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Ethology is the biological study of animal behavior. Any behavior can be explained at two levels: proximate causes (how the behavior works, through sensory signals, nerves, hormones and development) and ultimate causes (why it evolved, through its effect on survival and reproduction). Behaviors range from fixed action patterns, rigid sequences triggered by a simple sign stimulus, to forms of learning such as imprinting, habituation and associative learning. Karl von Frisch, Konrad Lorenz and Nikolaas Tinbergen shared the 1973 Nobel Prize in Physiology or Medicine for founding this field.
Pick one behavior, such as a bird singing in spring, and ask four questions about it: what triggers it, how it develops in the individual, how it helps the bird reproduce, and how it evolved. Then sort a set of example explanations into proximate and ultimate. Short videos of stickleback attacks on models or goslings following a person make classic experiments memorable.
Why does a bird sing? A physiologist might answer that longer spring days raise its hormone levels, which act on brain areas that control song. An evolutionary biologist might answer that singing attracts mates and warns rivals away, so birds that sang left more offspring. Both answers are correct. The first is a proximate explanation, about how the behavior works. The second is an ultimate explanation, about why it evolved.
In 1963 Nikolaas Tinbergen proposed four questions for any behavior. Two are proximate: what mechanism causes it, and how does it develop during the animal's life? Two are ultimate: what is its function, or how does it help the animal survive and reproduce, and how did it evolve over the history of the species? Ethology uses all four.
Some behaviors are fixed action patterns. These are sequences that, once started, run to completion, and they are triggered by a sign stimulus. Tinbergen showed that male three-spined sticklebacks attack a crude model with a red belly but ignore a lifelike model without one. Red bellies mark rival males in the breeding season. Tinbergen and Konrad Lorenz also studied greylag geese, which roll an egg back to the nest with the bill. If the egg is removed partway through, the goose often finishes the rolling motion anyway.
Other behaviors depend on learning. Lorenz found that newly hatched goslings follow the first moving object they see, and that object can be Lorenz himself. This is imprinting. It occurs during a sensitive period shortly after hatching, so it blends inheritance and experience. Habituation is a drop in response to a harmless repeated stimulus. In associative learning an animal links one stimulus with another, as in classical conditioning, or links a behavior with its result, as in operant conditioning. Tinbergen showed spatial learning in digger wasps: he placed pinecones around a wasp's nest, then moved them while the wasp was away, and the returning wasp searched where the pinecones now were.
Animals also communicate. Karl von Frisch decoded the waggle dance of honeybees, in which a forager's dance tells nestmates the direction and distance of a food source.
Ultimate explanations rest on natural selection. Behaviors that look selfless, such as alarm calls that draw predators' attention, can spread if they help relatives who share the caller's genes. This idea, kin selection, connects behavior to the genetics of populations. In 1973 von Frisch, Lorenz and Tinbergen shared the Nobel Prize in Physiology or Medicine for their work.
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