Chapter 2
Innate Behavior Patterns
Let us begin with a general statement such as the visitor
from another planet would make if he were interested in the nature of life
of our own. Every living creature, whether we call it plant or animal,
is characterized by a very particular physical shape which is always transmitted
by the individual to its descendants. There is no recorded instance of
a grasshopper begetting a mouse, nor has a sycamore seed ever produced
a violet. This sounds quite obvious, and it seems still more obvious that
no violet will spring from the seed of a grasshopper, nor a mouse from
that of a sycamore tree.
However, the position is not quite as straightforward
as it appears. Animals and plants always spring from a single cell, and
these germ cells possess the same essential structure. In many cases it
is far from easy to tell at first glance, even under the microscope, whether
a particular germ cell will develop into a plant or an animal.
The germ cell divides, and the resultant new cells separate
– which is how unicellular organisms multiply. Alternatively, division
proceeds as before, but the cells adhere to one another and differentiate
themselves in various ways by forming tissue and organs – which is the
origin of large and very dissimilar bodies such as those we call violet
or earthworm, sycamore or mouse. These multicellular organisms consist
of millions – indeed, billions – of individual cells, all of which issued
from single cells of fundamentally the same type.
(original book page 23)
Somewhere in the germ cells, therefore, reside very dissimilar
controlling "formulas" which impose a quite specific mode of behavior upon
the emergent daughter cells – though in this instance we speak not of behavior,
which it is in the strictest sense, but development. The questions of what
these developmental formulas actually look like, where they are located,
and how they operate have largely been solved. Thanks to one of the present
century's major feats of biological research, we know that these special
formulas are carried by the chromosomes which reside in the cell nuclei.
As we now know, these consist of minute threads arranged like a spiral
staircase, on which, as in a pearl necklace, are distributed various groups
of atoms (radicals). These may be likened to letters which spell out the
commands of heredity. Their sequence determines how individual cells behave
during development; in other words, whether they become a mouse or a sycamore.
We apply the word "behavior" in its true sense to something
else, namely, the movement of an entire organism in space. Such behavior
is seen principally in animals, but sometimes it occurs in plants as well,
though with plants movements are so slow that they can be seen with clarity
only in accelerated films. For instance, many flowers open during the day
and close when evening comes. Many leaves turn to face the light. The tendrils
of climbing plants describe spiral movements until they find a purchase
and anchor themselves. The leaves of some carnivorous plants snap shut,
in trap fashion. The best-known reactive movements are those of the mimosa,
which can be clearly seen with the naked eye. All these active movements
are caused by growth processes or by fluctuations in sap pressure within
the cells (turgor). There is no doubt that the regulation of such movements
in the plants in question is as much determined by heredity as is the conformation
of their various organs.
Many behavior patterns are likewise anchored in heredity
in the case of animals. The hereditary formula builds up those structures
which effect control of such movements. A duckling, for instance, has a
whole repertory of actions available and ready for use as soon as it leaves
the egg. It can already walk and swim excellently, it already dabbles in
mud with its beak and cleans its plumage in a characteristic manner. Each
of these
(original book page 24)
motor sequences entails hundreds – indeed, thousands –
of separate but coordinated commands which must travel by way of various
nerves to the numerous muscles which carry out the movements. Viewed as
a whole, the duckling's walk or preening movements appear extremely simple.
However, the true structural complexity of even such simple actions can
be gauged by observing a child as it laboriously learns the noninnate motor
sequence involved in eating with a spoon. Protracted experimentation is
required before the child manages to coordinate the muscular movements
of hand and arm in such a way that the spoon picks up food and conveys
it neatly to the mouth. Were it possible to draw a circuit diagram of what
underlies this procedure in the central nervous system, we should no doubt
be dumbfounded by its intricacy.
The butterfly is innately endowed with the far more difficult
art of flying. It is capable of taking the air as soon after it emerges
from the chrysalis as its wings have hardened. The young human has an innate
ability to find its mother's breast and suck. The tiny little slipper animalcule,
which lives in drops of water, is innately capable of propelling itself
forward with strokes from more than a thousand cilia, or minute hairlike
appendages, and of retreating when it meets an obstacle. When these unicellular
organisms divide, the new individuals thus formed can swim quite normally
– their cilia strokes are already coordinated properly. Each of these cases
poses the same question: Where are the formulas for these innate movements
located, and from what part of the body are they controlled?
Little is known about this in the case of unicellular
organisms. In that of multicellular creatures we know that the nerve cells
are the carriers of these controlling formulas. But we do not know what
these structures look like. They may depend on a "wiring" of individual
cells similar to that in an electrical circuit, or they may be molecular
differentiations resembling the developmental formulas in chromosomes.
