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Locomotion is an important behavior in the life history of animals and is characterized by discrete gaits, which may be adopted for optimal energetic efficiency, fatigue resistance, or maneuverability. We evaluated the kinematics and electromyography of Bluegill Sunfish (Lepomis macrochirus) swimming at different gaits to evaluate which factors might influence gait choice. When placed in the flume, Bluegill adopted a steady swimming gait until speeds reached 2.0 BL/s. When swimming volitionally, either in a laboratory pool or the field, Bluegill adopted an intermittent swimming gait (burst phase followed by a glide phase) and swam at average speeds of 1.0-1.3 BL/s. No statistical relationship was found between the kinematics of the burst and glide phases in either the lab or the field, so the phases were considered uncoupled. Furthermore, since the kinematics (tailbeat frequency, glide-duty factor) of lab and field volitional swimming were statistically identical, the EMGs of volition swimming in the lab likely reflect field effort. When relativized to volitional swimming speeds, the EMG intensities for both gaits were statistically identical. These results suggest that intermittent swimming may not reflect a strategy for energetic efficiency. Instead, the decoupling between the burst and glide phase may improve maneuverability, since 75% of 3D tracked intermittent swimming bouts (n=129) in the field involved a directional change. Although previous research suggests that intermittent swimming may also provide fatigue resistance, we hypothesize that intermittent swimming evolved in Bluegill as an adaptive gait for navigating their densely vegetated habitat. 

Field observations of animal behavior are essential for guiding the interpretations of laboratory data in order to ensure that they coincide with biological reality. Knowing how an organism behaves in its natural environment is a necessary first step in bridging the gap between experimental data collected in the controlled, artificial environment of the lab and explaining the adaptive significance of measured traits. Field observations also challenge assumptions about behavioral definitions and the apparent discreteness of behaviors measured in the lab. As part of an ongoing study in the locomotor performance of Bluegill Sunfish (Lepomis macrochirus), we illustrate the role field observations play in contextualizing and expanding interpretations of experimental data and standard assumptions about Bluegill behavior. A comprehensive field study of Lake Waban (Wellesley, MA) and its inhabitants was carried out using underwater cameras, fish finding sonar, and temperature/luminosity loggers to develop a behavioral profile of Bluegill relative to their habitat and interspecific interactions. Although previous experimental work assumed Bluegill adopted locomotor strategies that maximized energy efficiency, field observations demonstrate that swimming performance is driven by a myriad of abiotic and biotic factors. These factors include the need to navigate complex habitats, to flee from predators, to adopt context-specific foraging strategies, to ward off rivals, and to coordinate social interactions. These observations add an extra dimension for understanding why Bluegill adopt particular swimming behaviors and how those behaviors might be adaptively significant at each stage of their life history.