Fish use collective behavior to reduce energy costs

Researchers have made a major breakthrough in understanding how collective movement in fish leads to a reduction in energy costs. The study sheds light on a fundamental aspect of group behavior among animals.

Collective movement, observed in a plethora of species ranging from the smallest of fish to the mightiest of terrestrial predators, has long been recognized as a crucial element in navigating complex environments. 

What drives collective movement?

The underlying motivations driving collective movement have remained largely speculative, with theories suggesting a variety of evolutionary pressures including mating, safety, and energy conservation. 

Study lead author Yangfan Zhang is a postdoctoral researcher in the Department of Organismic and Evolutionary Biology (OEB) at Harvard University.

“The keyword is perhaps, because no one has actually measured this and compared it directly across all animal groups, mainly because it’s difficult to have a system that can measure not just a group, but individuals in that group. But, we do know that, evolutionarily, there is some pressure to optimize for efficiency of energy use,” said Zhang.

Focus of the study 

The researchers set out to investigate whether synchronized movements in aquatic environments can minimize the energy expenditure associated with locomotion.

They designed a sealed water “treadmill” that controlled water velocity. To calculate the rate of oxygen uptake by the animals, the experts measured the rate at which oxygen was removed from the treadmill.

“The system is designed to have the measuring sensitivity to capture the energetic cost of an individual fish compared directly to the cost for a group of eight fish,” explained Zhang. “By standardizing the biomass of the fish in the water treadmill with controlled water velocity, we can directly compare the cost of swimming between fish schools and an individual fish.”

Key findings

The study revealed that groups of fish exhibit significantly lower energy expenditure per biomass compared to solitary swimmers. The optimal speed seemed to be not too fast and not too slow. This median pace has been previously observed in migratory species, suggesting an evolutionary advantage.

“What we discovered is that the total cost for the group to move as a whole is much lower per biomass compared with an individual, and the group expended the least amount of energy at a median speed of one body length per second,” said Zhang. “When we look at studies that track wild animals, we find that a lot of animals migrate at a speed of around one body length per second.”

Broader implications 

The survival of fish species, which hold immense cultural and commercial value, is increasingly threatened by climate change and the ensuing shifts in biodiversity. 

Understanding the energetic efficiencies of collective movement becomes not just a question of biological interest but a critical factor in predicting the future dynamics of fish populations under environmental stress.

Critical new insights 

The researchers noted that moving quickly required more energy, but so did moving slowly. However, at a medium speed of one body length per second, they saw a dip in the energetic curve where swimming was at a minimum cost.

“Projections on the future abundance of fish species cannot be based only on the biology of the individuals, we also need a fundamental understanding of collective movement that accounts for the interactions among the individuals within a group,” said Lauder.

“Studying the energetics of aquatic locomotion under environmental constraints offers insight not only into highly conserved features of vertebrate physiology, but also into the inner workings of fluid dynamics principles and animal locomotion.”

Study significance 

“I think the beautiful thing about this study is that we captured the full spectrum of the energy expenditure in a holistic way that enabled us to account for an energy cost moving at high speeds,” said Zhang. 

“Scientists have been looking at this question for decades, but we found that the key lay in measuring not just the aerobic, but also the anaerobic costs. That is a huge part for any organism and, without measuring both, you get only half the story.”

The study is published in the journal eLife. 

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