Subjects

Chimpanzee observations were conducted in the Goualougo Triangle, located in the southern section of the Nouabalé-Ndoki National Park (E 16°51′−16°56′; N 2°05′−3°03′), Republic of Congo. The study area encompasses 380 km2 of evergreen and semi-deciduous lowland forest, with altitudes ranging between 330 and 600 meter. Rainfall is bimodal, with a primary rainy season from August to November and a short rainy season in May.

Data Collection

We placed remote video-recording devices with passive infrared sensors at termite nests to record chimpanzee visitation and tool-using behaviors30. Video footage was archived on hard drives and converted to MPEG for review after which we coded videos using INTERACT Version 1450. We screened 224 hours of footage and identified 96 fishing probe transfers, defined as the change of possession of a fishing probe from one individual to another. A subset of these transfers met criteria for inclusion in the present study. If multiple transfers occurred between the same individuals during the same visit to a termite nest, only the first transfer was included because subsequent transfers were considered nonindependent. On 4 separate visits, 2 transfers were coded in each and were included, because the transfers were separated by a minimum of approximately 10 minutes and by other intervening behaviors. Thus, we deemed each transfer event to be independent. Transfers were coded for age/sex class of donor and recipient. The resulting data set included 57 transfers of fishing probes from an older, more competent individual to an immature individual. There were two occasions in which there was a change of possession of a fishing probe between adults, four transfers from a subadult or older juvenile to an adult female, and two transfers between youngsters. These were not included in analyses due to their relative rarity.

Next we screened transfers for those in which the donor or recipient chimpanzee, or both, were continuously visible during the 30 seconds immediately before and after the transfer. We considered this time frame adequate for capturing representative behavior before and after transfers given the relatively short average duration (2.55 minutes) of termite nest visits by chimpanzees in this population51. In addition, because chimpanzees may go in and out of the camera field of view, coding clips continuously for the entire duration of chimpanzee presence at termite nests was not always feasible. For donors and recipients, respectively, 26 and 24 transfers allowed for determination of the proportion of time spent in active tool use. We coded behaviors including termite-gathering tool use (e.g., active insertion of fishing probes); and other behaviors such as play, inactivity, and locomotion. For a further subset of these clips, continuous visibility at a high degree of resolution for 30 seconds before and after the clips allowed for the coding of specific tool use and feeding behaviors. We further required that the donor must have initiated tool use by 30 seconds before the transfer. This was necessary in order to ensure that comparison of behavior before and after a transfer event was not systematically biased by a donor’s latency to begin termite gathering upon arrival at a termite nest. This criterion was not applied to recipients, given that immature chimpanzees often engage in a range of behaviors other than termite gathering while present at termite nests and the purpose was to discern how their behavior changed, regardless of the behavior immediately preceding the transfer. For donors, we coded fishing probe insertions and feeding events for 17 and 15 transfers, respectively; and for recipients, we coded fishing probe insertions and feeding events for 15 and 14 transfers, respectively. Fishing probe insertions involved the insertion and extraction of an herbaceous probe into an exit hole on a termite mound. Feeding elements included sweeping termites from tools, eating termites directly from the tool, or gathering termites by hand, wrist or lips from the termite nest surface.

Analysis

In order to test whether the duration of tool use differed before and after the tool transfer we used exact52,53 Wilcoxon tests, applied separately for donors and recipients. In case individuals acted repeatedly as donor or recipient, respectively, we used the average duration per individual and time period (before or after, respectively). We did not use mixed models (see below) for the duration since it showed bottom and ceiling effects, making it impossible to find an appropriate error distribution.

For testing whether the number of feeding events and fishing probe insertions differed between before and after the tool transfer, we used Generalized Linear Mixed Models34 (GLMM), fitted separately for donors and recipients (see Extended Data Tables 1 and 2). These included one fixed effect denoting whether the observation was made before or after the tool transfer (“time period”). As random intercepts we included the identity of the chimpanzee and also the particular transfer event. To keep Type I error rate at the nominal level of 0.05, we included the random slope of time period within chimpanzee identity whenever we had at least two tool transfer events for at least half of the individuals54,55; this random slope was included into the models for the number of fishing probe insertions of donors and the number of feeding events of donors. We did not include the correlation between the random intercept and slope to avoid overly complex models given the small sample sizes. We used either a Poisson error structure or, in case this revealed an overdispersed response, a negative binomial error structure. Specifically, we used a Poisson model for the number of fishing probe insertions for the donor (dispersion parameter = 1.17) and negative binomial models for the other three (dispersion parameters, number fishing probe insertions, recipient: 1.48; number feeding events, donor: 1.10; number feeding events, recipient: 1.33). We tested the significance of time period using a likelihood ratio test comparing the full model with a respective null model lacking the effect55,56.

The models were fitted in R57 using the functions glmer or glmer.nb of the package lme458 (version 1.1–10); and Wilcoxon tests were calculated using the function wilcox.exact of the package exactRankTests59.