Several years ago Doctor Hamilton reported to me observations which he
had made on preference for the right or left paw in dogs. He has not, I
believe, published an account of his work. Subsequently, Franz observed
a similar preference in monkeys which, according to his report, exhibit
marked tendency to be right-handed, left-handed, or ambidextrous.
My own observations, although they are wholly incidental to my other
work, seem worthy of description at this point. I noted, first of all,
that the orang utan Julius tended to use his left hand. He by no means
limited himself to this, but in difficult situations he almost
invariably reached for food or manipulated objects in connection with
food getting with the left hand. Figures 23 and 24 of plate V, show him
reaching for a banana with the left hand. Likewise, figure 34 exhibits
the use of the left hand in the draw-in experiment.
So marked was Julius's preference for his left hand that I became
interested in observing similar phenomena in the monkeys. Skirrl, when
driving nails, held the hammer with his left hand and the nail with his
right hand. The fact that he never was observed to reverse the use of
the hands is surprising, for other observations indicate that he
preferred the right hand for certain acts.
Stimulated by the obvious left-handedness, in certain connections, of
Julius and Skirrl, I tested the preference of several of the monkeys in
the following simple way. Standing outside the cage I would hold out a
peanut to a hungry animal, keeping it so far from the cage that the
monkey could barely reach it with its fingers. I noted the hand which
was used to grasp the food. Next I varied the procedure by placing the
peanut on a board in order to make sure that I was not definitely
directing the animal's attention.
With Sobke the following results were obtained. In forty trials given on
two different days, he reached for and obtained the food each time with
his left hand. Only by holding the bait well toward the right side of
his body was it possible to induce him to use the right hand. So far as
may be judged from these observations and from others in connection with
the experiments, this animal is definitely left-handed.
With Skirrl the results are strikingly different. As stated above, he
used the hammer consistently with his left hand, but in twenty attempts
to obtain food by reaching, he used his right hand seventeen times and
his left only three times. It was quite as difficult to induce him to
use his left hand for this purpose as it was to induce Sobke to use his
right. We must therefore conclude that Skirrl is right-handed in
connection with certain movements and left-handed in others.
The monkey named Gertie in the reaching experiment consistently used her
left hand, never once using the right.
Jimmie, so far as it was possible to make tests with him, also used his
left hand, but it should be said that the results are unsatisfactory
because he was at the time extremely pugnacious and paid attention to
the experimenter rather than to the food.
Scotty, in the first series of ten trials, used his right hand eight
times, his left twice. In the second series, given the following day, he
used the right hand three times and the left seven times. From this we
should have to infer that he is ambidextrous.
A female _rhesus_ monkey which had been brought to the laboratory only a
few days previously showed a preference for the right hand by the use of
it fourteen times to six.
In connection with these data which are, I should repeat, too scanty to
be of any considerable value, I wish to describe my own experience.
Although naturally left-handed, I am by training right-handed to the
extent of having been able to use my hands in writing and in various
other activities equally well at the age of twelve. I am at present
ambidextrous in that there are many things which I do with equal
readiness and skill with either hand. Delicate, exact, and finely
coordinated movements, such as those of writing and using surgical
instruments, I perform always with my left hand while grosser movements
involving the whole hand or arm, I am rather likely to perform with my
right hand.
It seems not improbable in the light of my own experience that we shall
find some specialization among the lower animals with respect to
preference for right and left hand or arm. I should not be at all
surprised to discover that it is the rule for animals to possess or to
develop readily definite preference for one hand in connection with a
given act of skill and to have quite as definite a preference for the
other hand in connection with a radically different kind of act.