In 1999, we observed six Père David’s deer calves (3 male and 3 female), which were born in the Beijing Milu Park to study the synchronization of action and grouping behavior in those calves. The observation began in the second week of their births until their weaning, altogether, we collected 4 320 behavioral records. We established two indexes, the Index of Activity Synchrony, A, and Daily Frequency of Calves in the Same Cohort, P, for measuring aggregate behaviour in calves. Before their weaning, the calves synchronised their activity (A=0.87±0.006), and they stayed in same cohort most of the time (P=0.86±0.012). No sexual segregation behaviour was found in the calves. There were no significant differences of A (F=0.075, P=0.788, df=1) and P (F=0.489, P=0.497, df=1) in the calves of the same sex or between opposite sex. However, Index A and P changed significantly as the ages of the calves increased. Daily frequency of calves stayed in the same cohort could not present aggregation tendency of calves exactly. At 4 week old, calves expressed the highest activity synchronization.
