Developmental time course of separation calls emitted by domestic kittens and mound-building mice. (a) Kittens. Average (± SE) number of calls emitted in separation tests by males (n = 19; black bars) and females (n = 14; grey bars) during four experimental sessions across the first postnatal month. (b) Mound-building mice. Average (± SE) calls emitted in separation tests by young mice tested on postnatal days 13 and 14 (n = 18) and postnatal days 14 and 16 (n = 59). Note that data in (a) are repeated measurements, whereas 2 sets of different individuals are presented in (b). In (a), the number of calls emitted by males was significantly lower than in females. Call frequencies differed across weeks; significant post hoc comparisons are indicated by different letters. In (b), differences are significant in both subsamples; see text for statistics.