Iguana 12.1


IGUANA  •  VOLUME 12, NUMBER 1  •  MARCH 2005 43HISTORICAL PERSPECTIVES

The rattle of the rattlesnake (all members of the genera
Crotalus, with small scales on the head, and Sistrurus, with

large plates on the head, are called rattlesnakes) is more or less
familiar to every one. Those who have not seen or heard it have
surely heard about it. It seems at first thought to be one of the
most difficult of developments to understand and explain.
However, when we consider the habit, which a vast number of
different species of snakes have, of vibrating the tip of the tail
when they are excited, together with the fact that the outer
horny layer of the skin is shed at frequent intervals, it is not dif-
ficult to look back over what have very probably been the steps
which led to the formation of the rattle. Many species of snakes,
such, for instance, as the bushmaster and the copperhead, have
a large horny spine at the end of the tail. It is only necessary to
suppose that some ancestor of the rattlesnake in the distant past
had such a spine, which perhaps swelled into a knob possessing
a slight constriction about its circumference. Now, since the
outer covering of this terminal scale, whatever its form, is shed
just as are the coverings of all other scales, the constriction
allowed the shed layer to be left hanging as a dry shell upon the
new layer which pushed it off. This is indicated on the diagram
(Fig. 38). The result is a string of shed castings of the terminal
spinal button, forming in the aggregate the rattle (Fig. 39).
Whenever this is agitated, it makes the familiar half metallic, half
insect-like sound.

Thus, in a few words, a long and slow process may be reca-
pitulated, but why, indeed, has the rattle been produced, or why
do snakes vibrate their tails when they are frightened or dis-
turbed? We know from experience that species living among dry
leaves make a considerable noise in this way, but the rattlesnake,
living in the open grassy plains, would not do so. Surely, the rat-

tle is not needed to warn away the snake’s prey. No such altru-
ism need be considered. The rattler lies in wait, striking the rab-
bit or bird upon which it feeds, then withdraws without rattling,
to wait for the prey to die. At first sight, man seems to be the
snake’s only enemy, but the rattler is, of course, far older than
man on this continent. What native animal, then, had the rat-
tlesnake to fear on a wholesale scale, which coincided in its dis-
tribution with the area where these snakes may conceivably have
been evolved? Ancestors of the wide-ranging bison fulfill these
conditions, and we know that there were earlier bisons, for their
fossil remains have been found. They were probably widespread
also, for the remains occur in Michigan, Kentucky, and many
other States. The hoofs of these ponderous animals, traveling
over the plains, must have been distinctly dangerous to snakes
living in the open, while the snake’s bite would be distinctly
unpleasant to the bison, although death would probably have
rarely ensued. The bison would gladly keep out of the snake’s
way if warned, and this warning the rattle gave.

It has been suggested that the rattle served some purpose
for signaling between the snakes, perhaps a mating call or an
assembly call for group hibernation. This is not a fact. In the first
place, abundant opportunity for observation has never shown it
to serve such a function; and, again, recent experiments have
shown that the rattlesnake’s hearing is confined to a range of
sounds of wave length far different from that of the rattle. Thus
the snake is deaf to the noise of its rattle.

H I S T O R I C A L  P E R S E C T I V E S

fig. 38. TIPS OF THE TAILS OF SNAKES
Upper two, non-rattling vipers; lower four showing rattle during

growth, lowest in cross section.
fig. 39. THE RATTLE OF THE RATTLESNAKE IN LIFE

A Neotropical Rattlesnake (Crotalus durissus) from Querétaro, México
in defensive posture. Photograph by Louis Porras.

Protective Devices of Snakes: Rattlesnakes1

Thomas Barbour

1 Excerpted from Barbour, T. 1926. Reptiles and Amphibians. Their
Habits and Adaptations. Houghton Mifflin Company, Boston and
New York.


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