However, the mechanism by which bipedalism increases predator evasion ability has not been identified. Previous research has suggested that bipedal locomotion increases predator evasion ability with respect to sympatric quadrupedal rodents 14, 15. In contrast, obligate bipedal locomotion has convergently evolved in desert rodents that are hunted via ballistic interception by owls and snakes 5, 6, 13.
Transient bipedal locomotion is often associated with prey escape trajectories, and is performed by a variety of terrestrial animals, including lizards and cockroaches when chased, to achieve high running speeds 12. We demonstrate the utility of this method by examining whether bipedal locomotion increases predator evasion ability in desert rodents via increasing trajectory unpredictability. We present the first quantitative method to measure the unpredictability of motion in three-dimensional space by calculating the differential entropy of animal trajectories. Rather, previous studies of prey escape trajectories are limited to qualitative descriptions of “evasive maneuvers” 6, 7, 8 or measurements of variance in speed or direction 9, 10, 11, neither of which is a comprehensive measure of unpredictability. Although the notion of unpredictability has been formalized in the field of Information Theory, this concept has not yet been applied to quantitatively characterize animal locomotion. Therefore, increasing the unpredictability of prey trajectories likely increases the chance of evading a predator’s ballistic interception 2, 4, 5. On the other hand, predation based on ballistic interception requires the prediction of prey movement to plan a predator strike. Often, the successful animal in a pursuit is the one with greater speed or endurance. For example, maintaining high uniform velocities can be useful for prey that are hunted by predators using a simple pursuit strategy 3. This mechanistic understanding of predator–prey interactions enables the inference of predator evasion ability, even without direct observation of predation events. The evasive success of prey locomotion can be defined in the context of the predation strategy encountered 2. However, understanding the aspects of locomotion that enhance predator evasion ability makes it possible to predict prey success in broader contexts, especially where direct observation of predator-prey interaction may not be feasible. Convergent locomotor behavior in prey species can indicate a broadly successful evasion strategy, and the success of a particular evasive maneuver can be measured by directly observing predator–prey interactions 1. Locomotion is an essential tool in the evolutionary “arms race” between predator and prey. Our unpredictability metric expands the scope of quantitative biomechanical studies to include non-steady-state locomotion in a variety of evolutionary and ecologically significant contexts. Consistent with this hypothesis, jerboas exhibit lower anxiety in open fields than quadrupedal rodents, a behavior that varies inversely with predator evasion ability. In field-based observations, jerboa trajectories are significantly less predictable than those of quadrupedal rodents, likely increasing predator evasion ability. Unlike the speed-regulated gait use of cursorial animals to enhance locomotor economy, bipedal jerboa (family Dipodidae) gait transitions likely enhance maneuverability. We then apply the method by examining sympatric rodent species whose escape trajectories differ in dimensionality.
We present methods based on the entropy measure of randomness from Information Theory to quantitatively characterize the unpredictability of non-steady-state locomotion. However, such movements are poorly characterized by existing biomechanical metrics. Predator evasion, a behavior that enhances fitness, may depend upon short bursts or complex patterns of locomotion. Mechanistically linking movement behaviors and ecology is key to understanding the adaptive evolution of locomotion.
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