The effectiveness of sampling methods in detecting copulatory behavior in Macaca arctoides

The copulatory behavior of a social group of 18 stumptail macaques was observed for 171 hours over a period of 5 months. The occurrence of copulation and several quantitative measures of copulation were compared using sampling periods of 2 hours daily and continuous dawn-to-dusk observation. In this environment copulation was found to occur in brief bouts of relatively short duration. Copulation was detected on 34% of the days observed with 2 hr sampling and 100% of the days with DTD sampling. Thus, brief limited-time samples of behavior are not adequate for reliably detecting copulation in this species under social group-living conditions.