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Energy efficiency plays a crucial role in shaping animal behavior and driving evolutionary changes. The remarkable variety of life we see today highlights the diverse biological strategies developed to obtain and manage energy. However, quantifying energy consumption remains a significant challenge, making empirical analysis complex.
Animals utilize energy for numerous functions, from growth to cognitive activities, but one of the most significant energy drains is movement. For many highly mobile species, examining movement provides valuable insights into overall energy expenditure.
Existing methods for assessing movement and corresponding energy expenditure are highly effective yet constrained by the size of the equipment involved. Recent research published in the Journal of Experimental Biology by a team from the Marine Biophysics Unit at the Okinawa Institute of Science and Technology (OIST), in conjunction with Professor Amatzia Genin of the Hebrew University of Jerusalem, presents an inventive technique for calculating energy usage during movement leveraging video analysis and 3D tracking through deep learning. “Until now, the most effective methods were often dismissed for approximately half of the world’s species due to dependence on wearable devices,” states Dr. Kota Ishikawa, the lead author. “Our video approach enables a broader examination of energy use within animal behavior and ecological contexts.”
Dynamic Body Acceleration (DBA) has traditionally been the preferred method for estimating energy expenditure associated with movement. DBA involves measuring the oxygen consumption of a specific species engaged in particular behaviors in a controlled environment, providing insight into energy usage. Oxygen serves as a reliable biomarker for energy since it is utilized in aerobic respiration to generate ATP, which powers most physiological activities, including muscle function. Acceleration data is concurrently collected using an accelerometer, and due to the strong relationship between acceleration and oxygen consumption, DBA is a dependable estimator of energy use.
Once the laboratory standards are established, DBA is measured in natural settings using wearable accelerometers, a challenging undertaking due to the difficulty in measuring oxygen consumption accurately in the field. However, reliance on physical devices creates significant limitations. “To ensure that measurement equipment doesn’t interfere with animal behavior, researchers opt for devices weighing no more than one-tenth of the subjects’ body weight. As a result, accelerometers and battery packs weighing 10-20 grams prevent the study of animals smaller than 100 grams — representing almost half of all vertebrate species. Additionally, the presence of a weight can alter movement patterns, especially in aquatic or avian species where drag is a factor,” explains Dr. Ishikawa.
The researchers’ solution is both innovative and straightforward. Instead of utilizing accelerometers, they employ two cameras to capture video from different angles, recreating the animal’s movement (demonstrated with a damselfish swimming in a tank) in three-dimensional space. Selected frames from the footage are used to train a deep learning neural network designed to follow the motion of distinct body features, such as the eyes, facilitating accurate measurements of movement-induced acceleration.
This video-based DBA technique allows for energy usage estimation both in natural environments and laboratory settings. It also opens up new possibilities for studying collective behaviors. “The energy dynamics of schooling fish have long been elusive,” Dr. Ishikawa notes. “For instance, are the leading fish expending more energy? And does schooling translate into more efficient movement? These insights may shed light on the ecological and evolutionary significance of fish grouping behavior.”
The introduction of video-based DBA paves the way for comprehensive energy consumption analysis during the natural behaviors of smaller vertebrates, potentially unlocking a wealth of research opportunities to explore the rich tapestry of life on our planet.
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