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IGUANA  •  VOLUME 14, NUMBER 2  •  JUNE 2007 115HISTORICAL PERSPECTIVES

For some years I have been teaching that the tail appendage
of the rattlesnake was not to be explained on the doctrine of

natural selection, inasmuch as it could contribute in no way to
the advantage of the animal. It seemed to me quite clear that it
was rather calculated to hinder rather than to help the creature
in the race of life by warning its prey of its presence. Nor did it
seem easy to account for its existence by supposing that it was
used as a sexual call and had been brought up by natural or sex-
ual selection for some such office. The burrowing habits of the
serpents would seem to make sexual calls almost unnecessary
and there is no evidence to make a reasonable basis for belief that
rattlesnakes exercise any such choice in pairing as would lead to
the development of this very singular appendage. Last summer,
however, I had a long desired opportunity of examining a little
into the habits of the rattlesnake and obtained some results
which have served to shake my confidence in the opinions I had
held as to the usefulness of his rattle. The observations are, as it
will be seen, rather insufficient for a determination of the points
in question, but it may be long before I am able to add to them,
so I give them now hoping that some one with better opportu-
nities for studying the ways of this interesting creature may
either confirm my opinion or refute it.

The first and only living and active rattlesnake which I met
on a carriage journey of some months’ duration made during
the past summer through that part of the Appalachian chain
where these serpents most abound was in the middle of a road
near the Kishicoquillas Valley, Pennsylvania. As the sound of my
carriage disturbed the surly fellow in his pleasant basking place
in the dusty way, he begun to sound his warning when we were
over a hundred feet from him. Although quite accustomed to
the sound, having had specimens captive for months at a time,

I mistook it for that made by our “locust,” the Cicada rimosa
Say, nor did I perceive the error until my companion, Mr. A. R.
Crandall, called my attention to the serpent when we were
within forty feet of it. My wife and child, a little girl of eight
years, who were in the carriage also mistook the noise for that
made by the Cicada, which was abundantly familiar as it had
been for a long time the most accustomed sound heard while we
were traveling through the wooded mountain country.

I have found that the note of the rattlesnake is recognized
by many persons as indistinguishable from the sound made by
the Cicada. Professor Brewer, whose long experience in the serv-
ice of the California Geological Survey gave him quite unrivalled
opportunities for becoming familiar with the sound made by
this reptile, tells me that he was on one occasion at least in great
danger of being bitten by one of these animals on account of
having supposed that its persistent rattle was only the whirring
of a locust. The range in pitch in the rattling sound of the snake
is quite great; it is even difficult to understand how sharp it can
be from a study of the sound made by the animals tamed by
captivity, but at the same time the note of the locust is also very
variable so that one is not able to discriminate by this means.
The reader will doubtless have caught the main point towards
which these facts so plainly tend, namely, that the imitation of
the note of the locust may possibly be of high value to the rat-
tlesnake. The Cicada furnishes the most satisfactory mouthful
to many of our birds. Almost every observer of the life of our

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

The Rattlesnake and Natural Selection1

Prof. N. S. Shaler

1 Reprinted from The American Naturalist 6:32–37 (January 1872).

Prof. N. S. Shaler suggested in 1872 that the purpose of the rattle was
to attract prey, specifically birds searching for cicadas, the call of which
was purportedly mimicked by the buzz of the rattle. His encounters
were with Timber Rattlesnakes (Crotalus horridus), which are widely
distributed in the eastern United States (although extirpated by
humans from many areas where they once were abundant). 

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The interlocking segments of the rattle generate a “buzz” when shaken
rapidly, such as by this Timber Rattlesnake (Crotalus horridus).
Although superficially similar to the call of a cicada, the likelihood that
rattlesnakes mimic an insect to attract avian prey is very slim, and
belied by the fact that rattlesnakes feed almost exclusively on mammals.

