Origins of Joy: Ultrasonic Vocalization in Selectively Bred Rats
Mentor:Nancy Dess, Professor of Psychology, Occidental College
Evolutionary psychology suggests an adaptive basis for the emotion ‘joy.’ This pleasurable feeling acts as a reward for performing pro-social behaviors as well as consuming the desired foodstuffs that sustain life. Consistent with the evolutionary tenet of variation, joy is experienced differently between individuals. This study seeks to discern the role of genetics in individual differences by comparing ultrasonic (50 kHz) vocalizations (indicating positive affect) between two lines of rats bred for high (HiS) and low (LoS) saccharin consumption. These two lines differ on anxiety, stress vulnerability, and social status, with LoS animals tending to be more anxious, more susceptible to stress, and submissive. This prior research indicates that LoS individuals express more negative affect. However, whether they also express less positive affect than HiS rats is unknown. In the present study, relative to HiS rats, LoS rats were expected to show lower levels of 50 kHz vocalization, indicative of less positive affect. A series of four experimental phases was conducted: habituation to a novel environment; anticipating and receiving a food reward; anticipating playful handling and being tickled; and injection of 1.25 mg/kg of amphetamine. Results indicate that all rats vocalized more when they should feel joy (anticipating/receiving food, social interaction). Surprisingly, LoS animals vocalized more than HiS when they explored a new environment. Additionally, HiS and LoS rats differed in vocalization quality with respect to pitch. This difference could be a result of the lines producing different call types. More research is needed to determine whether call types distinguish the lines and whether call quantity or quality covaries more strongly with individual differences in which genetics can reasonably be implicated (e.g. the saccharin phenotype).