Animal escapology I: theoretical issues and emerging trends in escape trajectories

Abstract

Escape responses are used by many animal species as their main defence against predator attacks. Escape success is determined by a number of variables; important are the directionality (the percentage of responses directed away from the threat) and the escape trajectories (ETs) measured relative to the threat. Although logic would suggest that animals should always turn away from a predator, work on various species shows that these away responses occur only approximately 50-90% of the time. A small proportion of towards responses may introduce some unpredictability and may be an adaptive feature of the escape system. Similar issues apply to ETs. Theoretically, an optimal ET can be modelled on the geometry of predator-prey encounters. However, unpredictability (and hence high variability) in trajectories may be necessary for preventing predators from learning a simple escape pattern. This review discusses the emerging trends in escape trajectories, as well as the modulating key factors, such as the surroundings and body design. The main ET patterns identified are: (1) high ET variability within a limited angular sector (mainly 90-180 deg away from the threat; this variability is in some cases based on multiple peaks of ETs), (2) ETs that allow sensory tracking of the threat and (3) ETs towards a shelter. These characteristic features are observed across various taxa and, therefore, their expression may be mainly related to taxon-independent animal design features and to the environmental context in which prey live - for example whether the immediate surroundings of the prey provide potential refuges.

Fig. 1.

(A) The geometry of escape trajectories. The shaded area has a radius D_prey, which is the minimum distance the prey needs to cover in order to exit the capture zone, and is the projection of W_t, the width of the predator's capture device. The prey will be safe if it reaches the escape point E before the predator, by travelling at an angle α from the line perpendicular to the predator's attack (i.e. β from the line of attack). The prey reacts to the predator at a distance D_pred. In order to reach E before the predator, the prey needs to cover distance D_prey+d_prey in a shorter time than the time the predator takes to cover D_pred+d_pred [based on fig. 7 from Domenici (Domenici, 2002), reproduced with permission from Taylor & Francis Ltd]. (B) The angles α and β as a function of the ratio of predator speed to prey speed (U_pred/U_prey) [based on fig. 8 from Domenici (Domenici, 2002), reproduced with permission from Taylor & Francis Ltd]. (C) Escape trajectories as a function of prey length, using a predator speed of 1.2 m s^–1 and a reaction distance of 2 cm. The dotted line indicates the optimal ET″, and the boundaries separate areas with different probability of capture P [fig. 10a reproduced with permission from Arnott et al. (Arnott et al., 1999)].

Fig. 2.

The various hypothetical ET distributions relative to a threat (grey arrows at 0 deg). (A) A circular normal distribution (generated with the random number generator at http://www.wessa.net/rwasp_rngnorm.wasp#output, using 180 deg as mean, 30 deg as s.d., N=1000). Each concentric circle represents a frequency of 30. (B) A distribution with manually generated preferred ETs. Each concentric circle represents a frequency of 6. (C) A random, 90–180 deg range-limited distribution (generated with www.random.org/integers/, number from 90 to 180, N=1000). Each concentric circle represents a frequency of 25. (D) A random distribution (generated with www.random.org/integers/, number from 1 to 360, N=1000). Each concentric circle represents a frequency of 10. (E) A circular normal distribution as in E, but directed towards a refuge, independent of threat direction. Each concentric circle represents a frequency of 30. (F) A circular normal distribution as in E, with the mode being a compromise between escaping towards a refuge and escaping directly away from the threatening stimulus. Each concentric circle represents a frequency of 30.

Fig. 3.

Schematic showing the potential effect of successive attacks (predator positioned at 0 deg, red arrows) and ETs on escape probability (represented by the length of the escape vector). (A) An ET with maximum escape probability (green) with other ETs (black) with lower probabilities of escape as a function of their angular distance from the optimal ET. (B) The ET with the highest escape probability is used in all four successive responses. Escape probability decreases as a result of predators learning to anticipate which ET will be used in successive responses. (C) Different ETs (both optimal and suboptimal) are taken in four successive responses; therefore, escape probability does not decrease with successive responses because predators cannot learn which ET will be taken.

Fig. 4.

Percent survival, at different values of ET″, in pike cichlid Crenicichla alta attacking guppies Poecilia reticulata [black squares; based on table 5 from Walker et al. (Walker et al., 2005)] and owl (Tyto alba) attacking an artificially moving prey (a dead mouse or chick) [red triangles; based on Shifferman and Eilam (Shifferman and Eilam, 2004)]. Data from Walker et al. (Walker et al., 2005) are plotted mid-way through the 30 deg range of ET″s observed (i.e. at 15 deg for the 0–30 deg ET″). Data from Shifferman and Eilam (Shifferman and Eilam, 2004) are plotted at the experimental ET″ imposed on the artificial prey.