Pick on someone your own size! Arctic grayling (Thymallus arcticus), do.

Abstract

Conflict and conflict resolution are processes that are integral to social evolution. Processed that are almost as old as multicellular life, driving evolution and major transitions in the history of life, such as eusociality (Bourke, 2023). Eusociality is at the extremes, as there is seemingly no conflict between conspecifics, due to process such as conflict dissolution (González-Forero and Peña, 2021). Despite eusociality arising in insects (famously, bees and ants), arachnids, crustaceans and mammals, it has never been found in the most diverse vertebrate group, fish (Stiefel, 2013). This is likely because aquatic environments are not conducive to building nests, which are key to defending and repeated feeding of offspring, the functional precursor of eusociality (Ruxton et al., 2014).

Despite fish not displaying eusociality, they still exhibit a wide variety of intraspecific conflict and conflict resolution. Whether that be the conflict between sexes leading to dazzling male morphology in live-bearers (Poeciliidae) (Furness et al., 2019) or the conflict over limited resources leading to egalitarian behaviours in cooperatively breeding cichlids (Fischer et al., 2024). These examples come from smaller freshwater species, which can be reared in laboratory settings with relative ease, making it easier to observe behaviour and construct conflict scenarios.

In this issue, however, Holubová et al. (2024) break free from fish that are popular in home tropical freshwater aquariums and assess conflict in the wilds of an Alaska River, focusing on the feeding aggregations of Arctic grayling (Thymallus arcticus). The river in question exhibits extremely low turbidity during the summer months, allowing for excellent visibility, which was combined with cutting edge three-dimensional underwater videography.

What Holubová et al. (2024) found from their footage is the perfect example of game theory in action, predicting the outcome of territorial conflict. Specifically, aggression between individuals in the dominance hierarchy of a grayling aggregation only occurred when conspecifics were of a similar size (±10cm), showing that challenging dominant individuals with the best positions in the river is only worth the risk when the potential reward outweighs the cost. However, winners of bouts were significantly larger than the losers, suggesting that the grayling could have imperfect size assessment of conspecifics, or that the reward of the conflict was worth the risk from smaller individuals, perhaps when prey may be scarce.

The results also showed a clear initiator advantage, irrespective of body length, with initiators up to 5cm shorter than receivers showing success. Initiating behaviour is, therefore, an important factor, and can be influenced by factors such as hormones (Neregård et al., 2008; Suter and Huntingford, 2002).

The results from this study illuminate the amazing conflicts, struggles, tradeoffs and strategies playing out every day, below the water line. It also demonstrates the value of assessing non-model organisms in their natural environment, capturing the fantastic diversity that fish are renowned for.