6. Departures

In all baboon societies, individuals appear to preferentially follow closely affiliated group members regardless of which individual initiates a collective movement27,40,67. Olive baboons follow close associates at the start of travel67 and hamadryas baboons follow other members of their one-male units and clans27,37. Therefore, we expect Guinea baboons will also follow their closest social partners (i.e. members of their units) during departures, and throughout collective movements.

6. Departures

We sampled a total of 146 attempted group departures, out of which 121 were successful (Table 2). Of the total attempts, 91 (62.3%) were led by adult males [55 by primary males (37.7% of all cases) and 36 by bachelor males (24.7%)], 52 (35.6%) by adult females, and three (2.1%) by juveniles. In two events, the group split as a result of two successful initiation attempts occurring during group departure. Of the 121 successful group departure events, 33 involved only one complete unit, 48 events involved more than one complete unit, and 40 events involved a complete party. Although the sex ratio across the two gangs was nearly 1:1 with 41 adult males and 42 adult females, males were almost twice as likely to initiate group departures than females (see below for statistics). Attempts to initiate group departures came from 58 different individuals: 28 adult males, 27 adult females, and three juveniles. The individuals that attempted initiations most frequently were two primary males, with 11 and seven attempts respectively; followed by four primary males and one bachelor male who each attempted to initiate a group departure six times. The two females that attempted initiations most frequently did so five and four times each.

We provide evidence that the Guinea baboons in our study population coordinate collective movements through partially shared consensus (distributed leadership), where most adult group members can successfully initiate group departures and move at the front of group movements68. Adult males attempted initiations more often than adult females, but members of both sexes were highly successful when attempting to initiate group departures. Primary and bachelor males attempted initiations with similar frequency and they were similarly successful. During group progressions, bachelor males were more likely to be found at the front, while primary males and females were found in all portion with similar probability. Individuals of the same unit typically departed and travelled together. Thus, sex, male reproductive status (primary or bachelor), and unit membership affected group departure and group movement patterns differentially.

The patterns of individual influence over collective movement decisions that we observed during group departures in Guinea baboons were overall more similar to those seen in uni-level baboon species such as chacma, yellow, and olive baboons12,14,45,46 rather than those of hamadryas baboons27,37. In Guinea baboons, males initiated group departures more often than females, but both males and females initiated group departures with similar degrees of success. This is precisely the pattern found in a group of chacma baboons12 and fits the general trend of partially-shared consensus observed in populations of olive, yellow, and chacma baboons14,45,46,54. The order in which Guinea baboons progressed during travel also reflected the shared influence of males and females over collective movement decisions. Female Guinea baboons occupied front, centre, and rear positions with similar likelihood. When in front, females could potentially influence ongoing movement decisions similar to females in some troops of yellow, olive and chacma baboons12,14,69,70. In addition, the male tendency to travel at the front was weaker in Guinea baboons compared to hamadryas baboons, where males were twice as likely to walk at the front of progressions than predicted by chance27. Positions at the rear of progressions were equally taken by individuals of all age, sex and male reproductive status categories, differing from patterns where males were more frequently observed toward the rear of hamadryas and chacma baboon progressions27,70. Thus, multi- or uni-level social organization per se does not directly translate into one or another type of leadership during collective movement.

Individuals assorting by unit while on the move could explain why classes of individuals that make up Guinea baboons units, i.e. primary males, females, and young, were found in all portions of progressions with similar frequencies. In contrast, bachelor males were more likely to be found in the front of the progression than in its middle. The tendency of bachelor males to be at the front of group progressions could indicate that, once on the move, they are choosing the direction of group movement. Yet, we found no evidence that bachelor males initiated departures more often or more successfully than primary males. Alternatively, bachelor males may be more likely to move at the front of progressions simply because they travel faster than their unit-bound party members (see Harel, Loftus, and Crofoot 202075).

We collected all data in 2016 and 2017 from January to August, respectively, 6 days per week. Observation days began before sunrise (at 6:00 or 6:30) in order to locate baboons at their sleeping sites. We recorded data on Samsung Galaxy Note 3 handhelds using forms created with Pendragon 7.2 (Pendragon Software Corporation, USA). D.M. took all data on departure and progression, and together with other team members, he collected census, ad libitum, proximity scan, and focal data of the baboons to investigate demography, reproductive success, association data, and behavioural patterns81. Observer reliability was regularly checked. We used focal follows and ad libitum data of grooming, copulations, contact-sitting, and aggressions to validate female-male associations, following the procedure described in Goffe and colleagues (2016)62. Group movement data were recorded from all instances of travel that occurred during daily observations (i.e. all-occurrence sampling81). We distinguished two types of events during the group movement process: group departures and group progressions.

Finally, we investigated the spatial association within the group progression to test whether interval times were influenced by unit membership, as with group departures. Using the time interval between successive individuals within a progression, we applied the same procedure as was used for the analysis of interval times in group departures. In brief, we used a linear mixed model (LMM85) that included unit membership as a fixed effect, and the identity of the following individual and event number as random effects. 041b061a72