Table 1 Experiment 1: Chimpanzee marking behavior at snake models with respect to receivers.

Experiment 1: Chimpanzee marking behavior at snake models with respect to receivers.. (A and B) The likelihood of subjects to initiate marking. (C) The impact of signal type on receiver behavior. Binomial GLMMs: Marking, reposition oneself to see both the snake and the receiver and gaze alternate between the snake and the receiver. H1, receiver has seen the snake before signaler calls or marks (yes/no); H2, subject is first to see the snake (yes/no); H3, receivers include a bond partner (yes/no); H4, subjects could see or hear receivers approaching when first to see the snake. Random factors include subject identity and event. Bold: P < 0.05. Model significance versus null model built of control predictors (for H2 to H4) and random effects: (A) χ² = 9.91, df = 1, P = 0.002; (B) χ² = 4.2, df = 1, P = 0.038. n = 37 cases, 22 subjects, and 12 snake placement events. We conducted two models instead of one due to model stability issues with four predictor variables. After running model (A), we removed the nonsignificant variable, replaced it with the remaining predictor variable, and reran the model [model (B)]. All significant variables remain significant with a Bonferroni correction [required across models (A) and (B)]. Model (C) included only receivers who had not seen the snake. n = 27 cases, 17 subjects, and 12 events: χ² = 6.41, df = 1, P = 0.011. See table S1 for the source data.

Predictor variableβSEχ²P
A. Response variable: mark (yes)
Intercept11.609.80
H1: Receiver knowledge: receiver has seen the snake (no)21.3410.49.910.002
H2: Habituation: subject is first to see the snake (yes)−1.775.700.070.70
H3: Receiver identity: bond partner present (yes)19.988.1311.600.001
B. Response variable: mark (yes)
Intercept10.767.20
H1: Receiver knowledge: receiver has seen the snake (no)19.137.204.280.008
H3: Receiver identity: bond partner present (yes)20.488.9813.450.022
H4: Receiver behavior: receivers approaching (yes)2.015.720.140.73
C. Response variable: receiver behavior cautious approach (yes)
Intercept0.410.91
Mark and call (yes)2.601.136.440.011
Call (yes)−0.691.220.320.57