Forced swimming test. The rat or mouse is placed into a cylinder partially filled with water from which escape is difficult. The longer it swims, the more actively it is trying to escape; if it stops swimming, this cessation is interpreted as depressionlike behavior, a kind of animal fatalism.
Sugar water preference. The preference an animal shows for sugar water is taken as an indication of its ability to derive pleasure, a quality that is missing in depression. Most rodents, when given two identical-looking sources of water, will drink much more of the sweetened water than the plain water. Rodents exposed to chronic stress or whose brains have been manipulated show no such preference.(Previously: Scientists create a mouse that's permanently happy.)
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