At all events, they are functional units which may fairly be compared with
organs. Like organs, they have a very definite function to fulfill within
the body's work-sharing system.
These structures, which thus prescribe a quite distinct
co-
(original book page 25)
ordination of muscular movements, mature in exactly the
same way as organs. In other words, they become complete and ready to function
on a particular day. This can be deduced from the fact that they sometimes
fail to reach maturity at the same time as the organs whose movements they
control. The cricket, for example, does not begin its characteristic chirping
until some days after the formation of its "musical instruments" because
it obviously lacks the requisite "score." Grasshopper larvae, by contrast,
describe typical "music-making" motions with their hind legs at an early
stage but fail to produce any sound because in this case their "instruments"
are not yet fully developed. It has even been possible to prove, in the
case of worms, crickets, bees, and fish, that formulas for the control
of movement conform to the Mendelian laws of heredity. If parents which
differ in their innate movements – the student of behavior calls them hereditary
coordinations – are crossed, all their offspring display either the behavior
of one parent or mixed behavior, whereas in the second generation the specific
motor characteristics of both grandparents recur.
Many biologists refer to such nerve structures as mechanisms
– a word easily misinterpreted by the layman. Far from being intended as
a comparison with the machine, the term was selected because these structures
function in a regular and predictable manner and thus conform perfectly
to the laws of physics and chemistry. What, then, do these mechanisms of
innate motor control actually achieve? Above all, what aspects of the behavior
of various animal species are actually rooted in heredity in the form of
hereditary coordinations?
Numerous students of behavior have devoted exhaustive
experiments to this question.1)
Controversy with American psychologists was particularly instrumental in
stimulating research on this subject. Its special difficulty lies in the
fact that many movements which are innate or genuine hereditary coordinations
in themselves cannot be performed at birth because the control structure
has yet to mature. This may create the impression that a creature has acquired
a particular motor pattern
(original book page 26)
by learning, whereas careful experimentation shows that
its behavior is probably innate but took time to mature. Grohmann, for
example, reared doves, some normally and the rest in cages too cramped
to allow them to move their wings. As soon as the normally reared birds
could fly well, he released the others. It turned out that the latter could
fly with equal ease. This clearly showed that flying, an extremely difficult
form of locomotion, does not have to be acquired by these birds and is
at their disposal complete, like their organs. Their control structure
matures somewhat later, however. The American researchers Carmichael and
Fromme carried out a similar experiment with tadpoles. They reared one
group normally, the rest under permanent anesthesia so that they did not
move and therefore could not learn. When the anesthetic was discontinued,
the drugged tadpoles proved to be able to swim almost as well as the others.
It has now been demonstrated that many animal movements
are rooted in the complete genetic apparatus (genome). Innate in the garden
spider is the spinning of its magnificent web, in honey bees the intricate
"tail dance" with which they communicate, in the collared turtledove the
method of feeding its young, in the common whitethroat its twenty-five
specific calls, in the rat its copulatory motions, in the duck its exceptionally
complicated mating movements, and so on. Experiments in isolated rearing
cannot, of course, be carried out with human beings, but corresponding
indications exist even here. Eibl verified the occurrence of normal smiling
in a congenitally deafmute child, although it could certainly not have
acquired the habit. We shall deal with the problem of human facial expressions
in due course.
The aptness of the term "mechanism" for these innate
structures of motor control becomes apparent when we take a closer look
at their performances in matching experiments. Fabre and many other students
of animal behavior were struck by how little the so-called instinctive
actions had to do with intelligence and how little associated they were
with a sense of purpose. The ease with which they can be simulated is often
deceptive.
