# Collective behavior diverges independently of the benthic-limnetic axis in stickleback

## TL;DR

This study finds that collective behavior in stickleback fish evolves independently of their habitat type, suggesting that behavior can change in more complex ways than previously thought.

## Contribution

The study demonstrates that collective behavior in sticklebacks diverges without being tied to the benthic-limnetic habitat axis.

## Key findings

- Collective behavior differences were observed across stickleback populations in a common garden experiment.
- These behavioral differences were not predicted by the benthic or limnetic ecotype classification.
- Morphological traits like latency to emerge were consistent with ecotype, but not correlated with collective behavior.

## Abstract

Comparing populations across replicate environments or habitat types can help us understand the role of ecology in evolutionary processes. If similar phenotypes are favored in similar environments, parallel evolution may occur. Collective behavior, including collective movement and social networks, can play a key role in the adaptation by animals to different environments. However, studies exploring the parallelism of collective behavior are limited, with research traditionally focusing on morphological traits. Here, we asked if collective behavior varies consistently across replicate populations of benthic and limnetic three-spined stickleback (Gasterosteus aculeatus). There were repeatable, population-level differences in collective behavior in a common garden experiment, with some populations forming groups that were more cohesive and with higher strength and clustering coefficients. However, these differences were not predicted by ecotype (benthic vs. limnetic). Latency to emerge and morphology did consistently differ between ecotypes, and there were no correlations between these traits and collective behavior. Together, these results suggest that collective behavior diverges, but not in a way that is associated with variation along the benthic-limnetic axis. By examining multiple traits, we show how phenotypic evolution can be occurring in parallel and non-parallel ways at the same time. This study further highlights that the classification of habitat types may only be relevant for certain traits, with collective behavior potentially changing in a more nuanced manner.

The online version contains supplementary material available at 10.1007/s00265-025-03599-z.

When animals are exposed to different environments, they may develop adaptations that are best fit for their local conditions. Understanding the effect of different environments on various traits including morphology and behavior can help us predict how future environmental changes might impact a species. Here, we compared shoaling behavior and social networks across eight populations of three-spined stickleback fish that tend to occur either in the open water or in the benthos (lake bottom) of lakes. We found that shoaling and social networks were heritable and repeatable, and differed across populations. However, these differences were not predicted by the habitat that the population was from. These findings suggest that behavior may evolve in a more nuanced manner relative to other traits associated with the benthic-limnetic axis. Future studies should explore other ecological conditions beyond this axis that might be driving these behavioral differences.

The online version contains supplementary material available at 10.1007/s00265-025-03599-z.