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Marian Dawkins at Oxford University has pioneered a number of studies that explore what animals prefer and how much they want it.

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To do this she has designed experiments where animals must work for access to different choices.

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Measurements of how hard the animal will work begin to tell us how valuable that resource is to the animal.

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This provides a way of measuring the animal's strength of preference.

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For example, animals can be trained to push against a door to open it to get access to one type of resource or another.

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By putting heavier and heavier weights onto the doors you can find out how hard the animal is prepared to work to reach its choice.

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Similarly you can make the animal do something it prefers not to.

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Chickens do not like to squeeze through narrow spaces, but they will if this gives them access to something they really desire such as a place where they can dust bathe.