Effects of constant daylight exposure during early development on marmoset psychosocial behavior.

Due to global industrialization, the light cycle is shifting to longer daytime. Mounting evidence indicates that social developmental disorders may correlate with longer periods of daytime in childhood. However, the exact mechanisms of this link remain unclear. To examine the impact of longer day-time on psychosocial development, we developed a novel non-human primate model, using the common marmoset (Callithrix jacchus) reared under constant daylight from birth. Marmosets were reared individually by human nursing under constant light (LL) during varying periods in juvenile development, and their behaviors were compared with those of normal day-night cycle (LD) marmosets by multivariate analysis based on principal component analysis (PCA). LL marmosets elicited egg-like calls (e-call) less in juvenile period, and displayed side-to-side shakes of the upper body with rapid head rotation through adulthood frequently. Based on the PCA, these behaviors were interpreted as 'alert' or 'hyperactive' states. Additionally, behavioral development of marmosets reared under constant dark (DD) was markedly different from both LD and LL marmosets, suggesting the fundamental importance of daylight-dependent neuronal and endocrine processes and entrainment by a constant 24-hour light/dark cycle on psychosocial behavior development.