Reliability of a New Method to Census Japanese Macaques Fixed Observers in Quadrats

Abstract

We devised a census method that uses fixed observers in quadrats to estimate the density of Japanese macaque groups or populations. A research area is divided into 500×500m quadrats. Observers are positioned in each quadrat at a fixed point where they can get a good view over the quadrat. The observers simultaneously search monkey groups both visually and auditorially, and map the movement of the groups. To avoid duplication in counting, the observers communicate with each other by radios. At the end of the census, the number of groups detected was summed. This method can also be used to estimate population density if the number of individuals in groups is counted. Unexperienced students conducted this census in 9.5km2 steep mountain area of Yakushima Island where a line census was impracticable. Individual censuses were conducted over areas of 2 to 4km2 from 7:00 to 17:00. This area included the 3km2 area where 10 or 11 groups were known to inhabit from studies since 1973. This area was used to measure the accuracy of the method. Three different areas were censured two or three times to evaluate the precision of the method. Furthermore, experienced observers moved with monkey groups to record the exact positions of the groups in order to measure the detection efficiency of fixed observes. Monkeys were mostly detected by loud calls, and 75% of groups were found within 3 hours after start of observation at 7:00 A. M, and after 8 hours no new groups were detected. Detection efficiency was measured as the ratio of the number of observers who found the position of groups to the number of all observers tested. The detection efficiency exponentially decreased as the distance between the observer and the group increased. The maximum observer-to-group distance was 720m. In the first day of the observation in an area where the number of groups had been known, unexperienced students detected about 20% of the groups, but in the second day they detected 60-70% in the same area as the first day. Repetition of censuses in the other areas resulted in the same counts of the groups in different censuses in each area. We conclude that this method can be successfully used in steep mountainous areas even by unexperienced field assistants, with sufficient accuracy and precision if a prior survey excercise of one or two days is conducted. If this method is applied in an area with different conditions, evaluation of accuracy is first needed.