Fig. 2.

Sex-dependent effects of age on pair bonding are apparent within the initial few minutes of the partner preference test. We used social preference index as a measure of partner or stranger preference during the first 20 min of partner preference tests comparing to a null hypothesis of no preference of 0. A) Male animals displayed a lack of partner or stranger preference during the initial 5 min of partner preference tests, with the exception of 6-month animals after 2 wks of cohabitation, which displayed a stranger preference. B) Similarly, most male animals did not display a partner or stranger preference in the following 15 min of partner preference tests, except for 12- and 18-month after 4 wks separation, which showed a significant partner preference. C) Unlike males, females across ages and timepoints tended to display a significant partner preference within the initial 5 min of partner preference tests, with the exception of 6-month animals after 2 days, which had a stranger preference. D) Female animals of all ages continued to show a significant partner preference in the following 15 min of partner preference tests. N = 7–10 pairs per group; *p < 0.05, **p < 0.01, ****p < 0.0001 for denoting significant main effects and partner preference compared to the null hypothesis of 0 (no preference). ^$p < 0.05 for denoting significant stranger preference compared to the null hypothesis of 0 (no preference).