It was previously believed that Java mouse-deer were nocturnal, but more recent studies have shown that they are neither truly nocturnal nor diurnal, but instead crepuscular, meaning they prefer to be active during the dim light of dawn and dusk. This behavior has been observed in both wild and captive Java mouse-deer. Although Java mouse-deer form monogamous family groups, they are usually shy, solitary animals. They are also usually silent; the only noise they make is a shrill cry when they are frightened. 