Motor development has been extensively studied in human infants and children, with several established scales for the evaluation of motor functions. However, the study of the neuronal mechanisms underlying human motor development is hampered by the lack of good animal models. The common marmoset (Callithrix jacchus), a small New World monkey, has recently attracted much attention as a potential nonhuman primate model for understanding human physiology and diseases. However, little is known about its gross motor development. In the present study, we found that marmosets have a critical period for motor development in postnatal weeks 2 to 5, and acquire most of their motor skills by 8 weeks of age. We also developed methods to assess their motor functions, which will be useful for the evaluation of motor performance in marmoset models of human diseases. In addition, we found that marmosets exhibit a "head-to-tail" sequence of motor development similar to that found in humans, further supporting the notion that they provide a good animal model for studying the neuronal mechanisms underlying human motor development.

The common marmoset (Callithrix jacchus) has attracted extensive attention for use as a non-human primate model in biomedical research, especially in the study of neuropsychiatric disorders. However, behavioral test methods are still limited in the field of marmoset research. The light-dark box is widely used for the evaluation of anxiety in rodents, but little is known about light-dark preference in marmosets. Here, we modified the light-dark test to study this behavior. The modified apparatus consisted of three compartments: one transparent open area and two closed opaque compartments. The closed compartments could be dark or light. We found that both adult and young marmosets liked to explore the open area, but the young animals showed more interest than adults. Furthermore, when one of the closed compartments was light and the other dark, the adult marmosets showed a preference for the dark compartment, but the young animals had no preference. These results suggest that the exploratory behavior and the light-dark preference in marmosets are age-dependent. Our study provides a new method to study exploration, anxiety, and fear in marmosets.