Lick-trading by rats: on the substitutability of dry, water, and saccharin tubes.

Scientists put thirsty rats in small boxes with three levers. Each lever had a different tube: plain water, sweet saccharin, or a dry tube with no liquid.

The rats worked on fixed-ratio schedules. They had to press a set number of times to earn a lick. The team watched which tube the rats chose.

Rats traded licks in a tidy pattern. When water cost more presses, they licked saccharin or dry tubes instead.

Two water tubes acted like perfect substitutes. The animals kept the same total licks no matter which water spout they used.

Dougherty et al. (1994) later showed that switching from water to sucrose changed only one number in the response-strength equation. The new study keeps the same rat model and agrees: reinforcer type shifts choice, not top speed.

Bradshaw et al. (1978) found that weaker sucrose moved the whole response curve to the right. Rast et al. (1985) adds a new angle: animals can swap between different liquids to keep total intake steady.

Michael (1974) saw rats repeat the lever that had just paid off, even when payouts were random. The lick-trading paper shows the opposite side of the coin: rats leave a lever when the cost rises, proving they track both price and payoff.

The data remind us that reinforcers are interchangeable goods. If you raise the response cost for one item, the client may switch to another reinforcer you did not plan for. Check what else is available in the room before you thin a schedule. Use this substitution rule to your advantage: offer a cheaper, acceptable backup reinforcer when you need to stretch the good stuff.