Fig. 4
Chimpanzee call detection probability on SPATUs versus number of indirect observations (fresh nests and chimpanzees signs) nearby. At Issa, the probability of detecting a chimpanzee call on a SPATU was predicted by the total number of chimpanzee signs found within a 500 m radius (est ± SE: 0.126 ± 0.047, X2 = 4.06, df = 1, P = 0.044, N = 2470). The fitted model results are depicted with a dashed line. Again, for plotting purposes only, data points were binned for every two observations of indirect chimpanzee activity to obtain a mean detection probability per bin (blue points). The relative area of the circles corresponds to the log of the number of data points (range: 1 to 2427) per bin