Biphasic effects of serotonin on tonic immobility in domestic fowl.

In two experiments, groups of 2 1/2 to 3 week old chickens were injected intravenously with various dosages of serotonin and tested for tonic immobility. Relatively low doses of serotonin produced significant increases in the duration of immobility, while high doses only produced slight decreases in duration. Serotonin also reduced the activity levels of chickens before testing for immobility and made the response easier to induce, but there were no apparent differences due to dosage for either of these behaviors. The effects by serotonin on the immobility response and activity level were found to persist for over two hours with little decrease in their potency. The data in this study are used to explain the contradictory effects produced by serotonins on immobility during past research and are discussed in terms of the importance of differential responding by prey during predatory encounters.