Figure 1. Overabundant emission of laughter-like 50 kHz calls in juvenile Ube3amat–/pat+ rats. A, Example images of the manipulations used to mimic social play and elicit USVs. B, Example spectrograms of USVs from a WT littermate control (Ube3amat+/pat+; mat+/pat+; top) and Ube3amat–/pat+ rat (mat–/pat+; bottom). C, Across 5 d of heterospecific play sessions, 50 kHz USV emission increased with repeated testing in both mat–/pat+ (n = 25) and controls (n = 25), but the emission rate was substantially elevated in mat–/pat+. D, On average, mat–/pat+ rats produced 50 kHz USVs at more than twice the rate of controls. E, Specifically, 50 kHz calling was abnormally high during the break and belly tickle phases, with trending increases during neck tickle, push and drill, and flip over. F, Before the onset of play, mat–/pat+ rats emitted anticipatory 50 kHz USVs at >3 times the rate of controls. G, Production of short 22 kHz USVs was low, did not differ between genotypes, and did not change over subsequent play sessions. H, The rates of 50 kHz and short 22 kHz calling during empty cage exploration were comparable between genotypes (mat+/pat+, n = 32; mat–/pat+, n = 29), as were the (I) 50 kHz and short 22 kHz calling rates in response to hearing playback of conspecific 50 kHz USV (mat+/pat+, n = 9; mat–/pat+, n = 12). Call features did not differ by genotype: J, The average duration and (K) peak frequency of spontaneous 50 kHz calls made during exploration of an empty cage were comparable between mat–/pat+ (n = 23) and mat+/pat+ rats (n = 25). L, The average duration and (M) average peak frequency of short 22 kHz calls made within an empty cage were also similar between genotypes (mat+/pat+, n = 6; mat–/pat+, n = 7). N, For 50 kHz USVs emitted in response to hearing playback of natural prerecorded 50 kHz rat USVs, average duration and (O) average peak frequency were comparable between mat–/pat+ rats (n = 12) and WT littermates (n = 9). P, There was no genotype effect on the average duration or (Q) average peak frequency of short 22 kHz calls made during USV playback (mat+/pat+, n = 3; mat–/pat+, n = 2). Of note, long 22 kHz USVs known to function as “alarm calls” were very rarely observed, indicating that our paradigms were not aversive. Data are mean ± SEM. C, **p < 0.01, repeated-measures ANOVA. D, F, ***p < 0.001, **p < 0.01, Mann–Whitney U test. E, *p < 0.05, #p < 0.06, repeated-measures ANOVA, Holm-Sidak's post hoc.