Game over: Conflict resolution through strategic growth in an invertebrate

In some multimember groups with unequal partitioning of reproduction and poor breeding opportunities outside the group, natural selection has favoured the evolution of adaptive mechanisms such as strategic growth modulation. Strategic growth is a form of body growth plasticity where individuals make growth adjustments calibrated to their closest competitor, establishing a size hierarchy that defines who has priority in reproduction.

In this study we explored the occurrence of strategic growth in an invertebrate, the marine annelid worm Ophryotrocha puerilis, and investigated its underlying mechanisms via growth curve analysis.

Size-matched juvenile worms exposed to different social environments established size hierarchies by following distinct developmental trajectories, with the intensity and duration of growth spurts correlated to the level of competition within their social environment. In monogamous environments, the onset of reproduction led to the weakening of the size hierarchy.

Conversely, in reproductively competitive environments, the onset of reproduction led to the stabilisation of size differences, supporting the idea that individuals actively regulate their growth relative to rivals to mitigate reproductive conflict and size hierarchies emerge as a result of these strategic adjustments.

This study provides the first evidence for strategic body growth in an invertebrate and explores both the establishment of the size hierarchy as well as its eventual dissolution upon conflict resolution.