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woods and fields, has seen a bird spying around a branch
whence came the whirring of a locust; suddenly there would be
an end of the monotonous sound, and a moment after the bird
would be seen, beating the wings off the insect by pounding it
against the ground. It is quite evident that any animal which
preyed upon birds would gain a decided advantage from being
able to imitate the sound made by the creatures on which the
birds fed, so that we can well imagine that those snakes endowed
with the power of attracting birds would be the best fed and
multiply the most rapidly. From this point of view we can also
understand how it is that birds have been seen fluttering around
a rattlesnake without calling into play the unreasonable agency
of fascination; the bird in this case being in search of his food
and decoyed into the range of the serpent’s spring.2 It is a well
known fact that birds, even those which have the best deter-
mined notes, are easily misled by sounds which approach, even
though imperfectly, the calls of their species or the sounds of
their prey, so that the imperfections in the note of a rattlesnake
when considered as an imitation of the Cicada cannot count
much as an argument against this view.

If this view be correct, then we must regard the rattle of the
rattlesnake as a useful appendage, and not an instrument calcu-
lated to do it injury by warning its prey of its presence.3 But it
is by no means so easy, even if we allow all that can be claimed
for natural selection, to account for the development of this
appendage. The following seems to me the most satisfactory
conception of its evolution, looking at the matter from Mr.
Darwin’s point of view. It is a fact well known, doubtless, to
those who have observed serpents, though I find no mention of
it in the works I have consulted, that many serpents, when in a
state of excitement vibrate the end of their tail just as the rat-
tlesnake does.4 This movement is likely enough the same in
character as that which occurs in the hinder part of the spinal
column among higher animals under excitement. The wagging of the dog’s tail is a rhythmical movement of essentially the same

character as the movement of the rattlesnake. Taking the same
line of argument as that adopted by Mr. Darwin with regard to
the monthly phenomena observable among the mammalia, it
could be claimed that the tendency to move the tail was expli-
cable on the following grounds. During more than half the life-
time of the group of vertebrates, from the point of their pre-
sumed origin at the close of the Silurian down to the present day,
the caudal portion of the body was used as a propelling agent.
Fishes, with slight exceptions, propel themselves with a recipro-
cating movement of the tail. All conditions of excitement at
once manifest themselves in the violent movement of this part
of the body. Whether in flight or chase or under the influence
of sexual excitement, this movement is the important element
of success. It is by no means surprising that the motion which
was for ages the point which natural selection operated most
intensely, for those forms which had the capacity for making this
alternate movement of the tail with the greatest rapidity would
be most successful in flight or chase, should have survived its
usefulness and remained as a mere feature of expression in most
of our animals. It may be remarked in passing that the obstinate
survival of the tail among the vertebrates may be accounted for
by the intense bodily inertia, if we may so call it, which causes
the energy of survival of useless structures to be proportionate
to the length of time which they have been of use to the groups

116 IGUANA  •  VOLUME 14, NUMBER 2  •  JUNE 2007 SHALER

The anatomy of a rattlesnake rattle: A. A complete rattle showing the
terminal “button.” B. A broken string of rattles showing a terminal rat-
tle (but not the original button). C. & D. Vertical and horizontal cross
sections through interlocking rattles. Adapted from Klauber (1972.
Rattlesnakes: Their Habits, Life Histories, and Influence on Mankind. 2nd

ed. 2 vols. University of California Press, Berkeley and Los Angeles).

2 I had an opportunity, recently, of seeing a living Cobra de Capello
in a state of excitement. The first impression was how entirely
unlike any other serpent it was. The broad, flat banner-shaped neck
upon the slender staff of the body would readily lead one to sup-
pose that it was something very different from a snake. I can read-
ily imagine that animals which had learned, by selection or other-
wise, to shun creatures shaped like serpents, would be easily misled
by the strange shape of the cobra and fail to avoid it or even be
attracted towards it by the curiosity which animals often show to
see closely or even smell any strange object.

3 Rattlesnakes of the genus Crotalophorus [= Sistrurus] make little or
no noise with the imperfect rattles of the tail. In this genus, there-
fore, there could have been no advantage derived from imitation.
It may be said, however, that the rattles which have little functional
value in this genus, if making a sound be their real function, are
even more irregular in number and shape than in the Crotalus. The
truth probably is that there is an inherent tendency to form rattles
in this groups of serpents and these structures are seized upon by
natural selection and made functional.

4 Since the above matter was put in type, I have learned that Prof.
Jeffries Wyman made a communication to the Boston Society of
Natural History, concerning the occurrence of this movement in the
tails of snakes other than the rattlesnake, some two or three years
ago. I have failed to find any notice of the communication in the
Proceedings of the Society, though there can be no doubt that this
eminent naturalist should be credited with the priority on this point.



