How group behavior reduces fish energy costs

Many animals, including top predators, move in groups. We know that this collective behavior is fundamental to the animal’s ability to move in complex environments, but less is known about what drives this behavior, as many factors underlie its evolution. Scientists wonder, however, whether all of these animals share a basic motivation, such as mating, safety, or perhaps even energy conservation.

“The key word is maybe,” said Yangfan Zhang, a postdoctoral researcher in Harvard’s Department of Organic and Evolutionary Biology (OEB), “because no one has actually measured this or directly compared all groups of animals, mainly because it is difficult to We have a system that can measure not only a group, but also the individuals in that group. But we know that, over the course of evolution, there is some pressure to optimize the efficiency of energy use.

In a new study published in eLifeZhang and co-author Professor George Lauder, also at the EPO and curator of ichthyology at the Museum of Comparative Zoology, wondered whether coordinated group movements of animals moving in a fluid could reduce the energy cost of locomotion.

By combining biomechanics and bioenergetics (simultaneous measurement of metabolic energy consumption and animal movements in a highly specialized experimental platform), researchers not only discovered a significant amount of energy savings, but also identified energy consumption reduced energy per tail beat.

Land vertebrates evolved from fish, changing from fins to limbs and changing respiratory organs from aquatic to air breathing. Despite having different environments and respiratory systems, all vertebrates and fish share the same metabolic pathways to produce energy. One pathway uses oxygen called aerobic metabolism. The other pathway, called anaerobic, is used when oxygen is limited or cannot provide enough energy to move at high speeds.

Together, they contribute to the total energy expenditure of the movement. Fish, however, need to optimize their locomotion more than animals moving in the air or on land. This is because water is a fluid 50 times more viscous than air and requires considerable energy to overcome the fluid’s resistance during movement.

Water also contains five times less oxygen per kilogram than air; meaning that aquatic animals are “squeezed” by a lower ceiling of oxygen availability and have a higher pressure on energy demand.

To test the energetic cost of fish locomotion, Zhang and Lauder designed a sealed water “treadmill” that controlled the speed of the water. By measuring the rate at which oxygen is removed from the sealed “conveyor belt,” the researchers were able to distinguish the rate of oxygen uptake by the animals.

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

The “treadmill” also used two high-speed orthogonal cameras to capture unique locomotion features: one from a side view, the other from below. This made it possible to measure the three-dimensional positions of the fish and allowed the researchers to measure the distance between fish in the school.

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

The researchers found that moving quickly required more energy, but so did moving slowly. However, at an average speed of one body length per second, they found a drop in the energy curve where swimming was at minimum cost, which increased at both faster and slower speeds, exhibiting a relationship J-shaped.

As the most diverse group of vertebrates, fish species have immense cultural and commercial value to human society. However, climate change poses a direct challenge to fish biodiversity.

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

“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 the principles of fluid dynamics and animal locomotion.”

“I think the beauty of this study is that we captured the full spectrum of energy expenditure in a holistic way, which allowed us to take into account a shifting energy cost at high speed,” Zhang said. “Scientists have been studying this question for decades, but we found that the key lies in measuring not only aerobic costs, but also anaerobic costs. This is a huge part of any organism, and without measuring both, you You’ll only get half the story.”