The digger wasp, for instance, seems to display highly
intel-
(original book page 27)
ligent brood-tending behavior. Having dug a nest, it flies
off in search of a caterpillar, overpowers and kills it, drags it into
the nest, and lays eggs on it. The emerging young are thereby provided
with the nourishment they need and find protection in the nest, which the
wasp seals. Interrupt the sequence of partactions, however, and it soon
becomes clear that no form of intelligence is at work here. Returning to
its hole with the caterpillar, the wasp first deposits it in the entrance
and inspects the interior, then reappears at the entrance, head foremost,
and drags its quarry inside. If, while the wasp is inspecting its hole,
the caterpillar is removed and deposited some distance away, the wasp will
continue to search until it has rediscovered the caterpillar and then will
drag it to the entrance again, whereupon the whole cycle-depositing, inspecting,
etc. –
begins all over again. Take away the caterpillar ten
or twenty times, and the wasp will still deposit it at the entrance and
embark on a tour of the hole, with which it is thoroughly familiar by this
time. The insect continues to be guided by the same commands, in computer
fashion, and evidently finds it hard to make any change in the overall
sequence. Only after thirty or forty repetitions will the wasp finally
drag the caterpillar into its nest without further inspection. Yet the
digger wasp shows a great aptitude for learning where other procedures
are concerned. While in flight, it memorizes the route which it must take
on the ground when returning to the nest with its prey – a very considerable
feat of learning. On the other hand, the burial of its prey is an instinctive
action and, thus, strongly programmed. The wasp is almost incapable of
influencing or altering this part of its behavior by learning, because
it is controlled by an innate and extremely incorrigible mechanism.
Once stimulated, whole cycles of action can proceed by
themselves. In the squirrel, food storing consists of the following part-actions:
scraping away soil, depositing the nut, tamping it down with the muzzle,
covering it over, and pressing down the soil. A squirrel reared indoors
will still perform these actions in full, even in the absence of soil.
It carries the nut into a corner, where it starts to dig, deposits the
nut in the (nonexistent) hole, rams it home with its muzzle (even though
it merely rolls away in the process), covers up the imaginary hole,
(original book page 28)
and presses down the nonexistent soil. And the squirrel
still does all these things even when scrupulous care has been taken to
ensure that it has never set eyes on a nut before or been given an opportunity
to dig or conceal objects.
Such observations lead directly to another question.
What is it that triggers off hereditary coordinations in the individual,
and what special stimuli activate the mechanisms that control them?
Simple reflexes are known in almost all multicellular
creatures. The organism responds to a very specific stimulus in a very
specific way. Nip a decapitated frog's toe, and it will retract its leg.
The reflex travels via the spinal cord, so the brain is not essential to
this performance. Sensory nerve fibers are linked with the motor variety
in such a way that a specific stimulus elicits a specific muscular movement.
In human beings the pupil of the eye is controlled by a similar reflex:
Dim light causes our optical aperture to expand, strong light causes contraction.
It was naturally assumed that hereditary coordinations, too, are attributable
to such reflexes – that they comprise a whole system of such reflexes.
Here, however, the question becomes far less straightforward, and this
brings us to a most significant discovery from the standpoint of behavioral
research.
The physiologist von Holst carried out the following
experiment: He severed an eel's head from its spinal cord – the center
from which its sinuous swimming motions are controlled – and also cut all
remaining (sensory) nerves leading to the spinal cord. This meant that
the spinal cord could receive no more extraneous sensory messages and was
completely cut off from the outside world. Von Holst had left the (motor)
nerves leading to the muscles intact, so that the spinal cord could still
issue commands to the muscles. The results were surprising. As soon as
the eel recovered from postoperative shock, it performed wellcoordinated
wriggling motions, an activity which continued without interruption until
the creature's death.
This discovery was extremely interesting from two points
of view. In the first place it refuted the view of the Russian biologist
Ivan P. Pavlov and his school that all instinctive actions are attributable
to chains of reflexes. According to this theory, which was then supported
by the majority of biologists, the
(original book page 29)
movement of one segment of muscle should have activated
the next in sequence by means of internal sensory impulses. The regularity
of the eel's swimming had hitherto been construed as a chain reflex of
this kind. However, von Holst's preparatory work had precluded the possibility
of return messages to the spinal cord, so the segments could not influence
one another. Second, and almost more important, his experiment showed that
this series of movements, which was obviously controlled from one point,
did not have to be activated by an external stimulus. On the contrary,
the motor cells which governed the wriggling motion were spontaneously
active.
Further experiments have since confirmed this crucial
realization. The nerve cells which operate the hereditary coordinations
are in a state of ceaseless activity and "fire off" their coordinatetd
commands constantly. Normally, however, they are inhibited. Another nerve
structure – another mechanism – blocks the movement and prevents nerve
impulses from reaching the executive muscles except at very specific, biologically
appropriate points in time. Ethologists, or students of behavior, call
this inhibitory nerve structure (which no one has yet seen and whose existence
can only be inferred) the innate releasing mechanism, or IRM for short.
It is this mechanism which determines when the incessant stream of commands
may reach the corresponding executive organs.
From this there arise the further questions: When does
the IRM issue its releasing commands, and to what stimuli does it respond?
1)Not
all authorities are mentioned by name in the text. Individual publications
may be identified by referring to the source key on p. 223 and checked
in the Bibliography.