IGUANA  •  VOLUME 14, NUMBER 2  •  JUNE 2007 117HISTORICAL PERSPECTIVES

to which they belong. It is natural enough that a part of the
body situated at one of the regions of manifold relations as the
tail is, and unappropriated to any special function, should be put
to use in various ways, as a prehensile instrument by some mon-
keys and other animals, or a building tool by the beavers, as a
fly-brush by many others, etc.

Mr. Herbert Spencer has already suggested that the wag-
ging of the dog’s tail and similar movements of that appendage
is in fact an escape of nervous force restrained from other modes
of expression at the moment. Looking at the matter from this
point of view, which is doubtless quite satisfactory, we may rec-
oncile it perfectly with the views which have just been presented
by supposing that the ancient and no longer functional channel
of escape for nervous force, the tail, has remained the way of
outlet for the suppressed energy of the animal. The older the
channel the less easy it is to close it either by volition or by nat-
ural selection.

Be the cause of the persistence of the tail and its movement
what it may, we are still justified in assuming as the starting
point, that the progenitor of the rattlesnake had the alternating
motion of the tail common among snakes. It is the opinion of
some herpetologists that the rattles are the remains of the skins
successively shed by the animal. The rate of development of the
rattles, together with the fact that the skins of some serpents are
more imperfectly detached from the region about the tail than

at other parts of the body, makes this view very probable. Let us
suppose that we had a group of poison-fanged serpents, acciden-
tally tending to keep the tail skin in the peculiar fashion of rat-
tlesnakes and that in some of these it was persistent enough to
make the whirring sound of the Cicada when the tail was rap-
idly moved under excitement. These would survive and breed
the most surely and so that feature would become hereditary.
The great variability in the number of rattles in the different
forms of rattlesnakes and the late time of their development,
even among those which differ in no other regard, would seem
to indicate that this structure has not yet been firmly fixed by
long inheritance.

The reader will please not suppose that because I have
boldly followed the lead of the most advanced of the champions
of natural selection that I am convinced of its sufficiency as an
explanation of the great diversities which exist among animals
or of its being sufficient basis for an explanation of the snake’s
rattle. But having been driven step by step from a decided oppo-
sition of the whole theory and compelled to accept it as a vera
causa, though I think one much more limited in its action
among animals than Mr. Darwin believes, I feel it to be my duty
to examine every one of those points upon which I have relied
for evidence against it.

It must be confessed that the case of the rattlesnake seems to
me no longer the bar to the acceptance of the theory it once did.

It seems that the singular structure from which the subject of
these notes derives its name, was intended as a special stum-

bling block in the path of antidarwinists, or to intensify the
“struggle for existence” which the Darwinian theory, like all
other theories must undergo.

In most notices I have seen of the rattles of the rattlesnake,
they have been mentioned as though they were of no advantage
to the possessor, and that natural selection would never produce
them but on the contrary would weed them out, if that theory
were correct. It seems to me that the whole trouble in the matter
arises from the assumption that the sound of the rattles, as a war-
cry, is a disadvantage to the reptile, by calling the attention of its
enemies to it and thus inviting its own destruction, and that con-
sequently the only way to reconcile the existence of the rattles with
the theory of Darwin, is to show that there is some other use made
of them and that in striking the balance between the profit and
loss sides of the ledger, the line falls on the side of the former and
for that reason natural selection produced and retains the rattles.
If I understand him rightly, this is the view of the matter taken by
Prof. N. S. Shaler in his paper in the January NATURALIST. He says
that for some years he has “been teaching that the tail appendage
of the rattlesnake was not to be explained upon the theory of nat-
ural selection, inasmuch as it could contribute in no way to the
advantage of the animal; that is seemed to him quite clear that it
was rather calculated to hinder than to help the creature in the race
of life by warning its prey of its presence.” But he intimates that
he is now ready to say, that this appendage can be explained upon

Use of the Rattles of the Rattlesnake5

J. G. Henderson

5 Reprinted from The American Naturalist 6:260–263 (May 1872).

J. G. Henderson, also in 1872, disputed Shaler’s assumption that rat-
tles were used to attract prey and proposed instead that they serve as a
warning directed at potential predators. Many of his observations were
of Western Massasaugas (Sisturus catenatus tergeminus), at that time
placed in the genus Crotalophorus. 

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