**Figure 5.** **Forced swim test to assay depressive-like behavior.** (**A**) Immobility in the forced swim test is considered an indicator of depressive-like behavior, and F2 hybrid females were found to be more immobile (p=0.0013, 2-way ANOVA), as compared to males. Values are mean ± SEM. (**B**) Due to no age effect found, all timepoints for each sex were pooled to demonstrate a 20% difference in immobility between females and males (p=0.0002, Mann-Whitney). Values are mean ± SEM. (**C**) Immobility plotted against fat mass for each animal. Females exhibited increased fat mass as compared to males, and immobility was found to correlate to percent fat mass. The correlation slopes were constrained to be equal since they were not significantly different (p=0.58, ANCOVA). (**D**) Bayesian mediation analysis separated the total effect of the difference between males and females in the forced swim test into effect mediated by fat mass or by unknown mechanisms. The sex difference in fat mass accounts for 30% of the sex difference in immobility (Bayesian p=0.96) and other mechanisms account for the remaining 70% (Bayesian p=0.99; Bayesian mediation analysis). Values are mean ±50% (thick lines) and 95% confidence interval (thin lines); n_{M7}=7, n_{M15}=10, n_{M22}=10, n_{F7}=9, n_{F15}=12, n_{F22}=7.