To avoid predation, many animals are required to appropriately switch between immobility for crypsis and fleeing for escape. We conducted two staged-encounter experiments using a frog and a snake to examine factors that affect the occurrence of immobility and fleeing, and to evaluate the efficiency of them. The first experiment demonstrated that frogs initially exhibit immobility, when snakes are moving at a long distance, and then switch from immobility to fleeing at a shorter distance even when snakes have not detected them. The second experiment demonstrated that snakes at 400-800 mm distance detect only fleeing frogs, whereas snakes at 100 mm or closer detect both immobile and fleeing frogs. Thus, the ability of snakes to detect motionless frogs depends on the distance, and the distance-dependent switching can be considered an adaptive strategy of the frog. However, a previous model predicts that cryptic prey should flee immediately on seeing a predator or not flee until being detected by the predator. To explain this discordance, we propose two factors: engagement of intensive searching mode by predator at short distance and effects of sudden fleeing at close distance. We suggest incorporating them in future theory for better understanding of anti-predator strategy.

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.

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.

Animal-borne accelerometers are effective tools for quantifying the kinematics of animal behaviors, such as swimming, running, and flying, under natural conditions. However, quantifying burst movements of small and agile aquatic animals (e.g., small teleost fish), such as during predatory behavior, or while fleeing, remains challenging. To capture the details of burst movements, accelerometers need to sample at a very high frequency, which will inevitably shorten the duration of the recording or increase the size of the device. 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 animal 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 (i.e., body beat cycles and maximum acceleration values). The field experiment using free-swimming yellowtail kingfish (Seriola lalandi) showed that the loggers trigger correctly (i.e., of the 18 burst movements, 17 were recorded by the loggers). We suggest that this new logger can be applied to measure the burst movements of various small and agile animals, whose movements may be otherwise difficult to measure.