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Manipulation of worker size diversity does not affect colony fitness under natural conditions in the ant Temnothorax nylanderi

Abstract

In social insects, within-colony worker diversity has long been thought to improve colony fitness. Recent studies start to question this assertion in species with moderate diversity. However, demonstrating an absence of effect is difficult as this absence could be due to the measurement of only few life history traits or to artificial conditions. In order to circumvent these limitations, we experimentally decreased worker size diversity within colonies of the ant Temnothorax nylanderi, with or without affecting mean worker size and we reintroduced them in the field for 4 months (spring and early summer). We then thoroughly measured their fitness based on survival, growth, and reproductive success. Our results show that our manipulation did not affect colony fitness. In addition, colonies did not restore diversity to its initial level, further suggesting that worker size diversity is not a key parameter to them. We found the classically observed positive relationship between colony size, colony growth, and reproductive success. Overall, our results confirm that worker size diversity within colony is not necessarily adaptive in species where it is moderate. We discuss the alternative mechanisms that could explain the evolutionary persistence of moderate worker size diversity.

Significance statement

Organisms that live in groups can greatly benefit from the emergence of novel group-level traits. For instance, social insects show significant variability in worker size within colonies. This size diversity increases the division of labor among workers and improves colony fitness. However, in species where size diversity is moderate, this relationship may not always be verified. Here, we manipulated both worker size diversity and mean worker size within colonies of the ant Temnothorax nylanderi, we reintroduced them in the field, and we extensively measured colony fitness after 4 months. We found no impact on colony survival, growth, and reproduction. We discuss how social life and its associated cooperation and conflicts could cause variation in worker size, without any positive effect on colony fitness.

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Data availability

The dataset analyzed during the current study is available from the corresponding author on reasonable request.

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Acknowledgments

We thank Romain Péronnet for technical assistance and ant collection in Vincennes. We are grateful towards Michel Neff and the Division du Bois de Vincennes de la Ville de Paris for letting us use an area of wood land of the Réserve Ornithologique du Bois de Vincennes as an experimental plot. We thank the two anonymous reviewers for their helpful comments on the manuscript.

Funding

This work was funded by the Institute of Ecology and Environmental Sciences—Paris and the Institut de la Transition Environnementale de Sorbonne Université.

Author information

Authors and Affiliations

Authors

Contributions

RH designed the study, collected and reared ants, performed the experiment and statistical analyses, and wrote the manuscript. CD designed the study, contributed to statistical analyses, and wrote the manuscript. MM designed the study, wrote the manuscript, and supervised the project. All authors read and approved the final manuscript.

Corresponding author

Correspondence to Romain Honorio.

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Conflict of interest

The authors declare that they have no conflict of interest.

Ethical approval

All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.

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Communicated by L. Keller

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Appendices

Appendix 1

Fig. 4
figure 4

Worker size diversity within one colony of Temnothorax nylanderi. In our experiment, the head width of the smallest worker produced was 0.384 mm and that of the largest was 0.661 mm

Appendix 2

Fig. 5
figure 5

Laboratory rearing setup. The nest consisted of two microscope slides separated by a 1-mm auto-adhesive plastic foam with three chambers, covered with black paper for darkness. It was placed in a plastic box (foraging area) with water (tube plugged with cotton) and food (mealworm)

Appendix 3

Fig. 6
figure 6

Field rearing setup. Eighty semi-buried buckets were installed in the “réserve ornithologique du Bois de Vincennes.” Each bucket contained local soil and was pierced at the bottom for water draining. Six nests (drilled corks) were available for the colony to choose. The foraging area was restricted to the bucket. Cut-off lids are not present on the photos

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Honorio, R., Doums, C. & Molet, M. Manipulation of worker size diversity does not affect colony fitness under natural conditions in the ant Temnothorax nylanderi. Behav Ecol Sociobiol 74, 104 (2020). https://doi.org/10.1007/s00265-020-02885-2

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  • DOI: https://doi.org/10.1007/s00265-020-02885-2

Keywords

  • Colony size
  • Social insects
  • Mean body size
  • Survival
  • Growth
  • Reproductive success