Biology concepts – venom, mammalian defenses, poison, toxin,
sequester, honest signals
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Humans
are much of a match for most large predators, we
have
to rely on our wits to get us out of dangerous
situations.
Or better yet, KEEP us out of dangerous situations.
Don’t
worry, no humans were harmed in the making of the
photograph;
it is a re-enactment for a Discovery
Channel
program.
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The answer to all – Homo
sapiens. We’re a mess when it comes to protecting ourselves physically. Most
mammals have defensive behaviors and some have built in protective anatomy,
but not us. If we had to survive by fighting off a tiger, our species would be
nothing but a footnote in history. We need some camouflage - or maybe a
superpower.
Some mammals have super sight. Most prey animals have their
eyes on the sides of their heads to scan 180˚ or more of the environment, but
ours are on the front of our faces in order to allow for binocular vision and
good depth perception. Don’t get cocky; even though we may be considered a
predator, think of all the animals you wouldn’t want to run into in a dark
alley.
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Sense
of smell in some mammals is quite developed.
Here
a tule elk raises it nose to catch chemicals in the
breeze
– always on guard for bears, and for good
reason.
Bears are the champion smellers. Their brain
is
1/3 the size of humans, but 5x as much volume
is
devoted to smell.
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But most mammals use other senses as well. Noses can be even
more important than eyes. Chemicals from skin lipids or urine can identify
predators before they are seen. Changing concentrations from spot to spot can
provide evidence of time and direction; merely sensing a predator's odor alone
would cause too many false alarms and wasted energy.
Added to the wealth of information prey mammals may pick up
is sound. Again, differences in timing and loudness can give clues as to
direction and distance, as we discussed with owls. Hearing is especially important for nocturnal
mammals, as sight is of less value in the dark… duh!
Many animals have a way of avoiding predators even if you
come into their range. Things like camouflage can help; predators may not
attack if they can’t see you, even if they know you are there somewhere. But if
they do spot you – it becomes decision time. Fight or flight are the two basic
choices, although there are variations on these themes.
Some animals will freeze, based on the idea that predators
are looking for grazing or some other motion. When motionless, camouflage has a
better chance of working. Others choose speed to survive. Rabbits, gazelles
look to outrun the predator, and turning on a dime is maneuver that a chaser may
not be able to follow.
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Pronking
(stotting) is a defensive behavior meant to
display
fitness. I am not aware of a study that shows
that
he who pronks highest is least apt to be attacked,
but
that is the idea. See the video for a good example.
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Likewise, elephants and rhinos will charge you with loud
noises and clouds of dust. Same with bulls. These are called honest displays. They are true
representations of fitness and/or aggressiveness. Honest displays can be used
defensively as well. Stotting (also
called pronking or pronging) is used by many species of gazelles, springboks
and deer ostensibly to impress predators with their fitness. They bounce as
high as they can using all four legs at once.
If they can expend this kind of energy when a predator is
near, it must mean that they are the most fit; predators shouldn’t waste time
trying to catch them. Many studies have been carried out to see if this type of
behavior is stable in a species. My explanation is – if it didn’t work, they
would all be dead or wouldn’t do it anymore. But other researchers want to be a
little more specific.
A late 2012 study used game theory to predict the stability of signaling in prey
animals. Their model showed that variable intensity signals would be stable;
those where greater energy is expended in signaling the nearer the predator is
to the prey. They also predict that fake signals (dishonest signals) would not be as stable because of wasted energy,
as would on/off signals where an intense response would be elicited no matter
the relative danger.
Sometimes signaling is not enough; pragmatic defenses are
needed. Gorillas are strong, elephants are huge, porcupines have quills. These
are all brilliant adaptations that serve their purposes, but I stand in awe of
the mammalian biochemists.
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Here
is an example of a dishonest signal. The squirrel
chews
up old rattlesnake skin and spreads it over its
fur.
Now he smells like a rattlesnake; predators are
much
more likely to leave him alone. Dishonest ---
but
smart.
