Yerkes Laboratories of Primate Biology, Orange Park, Florida
HE pronounced variability of behavior among a group of chimpanzees
was noted by the earliest students of this primate. R. M. Yerkes
called the chimpanzee ‘‘a rugged individualist,” and no one who knows these
animals has since found reason to question this description.
All animals are of course individually variable, but such differences are
certainly greater among the so-called higher animals; since they are more
complicated, structurally and behaviorally, there are more ways in which
they can differ. Man exceeds all other species in the combination of com-
plexity and polymorphism, and the diversity of human behavior is well known.
Variability among chimpanzees is perhaps less but, as
I shall try to show, is
still very impressive.
To avoid later confusion, let me first dispose of a preliminary question.
The early systematists divided contemporaneous chimpanzees into a number
a conservative four up to as many as fourteen-and
today disagreement persists as to the number of living species. All the criteria
of species differentiation have been of rather superficial characters. Skin color
is one of these; the designations “white-faced” and “black-faced” are in
common use today. But we now know that skin color is a function of at least
three variables: age per se, amount of exposure to sunlight, and heredity. All
chimpanzees are relatively light-faced when born, becoming darker as they
get older; the rate of this change is a function
how much they are exposed
to the sun. Some individuals never become as dark as others, but there is no
evidence for reproductive isolation related to skin color. A receding hair line
of the forehead is another irregular concomitant of advancing age which has
been used as a species-differentiating character.
I am no taxonomist and thus
speak without authority, but for present purposes
I shall assume with Schwarz
that our current population
chimpanzees, and all
apes that have been at the Yerkes Laboratories during the past
belong to the same species, Our discussion will therefore concern individual
rather than species differences.
the rare pygmy chimpanzee
from south of the Congo, shows many marked deviations from all others and
probably is a separate species. But since the behavior of
has not been
studied extensively, we cannot use it in a consideration of individuality.
Chimpanzees vary widely in respect to physical characteristics. Adult
weights range from about
pounds; males average some
heavier than females, but there is considerable overlap. In general, sexual
dimorphism is less pronounced in chimpanzee than in man. Body sizes and
proportions vary considerably. One quickly learns to identify our
by their faces; some have an Oriental “slant” to the eyes, others look like a
It is a favorite game at the Laboratories to