Positive effects of fast growth on locomotor performance in pelagic fish juveniles.

Many laboratory experiments on aquatic vertebrates that inhabit closed water or coastal areas have highlighted negative effects of fast growth on swimming performance. Nonetheless, field studies on pelagic fishes have provided evidence of survival advantages of faster-growing individuals. To reconcile this contradiction, we examined the relationship between growth rate and swimming performance as a continuous function for juveniles of chub mackerel (Scomber japonicus) using 3D tracking analysis. For experiments, 20, 24, 27, and 30 days post-hatch individuals within the size range of 14.5-25.3 mm were used. We found that the growth-swimming (burst speed) relationship in chub mackerel was substantially positive and it was supported by morphological traits such as muscle area, which were also positively related with growth rate. This finding is consistent with field observations showing selective survival of fast-growing individuals of this species, reconciling the current contradiction between laboratory experiments and field observations. A dome-shaped quadratic curve described the relationship between growth rate and burst speed better than a linear or cubic function, suggesting that growth may trade-off with swimming performance, as reported in many previous studies, when it is extremely fast. These results, obtained from the rarely tested offshore species, strongly suggests the importance of experimental verification using animals that inhabit various types of habitats in understanding the principles underlying the evolution of growth-locomotor relationship.

A miniaturized threshold-triggered acceleration data-logger for recording burst movements of aquatic animals.

Although animal-borne accelerometers are effective tools for quantifying the kinematics of animal behaviors, quantifying the burst movements of small and agile aquatic animals remains challenging. To capture the details of burst movements, accelerometers need to sample at a very high frequency, which will inevitably shorten the recording duration or increase the device size. To overcome this problem, we developed a high-frequency acceleration data-logger that can be triggered by a manually defined acceleration threshold, thus allowing the selective measurement of burst movements. We conducted experiments under laboratory and field conditions to examine the performance of the logger. The laboratory experiment using red seabream (Pagrus major) showed that the new logger could measure the kinematics of their escape behaviors. The field experiment using free-swimming yellowtail kingfish (Seriola lalandi) showed that the loggers trigger correctly. We suggest that this new logger can be applied to measure the burst movements of various small and agile animals.

Bats integrate multiple echolocation and flight tactics to track prey.

The ability of "target tracking," such as keeping a target object in sight, is crucial for various activities. However, most sensing systems experience a certain degree of delay due to information processing, which challenges accurate target tracking. The long history of studies on animal behavior has revealed several tactics for it, although a systematic understanding of how individual tactics are combined into a strategy has not been reached. This study demonstrates a multifaceted tracking strategy in animals, which mitigates the adverse delay effects with small implementation costs. Using an active-sensing bat to measure their sensing state while chasing natural prey, we found that bats use a tracking strategy by combining multiple echolocation and flight tactics. The three echolocation tactics, namely the predictive control of sensing direction accompanied by adjusting the sensing rate and angular range, produce a direct compensation effect. Simultaneously, the flight tactic, the counter maneuver, assists echolocation by stabilizing the target direction. Our simulation results demonstrate that these combined tactics improve tracking accuracy over a wide range of delay constraints. In addition, a concise rule based on the angular velocity between bats and targets explains how bats control these tactics, suggesting that bats successfully reduce the burden of multitasking management. Our findings reveal the sophisticated strategy in animals' tracking systems and provide insights into understanding and developing efficiently integrated strategies in target tracking across various disciplines.

Multiple preferred escape trajectories are explained by a geometric model incorporating prey's turn and predator attack endpoint.

The escape trajectory (ET) of prey - measured as the angle relative to the predator's approach path - plays a major role in avoiding predation. Previous geometric models predict a single ET; however, many species show highly variable ETs with multiple preferred directions. Although such a high ET variability may confer unpredictability to avoid predation, the reasons why animals prefer specific multiple ETs remain unclear. Here, we constructed a novel geometric model that incorporates the time required for prey to turn and the predator's position at the end of its attack. The optimal ET was determined by maximizing the time difference of arrival at the edge of the safety zone between the prey and predator. By fitting the model to the experimental data of fish Pagrus major, we show that the model can clearly explain the observed multiple preferred ETs. By changing the parameters of the same model within a realistic range, we were able to produce various patterns of ETs empirically observed in other species (e.g., insects and frogs): a single preferred ET and multiple preferred ETs at small (20-50°) and large (150-180°) angles from the predator. Our results open new avenues of investigation for understanding how animals choose their ETs from behavioral and neurosensory perspectives.

When a prey spots a predator about to pounce, it turns swiftly and accelerates away to avoid being captured. The initial direction the prey chooses to take ­ known as its escape trajectory ­ can greatly impact their chance of survival. Previous models were able to predict the optimal direction an animal should take to maximize its chances of evading the predator. However, experimental data suggest that prey actually tend to escape via multiple specific directions, although why animals use this approach has not been clarified. To investigate this puzzle, Kawabata et al. built a new mathematical model that better represents how prey and predators interact with one another in the real world. Unlike past models, Kawabata et al. incorporated the time required for prey to change direction and only allowed the predators to move toward the prey for a limited distance. By including these two factors, they were able to reproduce the escape trajectories of real animals, including a species of fish, as well as species from other taxa such as frogs and insects. The new model suggests that prey escape along one of two directions: either by moving directly away from the predator in order to outrun its attack, or by dodging sideways to avoid being captured. Which strategy the prey chooses has some elements of unpredictability, which makes it more difficult for predators to adjust their capturing method. These findings shed light on why escaping in multiple specific directions makes prey harder to catch. The model could also be extended to test the escape trajectories of a wider variety of predator and prey species, which may avoid capture via different routes. This could help researchers better understand how predators and prey interact with one another. The findings could also reveal how sensory information (such as sound and sight) associated with the threat of an approaching predator is processed and stimulates the muscle activity required to escape in multiple different directions.