People's responses to social situations varies considerably according to their personality type. We span the spectrum from loners to social butterflies and everything in between. And the same is true of other animals. Depending on their lifestyle, they may either be stressed or fulfilled by socializing. Megan Currier and colleagues from Widener University in Pennsylvania, USA, aimed to better understand this idea in wild animals by measuring the energy costs associated with movement in cooperative, socialite fish or antagonistic, loner fish.

Currier and colleagues suspected that the high energy investment needed to achieve movement may push some animals to maximize their efficiency through the generations. Using a fish species that is social (bluegill sunfish Lepomis macrochirus) versus a competitive fish with strong social hierarchies that can be found swimming alone (rainbow trout Oncorhynchus mykiss), the researchers set about figuring out how these animals maximize efficiency as they move through their habitat. Individuals could use what is known as ‘entrainment swimming’, in which they essentially hitch a free ride by swimming in the turbulence kicked up by obstacles such as rocks or chunks of wood in the flow. Alternatively, the fishes could take advantage of the benefits of swimming in schools, when the leader of a group essentially generates comparable turbulence through the swishing of its tail, which fishy followers can exploit to reduce their own energetic costs when moving.

To test these ideas, Currier and colleagues employed a swim tunnel, also known as a fish treadmill. This contraption allowed the researchers to pump up the water flow to test how the fish optimize their swimming efficiency, by measuring the amount of energy they consumed and the rate of tail swishing. For the social bluegill sunfish, they compared the swimming efficiency of groups of three fish with that of individual swimmers, at 4 different swim speeds (ranging from 25.5 to 51 cm s–1 for the 17 cm long fish). For the competitive rainbow trout, they examined the swimming efficiency of individuals versus 4 group sizes (composed of 2–8 fish), testing the fish's swimming efficiency with and without a cylindrical obstacle in the flow – for the fish to shelter behind – at a swim speed of 63 cm s–1 for the 21 cm long fish. The researchers suspected that fish would use whichever strategy (either schooling or entrainment swimming) that best suited their lifestyle.

And they were absolutely right. The more cooperative and social sunfish greatly reduced both the energy consumed and their rate of tail swishing when they could do it together in a school, rather than alone, regardless of how fast the flow pumped. For the more competitive and reclusive trout, group swimming actually increased their energy use when moving around, instead of making their movement more efficient, possibly because of the stress induced by the social setting. On the flip side, rainbow trout reduced their movement costs by sheltering behind a static obstacle in the flow, taking up positions in the eddies generated by the water's flow around the obstacle. This strategy allowed the trout to reduce both their energy consumption and rate of tail swishing.

Studies like this highlight the importance of taking an animal's lifestyle into account when speculating how it will respond to animate objects (like its buddies) and inanimate objects (think rocks) in its environment. Like humans, some animals will be more or less social naturally and, as such, will be more or less inclined to use sociality to enhance their own efficiency. Social distancing in the ongoing COVID-19 pandemic is becoming a prolonged way of life and this study shows that keeping your distance likely impacts some individuals (like the social butterflies) more than others (such as the loners). So, some species achieve more together, but being in a crowd can set others back.

D. J.
Group swimming behaviour and energetics in bluegill Lepomis macrochirus and rainbow trout Oncorhynchus mykiss
J. Fish Biol.