Chimpanzees behave prosocially in a group-specific manner

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Science Advances  24 Feb 2021:
Vol. 7, no. 9, eabc7982
DOI: 10.1126/sciadv.abc7982
  • Fig. 1 Experimental setup illustrating prosocial fruit-juice donating.

    Fruit juice could be provided to group members by pushing a button. The pushing actor could not benefit from the juice concurrently. The photograph shows an adult female (with infant) providing juice for a sub-adult group member. For a schematic illustration of the setup, see fig. S2. Photo credit: Clara Dubois (Leipzig University).

  • Fig. 2 Chimpanzees’ prosocial acts over the course of the experiment.

    Across the three groups, both the (A) frequency and (B) duration of the chimpanzees’ prosocial acts increased over time. The increase in prosocial behavior was observed in most of the chimpanzees (fig. S3). Dots represent individual data points, lines represent predicted values, and the gray zone around the lines represents 95% confidence intervals.

  • Fig. 3 Chimpanzees’ prosocial behavior in the test versus control condition.

    Chimpanzees provided more fruit juice when group members could benefit from their actions (test) compared to when the juice was released out of all chimpanzees’ reach (control), both as measured in the (A) frequency and (B) duration of pushing acts.

  • Fig. 4 Frequency of chimpanzees’ prosocial fruit-juice donations across the three study groups.

    Prosociality as measured by the number of fruit-juice donations per minute differed between the groups, both when (A) all pushing acts were included and (B) with only the non-egoistic acts analyzed. Medians are represented by the bold, horizontal lines within the boxes. The boxes represent the interquartile range (IQR); the vertical lines attached to the boxes represent Q1 – 1.5 IQR (lower) and Q3 + 1.5 IQR (upper).

  • Fig. 5 Chimpanzees’ prosocial dynamics map onto their group-specific social climates.

    Chimpanzees that (A) push more for others compared to solitarily and (B) provide benefits to more group members per prosocial act belong to (C) the most socially tolerant groups (i.e., Group_2 and Group_4: Kruskal-Wallis rank sum test, χ2 = 16.8, df = 2, P < 0.001). (A) and (B) represent averages per individual expressed in medians (bold horizontal lines) and interquartile ranges (boxes with vertical lines). (C) represents scan samples from a validated group assay on social tolerance (for more details, see the Supplementary Materials).

  • Fig. 6 Network visualizations of the prosocial interactions in the two study groups with more than one family unit: Group_1 (left) and Group_2 (right).

    The nodes denote the individuals who interacted in the experimental setup by either pushing or drinking and are colored by their maternal family group affiliation (i.e., maternal kin are represented by the same color). The edges represent the dyadic tie strength between individuals who pushed for each other, calculated as a simple ratio index: the number of times individual A pushed for B plus the number of times individual B pushed for A, divided by the total number of times A and B pushed. The network was generated with the R package “asnipe” (53). The diagrams were laid out using the Fruchterman-Reingold weighted algorithm (54). This layout increases the uniformity of edge length and minimizes edge crossings. The graphs display communities generated by the spinglass algorithm (55). This approach is based on the principle that nodes should prefer to be connected with other nodes of the same spin state, i.e. community, and disconnected from nodes of differing spin states. This algorithm has been found to be suitable for detecting communities in small networks (56). The graphs were generated using the R package “igraph” (57).

Supplementary Materials

  • Supplementary Materials

    Chimpanzees behave prosocially in a group-specific manner

    Edwin J. C. van Leeuwen, Sarah E. DeTroy, Stephan P. Kaufhold, Clara Dubois, Sebastian Schütte, Josep Call, Daniel B. M. Haun

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    The PDF file includes:

    • Supplementary Materials and Methods
    • Results
    • Figs. S1 to S9
    • Tables S1 to S4
    • Legends for movies S1 and S2
    • References

    Other Supplementary Material for this manuscript includes the following:

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