Rat 50 kHz calls reflect graded tickling-induced positive emotion (2020)

Hinchcliffe, J. K., Mendl, M., Robinson, E. S. J.

Abstract

Positive animal emotion (affect) is a key component of good animal welfare and plays an important role in stress-coping and resilience. Methods for reliably inducing and measuring positive affect are critical, but both have been limited in availability. In rats, one promising way of inducing positive affective states is by human-simulated rough and tumble play or ‘tickling'. However, in humans tickling induces both pleasure and displeasure. Rats also show individual differences in response to tickling, and this variation needs to be readily quantified if we are to ensure that tickling is only implemented where it generates positive affect. Here, we use a validated and objective measure of affective valence, the affective bias test, to show that 50 kHz ultrasonic vocalizations provide a quantifiable and graded measure of positive affect that accurately reflects the positive state induced by this human–rat interaction. Overall, our findings demonstrate that 50 kHz USVs provide an easy-to-use, graded, and real-time measure of positive affect in response to a short-term event (tickling). Our findings show that, at a population level, tickling induced a more positive affective state. However, rats varied in how strongly they preferred the tickling-associated substrate, implying that tickling induces stronger positive affect in some individuals than others. Therefore, not all rats like to be tickled and when employing tickling to enhance affect and welfare, care should be taken to identify these animals. Our results support the use of tickling to induce positive affect and welfare in rats, whilst confirming that rats, like humans, vary in how rewarding they find it.