Observation time and sampling intervals for measuring behavior and interactions of chimpanzees in the wild

This study examined two problems in the measurement of chimpanzee behavior: (1) comparability among data sets varying in length of total observation time; and (2) the longest interval for scoring reliable numbers of sample points with instantaneous sampling (this required procedures for evaluating the chi-square statistics of the sampled data). During a 4.5-month field study conducted at the Mahale Mountains National Park, Tanzania, one adult male was observed as a focal animal for about 300 hr with continuous recording. His behavior was classified into five categories. Data sets varying in total time were prepared by extraction from the raw data. Comparability among the data sets was evaluated using Pearson's correlation coefficients and Kendall's coefficients of concordance calculated from two kinds of measures obtained from the raw and simulated data sets: (a) the percentages of time spent by the focal animal in each behavior category; and (b) those of the time spent by adult males in his proximity. The results revealed that observation time of 25 hr was the critical length for scoring the above measures reliably. Sample points for the focal animal's behavior categories and for adult males in his proximity were simulated with intervals of various lengths for data sets differing in total time. The longest interval was measured by comparing the simulated scores with confidence limits calculated for the number of sample points to be scored with the respective intervals. It was found that the interval for sampling should be set at 3 min or shorter, and that chi-square statistics calculated from the data sampled with such an interval should be evaluated after their modification into the values to be obtained from the data sampled with a 5-min interval. These results may not be directly applicable to studies dealing with other behavior categories, other age/sex classes of focal animals, etc. However, the above problems should be examined widely in studies attempting to measure animal behavior, and the methods employed in this study are applicable to such studies.