Figure 2.

(a) Group-size effect on the observed collective proportion and the expected proportion under the independent scanning assumption. Open circles indicate the observed proportions in each group (P_obs) i.e. the proportion of time that at least one individual spent in vigilance within a group and the solid-line, the observed linear regression on the logit-transformed P_obs: Y=1−[1/(1+exp(−1.06+0.210X))^X]; F_1,40=21.577; p<0.0001). Filled triangles indicate the expected values of proportion of time and the dotted-line, the expected linear regression under the independent scanning assumption (linear regression on the logit-transformed P_exp: Y=1−[1/(1+exp(−1.086+0.246X))^X]; F_1,40=24.697; p<0.0001). (b) Difference between the observed collective proportion (P_obs) and the expected collective proportion of time (P_exp) under an assumption of independent scanning, for each observed group.