non-venomous snakes have rounder heads.
Let’s tear down each myth. About 99% of snakes have
triangular heads. So this is no help at all; although, if you stay away from
all triangular headed snakes you probably won’t get in trouble. Venomous snakes
do have a broader base to their triangular head, to account for the venom gland
volume and associated muscles. However, are you going to take the time to determine just HOW BROAD is
the head of the snake that’s about to bite you?
As for pattern versus solid color; this will also fail you. Ever hear of a black mamba (Dendroaspis polylepis)? Well, it’s the most venomous snake
in Africa and it’s solid colored. Strangely enough, the black mamba isn’t
black. Its name comes from the color of the inside of its mouth, but its body
is silvery. The Inland Taipan viper (Fierce Snake, Oxyuranus microlepidotus) is the most
venomous on land, but it has a solid dark tan body.
Maybe pupil shape matters. I am wondering why there would be
an evolutionary link between the shape of the pupil and whether a snake has
venom. Head shape – maybe, you have to account for venom glands. Body color –
maybe, patterns would warn a predator to stay away (aposematism). But pupil shape? How would that be linked to venom or
no venom?
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The
black mamba is one of the most aggressive of venomous
snakes.
It’s as if it goes out looking for things to bite. And
they
are fast, moving up to 20 ft./sec (that’s 14 mph, or 22.5
kph),
although I think Kobe Bryant can still move faster than
that
in short spurts. The mamba grows to over 10 ft in length
and
will attack a lion, so it backs up the deadly look of its
open
mouth – it ain’t just for show.
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So let’s find some real exceptions in the realm of venomous snakes.
Unfortunately, there are so many different combinations of fang type, venom
type, and venom gland type that it is difficult to call any kind of venomous
snake an exception – there aren’t many rules. There are true venom glands and
false glands, based on whether they can store venom. Then
there are rear fangs, front fangs, and front fangs that can fold up. And then
there are systems that deliver venom to several upper teeth, and those that
deliver venom only through the channels in front fangs or rear fangs.
There may be different ways to deliver and different venoms
to deliver, but a 2008 study says it all venomous snakes derive from a common
ancestor that lived about 60 million years ago. The study of Dr. Vonk looked at
front and rear-fanged venomous snake embryos, and saw that the venom gland
ducts ALWAYS start out attached to a rear tooth, but in the front-fanged snakes,
the tooth and duct move forward during fetal development!
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The
rear-fanged snakes have back teeth that deliver venom,
but
not by injecting it like a hypodermic. They are not hollow;
some
have grooves for the venom to run down, while others
are
just sharp, bigger teeth. Snakes with rear fangs have to
take
a bigger bite, and hold on longer to grind the venom into
the
wound. Doesn’t sound like the perfect system, but
evolution
is striving for perfection, it just uses what is there.
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It may be that this occurred with all snakes; those that
aren’t venomous just lost the ability to produce or deliver venom. This is part
of the toxicofera hypothesis of
which we have spoken. We don’t even know what percentage of snakes are
venomous. Scientists have focused on the highly venomous snakes for so long,
that the so-called non-venomous snakes have been ignored.
Many snakes that were once thought to be non-venomous are
now known to have venomous bites. Colubridae
snakes (garter snakes, hognose snakes and many more) were thought up to the
1950’s to be utterly non-venomous. But Dr. Fry showed that many of these snakes
do indeed deliver venom, though most may be harmless to humans.
As recently as ten years ago it was said that only 10% of
snakes were venomous; now that percentage is somewhere near 30%. Where might it
end – could most snakes be venomous?
Even though scientists now know more about the evolution of
venom, there are still mysteries. With millions of years to perfect a venom
system, why is it that some snakes have venom that is WAY TOO POTENT for its
purpose? With each bite the fierce snake delivers enough venom to kill 2000
mice or 50 humans – why so much? It must be advantageous in some way; or else
it doesn’t cost any more energy to make the venom that potent.
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This
is a juvenile tiger keelback snake. The raised part on the
back
explains the name. It is actually the nuchal gland that
stores
cane toad toxin as a defense. How could a small
juvenile
have toxin before it is big enough to start eating cane
toads?
It can be passed on from mother to offspring, if she
ate
a cane toad before egg format
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Debra Hutchinson published on the keelback snakes in 2007. Tiger keelback snakes (Rhabdophis tigranus) live in Asia, and enjoy a diet of cane toads – poisonous cane toads! Cane toads kill most things that try to eat them, but for some reason the keelback snakes don’t seem to be bothered by the toxin.
