Encyclopedia of Evolution.pdf - Online Reading Center

down the dark side of the stem, causing the cells on that side
to grow more. Animals respond to low blood sugar by feeling
hungry, which is a hormonal response. Animals respond to
hot temperatures by increasing blood circulation to the skin,
thereby allowing body heat to diffuse into the air. This is an
involuntary nervous response in which the muscles around
the animal’s arteries relax. None of these actions are behavior.
If the animal searches for food in response to hunger,
or stretches its limbs to expose more surface to the air, thus
allowing more heat loss, these actions would be considered
behavior.
Animals raised or kept in captivity often display unusual
actions that do not reflect their natural behavior. The study of
animal behavior under natural conditions is called ethology.
The study of behavior can occur at many different levels.
This is illustrated by the question, “Why does a bird sing?”
Several different answers could be given to this question.
First, a scientist could investigate the short-term causes of
the singing of a particular bird. What is it about the environment,
the genes, or the previous experience of this bird that
makes it sing at the time, at the place, and in the manner that
it does? These causes are called proximate causes, because
they are near or proximate to the organism. Second, the scientist
could investigate the evolutionary origin of the genes
that make or allow the bird to sing. Since these causes reach
far back in time, they are called ultimate causes.
Proximate Causes of Behavior
Proximate causes of animal behavior include stimuli, genes,
and learning.
1. Stimuli. A bird sings in response to stimuli (singular
stimulus) from its environment. The warmer temperatures
and increasing day length of spring act as external stimuli to
mating behavior in birds. An internal biological clock acts
as an internal stimulus to the bird’s activities. The external
stimuli (in this case, day length) continually reset the internal
stimulus (the biological clock).
Many stimuli come from the nonliving environment.
Sowbugs, for example, respond to the presence of bright light
by crawling toward the shade. Other stimuli come from animals
that did not intend to produce the stimulus; sharks, for
example, swim toward blood. Many stimuli are deliberately
produced as communication signals by other animals. Dominant
animals in populations use threat displays to subdue
other members of the population. Some animals use startle displays
or mimicry to avoid predators or to catch prey. Mating
behavior in many mammals occurs in response to the timing
of the female’s period of reproductive readiness (see reproductive
systems). For the female, it is an internal stimulus;
for the male, an external one; for both, it causes physiological
changes and behavior that precede or accompany copulation.
A bird can respond to environmental stimuli only
because it has sensory structures that intercept, and a nervous
system that can interpret, the stimulus. The sensory structures
and the nervous system are the products of gene expression.
In this way, all animal behavior has a genetic basis.
2. Genes. A bird sings because singing is a behavior
encoded in its genes and is therefore instinctive. Instinct
behavior, evolution of
allows an animal to perform behaviors that it has never
seen. Instinctive behavior patterns have a genetic basis, in
two ways. First, instinctive behavior patterns may be specifically
coded into the animal’s brain, the structure of which
is encoded in the animal’s DNA. The genetic basis of many
animal behavior patterns, including a number of human
behavioral disorders, has been determined by the study of
inheritance patterns. Second, instinctive behavior patterns
may be controlled by hormones. Genes regulate and are regulated
by hormones, and hormones can influence behavior.
During the winter, the days are short; this is the environmental
stimulus. In response to the short days, the pineal glands
of birds produce high levels of the hormone melatonin. High
levels of blood melatonin inhibit birds from singing. When
the days become longer in the spring, the pineal gland produces
less melatonin, and the birds sing.
Once they have been stimulated, instinctive behaviors
continue to completion. For this reason, instinctive behaviors
are called fixed action patterns. Fixed action patterns may
be very complex but are still performed without thinking.
Examples include the egg-laying behavior of the female digger
wasp, the foraging behavior of honeybees, and the parental
behavior of female blue-footed boobies.
Consider the example of the digger wasp. The female
digger wasp stings her prey (usually another insect) enough
to paralyze but not to kill it. Then she brings the prey to
the mouth of a burrow that she has previously dug in the
ground. After dragging the prey into the burrow, the wasp
lays eggs on it. When the eggs hatch the larvae eat fresh
food, as the prey is still alive. Before dragging the prey into
the burrow, the wasp enters the burrow to inspect it. In one
case, a scientist was watching; while the wasp was inside
the burrow, the scientist moved the prey to a nearby location.
When the wasp emerged, she noticed that the prey
was missing and quickly located it. She dragged the prey
back to the mouth of the burrow. Only a few seconds had
passed since she last inspected the burrow. It would seem to
a human observer that the wasp should now drag the prey
into the burrow and finish laying her eggs. However, the
wasp dutifully left the prey outside the burrow and went
inside for another inspection. The scientist again moved the
prey. When the wasp emerged again, she relocated the prey,
dragged it back, then inspected the burrow. The scientist
repeated this activity many times.
The digger wasp’s behavior involved both elements of
response to stimulus and of instinct, but not of reasoning. The
wasp responded to environmental stimuli in order to locate
the prey and the burrow. The timing of the wasp’s reproductive
activity, and of all of the stages of its life, occurred at
the appropriate season of the year because of responses to
environmental stimuli such as temperature and day length.
The sequence of events that occurred during egg-laying was
instinctive, similar to the operation of a machine. When the
scientist moved the prey, the wasp’s egg-laying program
was reset to an earlier stage. The wasp was like a washing
machine that had been set back to an earlier stage in its program;
the washing machine does not know that it has already
washed the same load of clothes before.