Observations

We tried to locate parties of E1 on 6 days of each week. We made direct observations of bonobos on 484 days, which represented 68% of the working days during the study period. We followed the parties from sleeping site to sleeping site when possible. On average, we observed bonobos directly for 4.6 h per day on days when we conducted direct observations.

We started a tracking session when we found bonobos at a sleeping site in the morning or while they were ranging in the forest. While tracking a party, we recorded their position at 30-min intervals via a global positioning system (GPS) receiver. When we lost sight of the bonobos, we followed their tracks or vocalizations. If we could find them again within 1 h, we assumed that we had been successfully tracking them on their ranging route. If >1 h passed, we assumed that we had lost them, and began a new tracking session when we found them again. We calculated the daily mean ranging rate by dividing the total ranging distance by the total time spent for all tracking sessions of the day. Due to some mechanical problems with the GPS, data for ranging rate were only available for September 2003-January 2004, March-November 2004, and October-December 2005.

While following a party, we recorded members using a 1-h party method (Hashimoto et al. 2001). We recorded all bonobos in sight at the beginning of each 1-h segment and continued recording bonobos that appeared in sight until the end of that hour. We calculated the party size and composition for each day by averaging the data recorded for each observation hour on that day. Data on party size and composition are available for the whole study period.

To census fruit abundance, we established five trails totaling 22,550 m, which covered the home range of E1, and we walked the trails twice per month. When we found a cluster of fallen fruit within 1 m on either side of the trail, we recorded the name of the species and whether the fruits were ripe or unripe. We estimated fruit abundance from the number of clusters of ripe food fruit per km of trails (Furuichi et al. 2001). The food fruit included only the species that we observed bonobos eating in the current or past study periods. Data for fruit abundance are available for June 2004 and from August 2004 to December 2005. Further details on the observations of the 1-h party size and fruit abundance are described by Furuichi et al. (2001), Hashimoto et al. (2001), and Mulavwa et al. (2008).

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