Emotion and phylogeny
Gentle handling of mammals (rats, mice), and lizards (Iguana), but not of frogs (Rana) and fish (Carassius) elevated the set-point for body temperature, i.e., produced an emotional fever, achieved only behaviourally in lizards. Heart rate, another detector of emotion in mammals, was also accelerated by gentle handling, from ca. 70 b/min to ca. 110 b/min in lizards. This tachycardia faded in about 10 min. The same handling did not significantly modify the frogs’ heart rates. The absence of emotional tachycardia in frogs and its presence in lizards (as well as in mammals), together with the emotional fever exhibited by mammals and reptiles, but not by frogs or fish, would suggest that emotion emerged in the evolutionary lineage between amphibians and reptiles. Such a conclusion would imply that reptiles possess consciousness with its characteristic hedonic dimension, pleasure. The role of sensory pleasure in decision making was therefore verified in iguanas placed in a motivational conflict. To be able to reach a bait (lettuce), the iguanas had to leave a warm refuge, provided with standard food, and had to venture into a cold environment. The results showed that lettuce was not necessary to the iguanas and that they traded off the palatability of the bait against the disadvantage of the cold. Thus, the behaviour of the iguanas was possibly produced, as it is in humans, through the maximization of sensory pleasure. Altogether, these results may indicate that the first elements of mental experience emerged between amphibians and reptiles.
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Document Type: Research Article
Affiliations: Department of Physiology, Universite Laval, Quebec, Canada, G1K 7P4.
Publication date: 1999-06-01