Table 1.

Estrous phase of the resident B6 female mice and the CD1 stimulus female intruders for each testing session.

Experiment	Estrous phase	Session 1		Session 2		Session 3
		Estrous	Diestrous	Estrous	Diestrous	Estrous	Diestrous
1	Resident	5*	5	4*	6	4*	6
1	Intruder	5	5*	4*	6	5*	5
2	Resident	4	6	7	3	7	3
2	Intruder	5	5	6	4	5	5

While the female CD1 intruders used for assessing males' USVs and social behaviors in male-female interactions were all in diestrous phases, the intruders used for female-female interactions were either in diestrous or estrous phase. Their assignment on each testing day was counterbalanced according to the estrous phase of the B6 experimental female mice (i.e., approximately half of the intruders in estrous phase encountered a resident in estrous and the other half was assigned to a resident in diestrous; the same design was applied to the intruders in diestrous). The female CD1 stimuli at the time of the first testing session were naïve to encounters with mice of different strains (i.e., they have had pre-testing social interactions only with their same-sex and same–strain cagemates); they were used for a total of 3 times on the 3 subsequent testing sessions, but they were tested only once for each session. Separate batches of stimulus females were used for female-female interactions and male-female tests, and for each experiment (for a total of 40 CD1 female and 10 B6 male + 10 B6 female mice for each experiment).

^*One female B6 was excluded from the analysis of social behaviors because she was a statistical outlier on the time spent in affiliative behaviors (based on Grubbs' ESD test).