A Tale of Two Species
by Karl Peterson
T
he Big Bend region has six rat-
tlesnake species living within its
confines. The two most widely
distributed and commonly-seen
species are the western diamondback
and Mojave rattlesnake. I have seen
neonate (baby) and adult western dia-
mondbacks dead on the road (DOR)
within the city limits of Alpine on
many occasions. Last year I saw and
was seen by a nervous, aggravated, rat-
tling, roughly-four-foot western dia-
mondback under my trailer. I know
several people who live near me who
have seen a range of age classes. Just a
few weeks ago we found an adult
female Mojave DOR on Hwy 90 next
to the Oriental Express restaurant.
The fact that large, three- to four-foot
long, potentially dangerous rat-
tlesnakes are common in the Alpine
area but are rarely seen and even more
rarely bite anybody is a testament to
their generally reclusive nature.
The western diamondback and
Mojave rattlesnakes are very similar in
appearance, and the only practical
way to safely differentiate between
them is to look at the bands on the tail.
The western diamondback's tail has
black and white bands of roughly
equal width; the Mojave rattlesnake
has white bands that are roughly twice
the width of the black bands.
I do not like to categorically state
anything about “reliable” characteris-
tics that can absolutely be used to iden-
tify any given snake species, because
snake patterns can vary hugely. When
I was at the Houston Zoo, we had to
be very careful when identifying local
snakes over the phone. The local cop-
perhead population had extremely
variable patterns. If a phone descrip-
tion left too much uncertainty, we were
allowed time to travel to the location to
make a positive identification.
As for the two species under discus-
sion, there are also differences in scale
number, size and arrangement on
both the heads and bodies of the two as
well as subtle differences in the head
patterns of each. If you are looking at
8
any of these characteristics to differen-
tiate between the two, you are way too
close! Many field guides go into detail
discussing these characteristics in their
discussion of the two species. I have
always found that rather amusing. To
use these you must have caught and
safely restrained the snake (always an
exciting endeavor, even if one has
done this many times), or you will have
had to kill the snake. There are many
case histories of bites inflicted by a
snake someone was trying to kill or by
a snake that appeared dead until it bit
someone. Thus, ignore every “aid” to
identification except the difference in
the bands on the tail. You can see that
from a safe distance. If you are looking
at one of the two out in the wilds of the
Big Bend region, be happy you got to
see a rattlesnake, then go your separate
ways.
If there is one in your backyard, you
may be skeptical of the reclusive, non-
aggressive nature of these species—
you may want it GONE. Either try to
safely kill it or call the Sheriff’s
Department or Animal Control. I hate
to tell anyone to kill a rattlesnake, but I
am a realist. Study after study on relo-
cated rattlesnakes (and much wildlife
in general) shows the same thing: a
specimen released within a certain dis-
tance from its home, your yard, invari-
ably returns home. One taken well
beyond that distance (for most rat-
tlesnakes over a mile) will almost
invariably die because they are not
familiar with the new area. I am a
great fan of rattlesnakes but any parent
would view one as a potentially very
serious, if not fatal, threat to a child. I
also don’t like the idea of one slowly
withering away, horribly stressed and
utterly lost, in a strange area.
The venoms of the two species have
evolutionary, taxonomically, and med-
ically significant differences. Very sim-
ply stated, the venom of western dia-
mondbacks destroys tissue and can
have an anti-coagulant effect, and
rarely, systemic effects like shock that
may be life threatening in children.
Cenizo
Fourth Quarter 2016
Depending on the amount of venom
injected, bite symptoms may range
from very mild swelling and pain to
massive swelling, severe tissue damage
and massively severe pain. In contrast,
Mojave venom is primarily a neuro-
toxin (affecting the nervous system). It
depresses respiration and conscious-
ness. The amount of swelling and pain
involved in a Mojave bite is normally
much less. In some areas, Mojave rat-
tlesnakes lack the dangerous neurotox-
ic element, like in south-central
Arizona. The Mojaves in Big Bend
actually are on record as having the
most toxic venom of any U.S. popula-
tion. All things considered, a bite by a
Mojave rattlesnake is the more serious
of the two. Western diamondback bites
are rarely fatal to livestock (a nose bite
can cause suffocation). Livestock is
killed by the bite of Mojave rat-
tlesnakes. I know this from both litera-
ture reports and discussions with
ranchers in Mexico.
Given all this potential biochemical
mayhem, I think it is important to
remember how rare deaths from rat-
tlesnake bites are in the United States:
generally less than about 20 deaths per
year. The statistical chance of dying
while taking a bath or a shower is
much higher than it is from dying as a
result of a rattlesnake bite.
Venom is a crucial resource for
obtaining prey as well as self-defense;
the amount of venom injected at any
given time is actually determined by
the snake. Studies have shown that
generally, a defensive bite involves
less venom than a prey-obtaining bite.
Reptiles and amphibians are more
than automatons, reacting to external
stimulus in a somewhat unsophisticat-
ed manner. Using their voluntary
venom metering as an example shows
a rattlesnake must weigh different fac-
tors very quickly; food or predator,
relative size or mass, to determine the
amount of venom used. In the case of
a hopping type of prey animal, like a
kangaroo rat, rattlesnakes sometimes
must do vector analysis to bite the
prey mid-hop, which is no small feat.
Lower vertebrates are actually much
more intelligent than generally per-
ceived, as recent research has shown.
Years ago all the books on reptiles I
read held a common belief, the idea
that baby rattlesnakes were more ven-
omous than the adults was nothing
more than a myth. In the 1980s,
venom researchers began to find that
in some rattlesnake species, including
the western diamondback, this is actu-
ally true. I often listen to “myths”
because on rare occasions there can
be a kernel of truth at their founda-
tion. I also listen to what kids have to
say; they are often sophisticated
observers of objective reality, even
when they are unaware of it.
There is an interesting belief about
both the western diamondback and
Mojave rattlesnake that I have only
seen published once – not in a book
about reptiles – and heard it on two
occasions. It is believed that when a
western diamondback and Mojave rat-
tlesnake hybridize the resultant off-
spring is identical in appearance to the
Sonoran gopher snake, aka bull snake,
another fairly common snake in our
area. The hybrid offspring is believed
to be far more venomous than either
parental species, is even said to be lack-
ing a rattle at the end of its tail,
although it rapidly vibrates the tail
anyway. Sonoran gopher snakes do
have a blotched pattern like both rat-
tlesnake species. Like most non-ven-
omous snake species they do vibrate
their tails against the substrate when
threatened. This leads to the sugges-
tion that they are mimicking the rat-
tling of the rattlesnake tail. Vibrating
the tail is a common defensive behav-
ior of many snake species across the
globe, including areas where there are
not and have never been any rat-
tlesnake species. *
Western diamondback and Mojave
rattlesnakes have very rarely been
recorded as hybridizing in the wild,
and interestingly, there is one refer-
ence that conveys an air of perhaps