In fact, they sequester the cane toad toxin to two nuchal
glands, located on the back of their necks. Then, when the snake is threatened
by a predator, they turn their back to the aggressor and dare them to bite down
on the nuchal glands! Most predators have learned not to take the bait.
The nuchal glands are purely for storage. They don’t have
ducts or deliver the poison to the skin or a fang. The sequestered toxin is
purely defensive. But the keelbacks also have venom glands, of the false gland
type, delivered to the base of several upper teeth. The keelback then bites, chews and
grinds the venom into the wound.
This is the way it goes for many rear fanged snakes.
Delivering venom by the front fangs is 100x more efficient than the rear fangs,
so rear fang snakes must hang on longer to their target in order to envenomate
them. This means that they are more vulnerable to being bitten when the target
fights back. The keelback has made this less likely by storing another toxin in
its neck. Pretty smart, huh?
Now for one more venomous snake exception. Your parents
always told you not to spit, but a few snakes are expectorating geniuses. The
spitting cobras (genus Naja, and a
couple others) have an additional modification to their front fangs that gives
them the ability to spit their venom, in some cases, over twenty feet. |
When
threatened, the tiger keelback assumes a familiar neck
arch
position. This was described in Akira Mori’s 2012 paper.
If
disturbed, it will also try to bump the nuchal glands into the
aggressor,
They will spray toxic contents if bitten or pinched.
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Injecting venom from front fangs is controlled by specific
muscles around the venom gland. Spitting snakes combine this quick delivery
under pressure with a targeting system. Instead delivering venom from the tips
of their fangs, they have an aperture (hole) in the front face of their fang
(see picture). Some cobras aim for the eyes of their targets, while others aim
for mouth, nose or skin.
The aim is incredible in all, but it is even better in some
species. I will use guns as a model. Most guns and cannons up to the time of
the US Civil War were very inaccurate. By gouging curved grooves down the
barrel, a spin was placed on the cannon ball, and the spin made it much more
accurate. This “rifling” was invented in the 1500’s, but didn’t become common
until the 1800’s.
The same is seen in the African (not so often in the Asian)
spitting cobras. The fang and aperture have rifling grooves that make them even
more accurate. I would say that humans stole the idea from nature (like we so
often do), but I don’t think we knew about spitting cobras when guns barrels
started being rifled.
But how does spitting (really squirting, no saliva is
involved) venom at an aggressor help, other than grossing them out? We know
that venom must be injected below the skin in order to be effective, but a
spitting cobra’s toxin can be cytotoxic (lots of inflammation and tissue
destruction) to the skin, and can blind if it hits the eyes. The black-necked
cobra and the red Mozambique cobra have been shown to aim only for eyes.
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This
is a Mozambique spitting cobra. Notice how the spray
comes
straight out from the front of the fangs and is directed in a
narrow,
pointed direction. This isn’t strafing fire, it’s sniper work.
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Most spitting cobras actually have a mix of toxins; some
neurotoxic, some hemotoxic, some cardiotoxic, and some cytotoxic. Somewhere
along the way, evolutionarily speaking, the spitting cobras concocted a toxin
that has both the ability to harm by surface contact, and the ability to harm
on contact. Evolution at its best.
Next week, is there a group of animals where every species
is venomous? And how about a group where none of the species are venomous.
Vonk, F., Admiraal, J., Jackson, K., Reshef, R., de Bakker, M., Vanderschoot, K., van den Berge, I., van Atten, M., Burgerhout, E., Beck, A., Mirtschin, P., Kochva, E., Witte, F., Fry, B., Woods, A., & Richardson, M. (2008). Evolutionary origin and development of snake fangs Nature, 454 (7204), 630-633 DOI: 10.1038/nature07178
Mori, A., Burghardt, G., Savitzky, A., Roberts, K., Hutchinson, D., & Goris, R. (2011). Nuchal glands: a novel defensive system in snakes Chemoecology, 22 (3), 187-198 DOI: 10.1007/s00049-011-0086-2
Hutchinson, D., Mori, A., Savitzky, A., Burghardt, G., Wu, X., Meinwald, J., & Schroeder, F. (2007). From the Cover: Dietary sequestration of defensive steroids in nuchal glands of the Asian snake Rhabdophis tigrinus Proceedings of the National Academy of Sciences, 104 (7), 2265-2270 DOI: 10.1073/pnas.0610785104
For
more information and classroom activities, see:
Snakes
–
keelback
snakes and nuchal glands –
spitting
cobras –
