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Cooperation and population dynamics in the Sociable Weaver

The aptly named Sociable Weaver Philetairus socius is a highly social species that is endemic to the Kalahari region of southern African. As the common name suggests, these weavers work together to accomplish diverse tasks, from building their highly distinctive thatched nests to help raising the chicks and defending the nest and colony mates from predators. Their fascinating social structure and different types of cooperative behaviour make them an ideal study model to investigate the benefits and costs of sociality and the evolutionary mechanisms that allow cooperation to evolve and be maintained.

Cooperation represents an evolutionary puzzle because natural selection is thought to favour selfish individuals over co-operators. However, theory and studies in humans suggest that co-operators are preferred as social and sexual partners. Partner choice may therefore provide a powerful explanation for the evolution and stability of cooperation, alongside kin selection and self-serving benefits, but we lack an understanding of its importance in natural systems. This has become an important focus of the Sociable Weaver project, led by Rita Covas and Claire Doutrelant, and a recent grant from the European Research Council (ERC) will allow us to focus on this topic over the next five years by using new methodological developments to implement automatic data collection of cooperative behaviour and run field experiments.

The sustained funding is also crucial to continue our long-term study of the costs and benefits of sociality and weaver population trends. In particular, under the current scenario of increasing environmental variation, we are attempting to establish whether sociality may lessen the impacts of these adverse conditions.

Activities in 2019

  • 2019 was another record year, with late rains and high temperatures resulting in a very late onset of breeding (February instead of September). The late rains led to a long breeding season that continued until July and a very short winter break. By the end of August, the birds were starting to breed again! As a result, almost 500 chicks had fledged by the end of December 2019! The Sociable Weaver is an opportunistic breeder and the increasing unpredictability of climate patterns is causing increasingly irregular breeding seasons.
  • Keeping track of all the individuals in our population and collecting the behavioural data needed to test different hypotheses has represented a challenge that we have been attempting to address in creative ways. Over the last year, PhD student André Ferreira developed a method that uses artificial intelligence to identify individual Sociable Weavers based on the feather patterns on the individuals’ back allowing us to speed up data collection and analyses. This method is already being applied to the study of nest building behaviour in our population and we intend to apply it also to the study of vigilance and mobbing behaviour.
  • One of the advantages of long-term studies is to be able to monitor how individuals adapt to the variation in their environment over several years. PhD student Rita Fortuna analysed an 8-year data set to investigate whether female Sociable Weavers vary their reproductive investment to match the variation in ecological and social factors. The results revealed that females are highly consistent in the size of their eggs, but clutch size varies in relation to rainfall.
  • Another analysis of long-term data by BSc Hons student Delphine Duval suggests that infanticide increases under adverse climactic conditions.
  • A collaboration with researchers at the CNRS and University of Strasbourg (France) allowed us to investigate whether physiological benefits arise from the presence of helpers. Using a cross-fostering experiment, PhD student Martin Quque analysed telomere length in nestlings in relation to the presence of helpers during egg production and the nestling stage. Chicks receiving additional care of postnatal helpers had longer telomeres than chicks that never had helpers, and chicks with longer telomeres at day 9 were more likely to survive. However, this result was not maintained at fledging.
  • Unlike most other birds, Sociable Weavers are sexually monogamous and most individuals pair for life. Post-doc Pietro D’Amelio has been investigating the benefits of these long-term associations and found that individuals that have been together for longer start to breed earlier in the season and continue until later, and as a result fledge more chicks.
  • Pietro also concluded an experiment to investigate whether male helpers are more cooperative in the presence of females by simulating the presence of different audiences using playbacks.

Highlights

  • Rita Covas received an ERC grant (EU) to study the role of partner choice on the evolution of cooperation.
  • Claire Doutrelant received a French National Research Agency (ANR) grant to study the role of sexual and social selection in the evolution of nest building in Sociable Weavers.
  • Rita and Claire published a paper in Trends in Ecology and Evolution about the role of sexual and social selection in cooperation.
  • Former PhD student Dorine Jansen published a paper in Population Ecology which uses an integrated population model to illustrate the environmental factors influencing population dynamics in the Sociable Weaver.
  • André Ferreira gave a talk at the ASAB conference in August (Konstanz, Germany) about a new method that uses artificial intelligence to identify small birds based on individual feather patterns. A manuscript describing this method is under review.
  • MSc student António Vieira graduated from CIBIO, U. Porto with a project on the role of helpers on reproductive output.
  • MSc student Louis Bliard graduated through the European ‘Darwin’ programme with a project on the effects of group size on helper survival.
  • A new student, Bruna Fonseca (CIBIO, U. Porto) joined the project to investigate the role of colony size on agonistic behaviour.

Impact of the project

The long-term nature of this project allows unique insights to understand the evolution of cooperation and the mechanisms that allow it to persist. The demographic data allow for examination of the factors affecting population dynamics in relation to environmental change.

Key co-supporters
The Foundation for Science and Technology (FCT), Portugal; French Research Agency; ANR, France; DST-NRF CoE grant.

Research team 2019
Dr Rita Covas (FIAO, UCT and CIBIO, U. Porto)
Dr Claire Doutrelant (FIAO, UCT and CNRS, France)
Dr Fanny Rybak (U. Paris-Sud, France)
Dr Pietro D’Amelio (FIAO)
Dr Julien Renoult (CEFE-CNRS, France)
Dr François Criscuolo (CNRS, Strasbourg, France)
Dr Damien Farine (Max Planck Institute, Germany)

Students: André Ferreira (PhD, Montpellier); Rita Fortuna (PhD, Porto); Martin Quque (PhD, Strasbourg); Louis Bliard (MSc, Montpellier); António Vieira (MSc, CIBIO); Claire Shigo (Hons, Montpellier); Delphine Duval (Hons, Montpellier).

Research assistants: Franck Théron, Liliana Silva, Cécile Vansteenberghe, Jess Berndt, Colleen Lindberg, Baudoin des Monstiers, Michelle Schroeder, Corisande Abiven, Annick Lucas, Samuel Perret.