Research ArticleECOLOGY

Persistent producer-scrounger relationships in bats

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Science Advances  07 Feb 2018:
Vol. 4, no. 2, e1603293
DOI: 10.1126/sciadv.1603293
  • Fig. 1 Bats exhibit two alternative foraging strategies—the Established Colony.

    Individual PIs for all bats in the Established Colony with males in black (n = 6) and females (n = 9) in gray. Symbols on the x axis depict bats’ IDs. All PIs in this panel were calculated over the entire study period (January 2012 to April 2013).

  • Fig. 2 Temporal consistency of strategy use—the Established Colony.

    (A) Average PI + SE for males (black, n = 6) and females (gray, n = 9). The female reproductive periods are denoted along the x axis. Notice that the average female PI is positive in July-August-September. (B) There were high correlations between monthly PI ranks (n = 16), implying that even when bats changed strategy use, they maintained a relative order; that is, individuals that were stronger producers remained stronger and vice versa. Left: An example of the correlation between the first and second time bins (January and February 2012; r = 0.9, P = 0) and between the first and last time bins (r = 0.78, P = 0.001, January 2012 and April 2013). Right: Temporal correlation matrix. Each cell denotes the Spearman correlation coefficient between two time bins (1 to 2 months each). The color indicates the strength of the correlation. All correlations were significant [P < 0.05, following a false discovery rate (FDR) P value adjustment for multiple comparisons] except for those of bin 3 with 6, 7, 9, and 12 representing March 2012 and June 2012/July 2012/August 2012/February 2018) which had P values between 0.1 and 0.06.

  • Fig. 3 Bats exhibit individually specific social preferences—the Established Colony.

    (A) The number of observed and expected dyadic interactions (y axis) for each of the 240 possible directional dyads (x axis). Expected interactions (mean + SD) are depicted by gray bars and are sorted by their values. The corresponding observed number of interactions is depicted by a black dot. Note that dyads are directional. (B) The social foraging network was stable over time. There were significant correlations between the temporal interaction matrices representing the different reproductive periods except between lactation and postweaning. Each circle represents a reproductive period, with thickness of the connective arrows representing the strength of the correlation coefficient between the social foraging networks of two reproductive periods. *P < 0.001 (Mantel test with a Bonferroni correction). (C and D) Preference and avoidance social foraging networks containing only those ties that significantly exceed (that is, preference) or fall below (that is, avoidance) their expected values. Black nodes, males; gray nodes, females. Networks are weighted and directed. Arrows indicate the directionality of the relationship with the arrowhead pointing toward the producer in the dyad and the thickness of the arrows representing the relative strength of the relationships (in the case of double-sided arrows, the thicker arrowhead represents the stronger of the directional ties). The numbers on the left of each network represent the proportion of dyads out of the total 240 possible dyads in the full network.

  • Fig. 4 Flexibility in strategy use in social bats.

    Experimental PI as a function of the producer-to-scrounger ratio. The average (+SD) for all producers (black) or scroungers (gray) at each ratio is shown. Notice how some of the scroungers start producing (PI > 0) when the ratio is above 2:2, as denoted by the fact that the SD crosses the 0 line. The overall average scrounger PI does not cross zero because the individual strategy diverges with some individuals maintaining their old scrounging tendency and others switching to producing.

  • Fig. 5 Collecting-to-landing ratio as a factor of PI.

    The y axis shows the average overall number of fruit items collected divided by the overall number of landings on the food source, and the PI of the bats in Wild Control Colony 2 (n = 32) is presented on the x axis. The average + SE collection-to-landing ratios for scroungers “S” and producers “P” are inserted in the upper left corner. The asterisk denotes significant differences (P < 0.01).

  • Fig. 6 Fine-scale temporal analysis of producing/scrounging.

    We used an Elo rating–inspired algorithm to examine the temporal dynamics of strategy use (84). The x axis represents time, with each bin representing one interaction event. The y axis represents the cumulative producing score, and each line represents one individual (notice that the graph only includes positive scores, that is, of producers). The analysis above shows cumulative producing scores of all male producers, which consistently increase, indicating the consistent use of the producer strategy. In red, the producer score of individual “B” initially increases as in the case of producers, until he was injured (the date of injury is denoted by an arrow), after which his score consistently drops, reflecting a change in strategy from producing to scrounging.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/2/e1603293/DC1

    fig. S1. Wild Control Colony 1.

    fig. S2. Wild Control Colony 2.

    fig. S3. The cumulative producing rates over time for the Established Colony and Wild Control Colony 1.

    fig. S4. Temporal consistency of strategy use—Wild Control Colony 2.

    fig. S5. Temporal consistency of strategy use—Wild Control Colony 1.

    fig. S6. Bats exhibit individually specific social preferences—Wild Control Colony 1.

    fig. S7. Bats exhibit individually specific social preferences—Wild Control Colony 2.

    fig. S8. Females’ rise in producing corresponded to parturition.

    fig. S9. Bats avoid foraging on the lower branches of trees.

    table S1. Strategy indices (PIs) across individuals within the Established Colony (n = 16).

    table S2. Strategy indices (PIs) across individuals within Wild Control Colony 1 (n = 17).

    table S3. PIs across individuals within Wild Control Colony 2 (n = 31).

    table S4. Results of permutation-based t tests between female PIs across reproductive periods.

    table S5. Reproductive periods.

    table S6. Study 3 experimental groups.

    movie S1. An example of an aggressive scrounging attempt on a bat returning to its colony with food.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Wild Control Colony 1.
    • fig. S2. Wild Control Colony 2.
    • fig. S3. The cumulative producing rates over time for the Established Colony and Wild Control Colony 1.
    • fig. S4. Temporal consistency of strategy use—Wild Control Colony 2.
    • fig. S5. Temporal consistency of strategy use—Wild Control Colony 1.
    • fig. S6. Bats exhibit individually specific social preferences—Wild Control Colony 1.
    • fig. S7. Bats exhibit individually specific social preferences—Wild Control Colony 2.
    • fig. S8. Females’ rise in producing corresponded to parturition.
    • fig. S9. Bats avoid foraging on the lower branches of trees.
    • table S1. Strategy indices (PIs) across individuals within the Established Colony (n = 16).
    • table S2. Strategy indices (PIs) across individuals within Wild Control Colony 1 (n = 17).
    • table S3. PIs across individuals within Wild Control Colony 2 (n = 31).
    • table S4. Results of permutation-based t tests between female PIs across reproductive periods.
    • table S5. Reproductive periods.
    • table S6. Study 3 experimental groups.
    • Legend for movie S1

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mov format). An example of an aggressive scrounging attempt on a bat returning to its colony with food.

    Files in this Data Supplement: