The effects of cage color and light intensity on rat affect during heterospecific play

•Rats were raised in high or low light intensity (200 vs 25 lx) & red or clear cages.•Rat affect was assessed with heterospecific rough-and-tumble play or “tickling”.•At high light intensity (200 lx), rats produced more positive calls in red cages.•At low light intensity (25 lx), rats produced more positive calls in clear cages.•Results suggest lighting and spectral environment impact rat affect. Environmental enrichment provides physiological and emotional benefits to laboratory rodents. Red tinted shelters are a common enrichment found in laboratories that provide rodents with a hiding space shielded from bright light. Red tinting alters the light’s spectral make-up which reduces the amount of blue-green wavelengths while also reducing overall light intensity. However, it is unknown if red tinting has behavioral affects. Heterospecific play, or “rat tickling,” is a technique that mimics aspects of rat rough-and-tumble play and elicits 50-kHz ultrasonic vocalizations (USVs). These USVs are indicative of positive affect and reward in rats, which could be used to evaluate the impact of environmental enrichment. Our objective was to determine the effect of housing rats in different cage colors (red or clear) and light intensities (25 or 200 lx) on USV production during tickling. Long Evans and CD rats (N = 48) were born, raised, and tickled in a randomly assigned caging treatment: red 200 lx, red 25 lx, clear 200 lx, or clear 25 lx. Rats were co-housed at weaning into same sex pairs (1 Long Evans, 1 CD). Rats were tickled for seven days and then once weekly for 10 weeks total. Recordings of USVs were made during each session and data were analyzed using general linear models. Our results showed an interaction effect between cage color and light intensity. For our primary outcome of 50-kHz USVs during weekly tickling sessions, rats produced more 50-kHz USVs in red 200 lx cages than in either clear 200 lx cages or red 25 lx cages (p’s < 0.0001). Additionally, rats produced more 50-kHz USVs in clear 25 lx cages than either clear 200 lx cages or red 25 lx cages (p’s < 0.0001). In conclusion, our results indicate that the environment rats are raised, housed, and tickled in – specifically light intensity and spectral make-up - as influenced by cage color – significantly impacts rat positive affect. This conclusion is based on increased 50-kHz USV production during tickling in red 200 lx cage and clear 25 lx caging. It appears that at high light intensity, red cagi