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A Cross-Species Comparison of Task Partitioning in Ants Lacking Discrete Morphological Worker Subcastes

  • Author(s): West, Mari
  • Advisor(s): Purcell, Jessica
  • et al.

Task partitioning allows for coordination of behavior in animal societies, potentially enhancing task efficiency. Many task allocation studies focus on social insects with discrete morphological worker subcastes, such as those possessing major and minor workers with strongly differentiated body plans. Much less is known about task partitioning among size-variable workers lacking discrete morphological subcastes, like in Formica ants. Through a large-scale mark-recapture study and a controlled laboratory experiment, we investigated how worker size affects task fidelity and proficiency across Formica species with differing degrees of body size variation. Additionally, we carried out genomic analyses to identify any genetic underpinnings of size-based task partitioning in these species. In species with high levels of worker size variation, a worker’s body size is strongly correlated with the tasks it performs. Specifically, large workers specialize in nest building or protein foraging, while small workers specialize in honeydew collection. This size-task correlation is weaker, but still present, in species with less size variation among workers. Interestingly, our laboratory experiments suggest that, in Formica species with substantial intracolony size variation, large workers outperform small workers at both nest building and sugar-water collection. It is unclear whether small workers’ relatively poor performance at a task they typically perform in nature is due to limitations of our experimental design, or if small workers make other important contributions to colony efficiency. Genomic analyses reveal that both worker size and task may be under genetic control, although this is variable across species. The two phenotypes are not always genetically linked, although they appear to share some genetic associations in the most size-variable species analyzed. Combined, these studies suggest that Formica ants utilize a size-based task partitioning strategy, but the reliance on, benefits of, and genetic underpinnings of this strategy vary considerably across species. We expect social insects with varying degrees of morphological task specialization to differ in ontogeny, evolutionary history, and behavioral flexibility. Additional comparative studies will help us understand the potential costs and benefits of alternative strategies.

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