It is also noteworthy that the mean departure latency of the whole group did not vary with the experimental group size.
The experimental probability per unit time to follow (the following rate expressed in s−1) is the inverse of the mean departure latency.
The departure latency of a follower j is the time between the departure of follower j and the departure of the previous follower j - 1. Accompanied by a field assistant, I scored the location of the baboon group from dawn (about 7 00) to dusk (about 17:00) throughout the study period.
The evolutionary advantages of the distribution of departure latencies for species subject to predation are now well known [4], and survival typically increases with an increasing group size [20].
The shape of the mean departure latencies distribution per rank was analysed using the same test.
Joining and order during progression: A survival analysis was carried out on departure latencies; the shape of the inverse cumulative distribution of departure latencies was analysed using curve estimation tests.
The distribution of departure latencies according to the rank of joiners follows a parabolic curve (R2 = 0.43, F1,9 = 6.6, P = 0.030, y = 0.0067x2 - 0.7489x + 33.081, Figure 3d).
The inverse cumulative distribution of departure latencies in the small group is correlated to that of the large group (R2 = 0.86, F1,808 = 5290, P < 0.0001, 1.2222x + 0.0127).
These "rules of thumb" are based on physiological needs and social relationships [ 4, 40], and they have explained departure latencies, order and associations of individuals during movements in macaques [ 41] and lemurs [ 42].
The inverse cumulative distribution of departure latencies of joiners follows an exponential law (R2 = 0.90, F1,153 = 1269, P < 0.0001, y = 0.1383e-0.009x, Figure 3c) better than a linear one (R2 = 0.25, F1,153 = 52.28, P < 0.0001 y = -0.0004x + 0.1402): the probability of joining the movement is constant per time unit and equals 0.009.
However, birds were more reluctant to leave their first position and flew a shorter distances in response to the 50m-propagated signal than to the non-propagated song (Fisher's exact tests: latency of departure from the first loudspeaker position, p<0.01, n = 18; approaching distance to the second speaker, p<0.05, n = 18).
