The Strategy of Predator Evasion in Response to a Visual Looming Stimulus in Zebrafish (Danio rerio)

Synopsis A diversity of animals survive encounters with predators by escaping from a looming visual stimulus. Despite the importance of this behavior, it is generally unclear how visual cues facilitate a prey’s survival from predation. Therefore, the aim of this study was to understand how the visual angle subtended on the eye of the prey by the predator affects the distance of adult zebrafish (Danio rerio) from predators. We performed experiments to measure the threshold visual angle and mathematically modeled the kinematics of predator and prey. We analyzed the responses to the artificial stimulus with a novel approach that calculated relationships between hypothetical values for a threshold-stimulus angle and the latency between stimulus and response. These relationships were verified against the kinematic responses of zebrafish to a live fish predator (Herichthys cyanoguttatus). The predictions of our model suggest that the measured threshold visual angle facilitates escape when the predator’s approach is slower than approximately twice the prey’s escape speed. These results demonstrate the capacity and limits to how the visual angle provides a prey with the means to escape a predator.

ESCAPE TRAJECTORIES IN ANGELFISH (PTEROPHYLLUM EIMEKEI)

Although fish escape trajectories are linearly related to stimulus direction, they vary considerably after the initial turn away from the stimulus. Past studies of escape trajectories in fish and other animals have been analysed by employing linear plots of stimulus angle versus body turning angle. Here, we define escape trajectories as a circular variable, with 0° as stimulus direction. Angelfish (Pterophyllum eimekei) escape in non-random trajectories when the stimulus is presented laterally, within an angular zone of approximately 30–120° (discriminating zone). The circular plot of escape trajectories shows a bimodal pattern that cannot be revealed by linear analysis. Angelfish escape preferentially at 180° and 130° away from the stimulus, maximizing the distance covered from the stimulus and escaping at the limit of their discriminating zone, respectively. Angelfish correct their responses when turning towards the stimulus, suggesting that escape trajectories are modulated by sensory feedback. Re-analysis of published work on other animals, by employing circular histograms of escape trajectories, reveals multimodal patterns which are also not apparent from the linear plots. We suggest that the presence of multiple preferred trajectories may be adaptive in preventing predators from learning any single fixed pattern of response and compensating for it.

How stimulus direction determines the trajectory of the Mauthner-initiated escape response in a teleost fish

Fishes use the Mauthner-initiated C-start for short-latency evasion of predators. C-starts consist of a sudden turn (stage 1) and a rapid acceleration (stage 2). We analyzed high-speed cine films of goldfish C-starts elicited by dropping a ball into the water. It was previously thought that stage 1 angle does not vary concomitantly with the angle of the threatening stimulus relative to the position of the fish. We found, however, a significant inverse relationship between the direction of the impact of the ball and the angle turned by the end of stage 1. When starting near a wall, or when its usual trajectory was blocked by a wall, the fish used an escape route that was not predictable from the stimulus angle. The fish did not appear to correct its trajectory if it began to turn towards the ball. This behavioral evidence supports the previous notion that the underlying neural command is ballistic and does not use sensory information from the stimulus once the movement begins. If this is so, the fish probably utilizes information on obstacle location in the interval leading up to the trigger stimulus.

Three-Dimensional Sets in Estimation of Angles

The hypothesis was that an acute angle preceded by a three-dimensional set will increase the likelihood of overestimation of that angle both absolutely and relative to the same angle preceded by a two-dimensional set or none. 60 subjects, using a mechanical estimator, estimated a 34° stimulus angle from a photograph in one of the three conditions in a one-way analysis of variance design. The three-dimensional set generated significantly larger estimates of the angle.