Food concentration-effect functions under a FR 1 schedule of reinforcement in wild-type (WT), heterozygous (HET), and DATki mice. The abscissa shows the percentage of liquid food in water; W designates water. The ordinate shows the food reinforcers earned per hour. Food maintained comparable rates of nose poking in wild-type, heterozygous, and DATki mice across a range of liquid-food dilutions. ∗∗, p < 0.01; ∗∗∗, p < 0.001 versus water, two-sided paired-sample t test. Data are group means; bars represent one S.E.M.