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Skunks are very confident in their chemical defense, as well
they should be. However, they may be a little over confident. It is
hypothesized that many skunks are hit by cars because they see oncoming cars as
just another predator that will cringe and run when faced with their backside
and raised tail – oops.
Some mammalian chemical engineers are true exceptions. Did
you know that there are venomous mammals? For one, there is the solenodon (solen = slotted, and don
= tooth), a mammal that we were sure was extinct. Two species are now known to still exist, the
Cuban solenodon, and the Hispaniola solendon, but they are exceedingly rare and
are usually spotted only years apart.
The solenodons are very old species and have retained their
ancient traits, this makes them interesting as example of what mammals were
like during the dinosaur age. They have poisonous saliva that they grind into
their prey with their slotted teeth, but this does not save them from their own
predators. They have a tendency to stop and hide their heads if attacked; this
is a less than optimal defense. Therefore, it's a good thing that they are
nocturnal.
There is also the male platypus. We met the platypus when we discussed genomic imprinting, but being egg layers with the potential for
parthenogenesis are just some of their exceptions. On each hind leg they have a
talon or spur that is connected by a duct to the crural gland in their thigh.
The venom, which is actually a mixture of many toxins, seeps out onto the spur
and is transferred into the wound when the platypus kicks at a target.
But only the males have the spurs and toxin in adulthood.
Females are born with the spurs, but they soon fall off. This is related to the
notion that the venom is not fatal, it just hurts very badly, and that the
venom is made mostly in the mating season. Put these three clues together, and the answer says that the venom is used in mate selection. The platypus has few predators, and they don’t need to
subdue the worms and tiny shrimp they eat. But they do have rivals for
females. A 2009 study
speculates that the non-lethal venom probably developed as a mating selection
device. The male who isn’t cringing in pain wins the girl. This is the only
instance known of a temporal cue for venom production.
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Here
is a slow loris that is more than willing to show
you
his toxin glands. They are not covered with fur
and
are located halfway between his armpits and his
elbows.
I’m not sure I would be handling
him
without
gloves.
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The slow lorises (all 9 species, living in southern Asia)
will lick their young before stashing them away to go find food, protecting
them from potential predators in a poisonous kind of way. They will also bite
strongly and hold on, passing the toxin into the wound, which makes it a kind
of venom.
To add to our mammalian exceptions, we should spend a minute
talking about the mammals that are strictly poisonous. Two examples are known,
both being toxin sequesterers. They
don’t make their toxin, they gather it from another natural source and then use
it for their defense.
One example is the southern vole (Microtus levis). They eat grass,
and sometimes the grass is infected by a poisonous fungus. For most voles, the
fungus is lethal, helping the fungus protect its grass habitat, but the southern vole
is immune to the poison. In fact, the fungus toxin protects the voles from their main predator,
the least weasel. One study
has investigated why, with mixed results.
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On
the left is the Acokanthera tree. It looks harmless but is deadly.
However,
the fruit is said to he edible and is used as medicine –
not
for me it isn’t. In the middle is the crested rat. He has adopted
black
and white colors, much like the skunk. Bright colors don’t work
so
well when you are a crepuscular animal. On the right is one of the
hollow
flank hairs of the crested rat that holds the oubain toxin.
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Lastly, there is the African crested rat (Lophiomys imhausi). A recent study has shown that
this small mammal likes to moon its potential predators. It spends a lot of its time gnawing
on the bark of the Acokanthera tree, which contains oubain, a curare-like
toxin. It spreads this chewed up mess on its flanks, which contain specialized,
hollow hairs (see pictures above). The hairs soak up the toxin, and then when threatened, the rat turns
its flanks to the predator. This, along with its coloring and thick fur and skin
in that area, is enough to keep the crested rat alive until the predator learns its lesson –
it dies from the toxin.
Next week, how about discussing more exceptional animals - venomous amphibians
and lizards?
For
more information, see:
Mammalian
defenses –
Signaling
theory –
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