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.