Indications that behavior has a
biological basis
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Behavior often is species specific. A chickadee, for example, carries one
sunflower seed at a time from a feeder to a nearby branch, secures the
seed to the branch between its feet, pecks it open, eats the contents,
and repeats the process. Finches, in contrast, stay at the feeder for long
periods, opening large numbers of seeds with their thick beaks. Some mating
behaviors also are species specfic. Prairie chickens, native to the upper
Midwest, conduct an elaborate mating ritual, a sort of line dance for birds,
with spread wings and synchronized group movements. Some behaviors are
so characteristic that biologists use them to help differentiate between
closely related species.
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Behaviors often breed true, that is, we can reproduce them in successive
generations of organisms. Consider the instinctive retrieval behavior of
a yellow Labrador or the herding posture of a border collie.
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Behaviors change in response to alterations in biological structures or
processes. For example, a brain injury can turn a polite, mild-mannered
person into a foul-mouthed, aggressive boor, and we routinely modify the
behavioral manifestations of mental illnesses with drugs that alter brain
chemistry. More recently, geneticists have created or extinguished specific
mouse behaviors—ranging from nurturing of pups to continuous circling in
a strain called "twirler" by inserting or disabling specific genes.
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In humans, some behaviors run in families. For example, there is a clear
familial aggregation of mental illness.
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Behavior has an evolutionary history, as demonstrated by the persistence
of some behaviors across related species. Chimpanzees are our closest relatives,
separated from us by a mere two-percent difference in DNA sequence. We
and they share behaviors that are characteristic of highly social primates,
including nurturing, cooperation, altruism, and even some facial expressions.
Genes are evolutionary glue, binding all of life in a single history that
dates back some 3.5 billion years. Conserved behaviors are part of that
history, which is written in the language of nature's universal information
molecule—DNA.
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