Optimal running speeds when there is a trade-off between speed and the probability of mistakes

Summary Do prey run as fast as they can to avoid capture? This is a common assumption in studies of animal performance, yet a recent mathematical model (Wheatley et al. Integrative and Comparative Biology, 55, 1166–1175; ) of escape behaviour predicts that animals should instead use speeds below their maximum capabilities even when running from predators. Fast speeds may compromise motor control and accuracy of limb placement, particularly as the animal runs along narrow structures like beams or branches. Mistakes decrease speed and increase the probability of capture. We tested several key assumptions and predictions of Wheatley et al.'s () model using wild‐caught northern quolls (Dasyurus hallucatus), a squirrel‐sized marsupial carnivore. We quantified the speeds of quolls as they traversed beams of differing width and expected animals should balance the benefits of higher speeds against the increased probability of mistakes when selecting speeds. We first explored whether the probability of mistakes when running along a beam increased at faster running speeds (speed‐accuracy trade‐off) and when the difficulty of a task was greater (narrower beam). In addition, we quantified the costs of locomotor mistakes to test the assumption that mistakes decreased overall running speed. Finally, we tested whether individual northern quolls modulated their running speeds when moving on narrow beams, which would decrease the probability of making catastrophic mistakes. We found quolls were more likely to make mistakes when running faster and on the narrower beam. Locomotor mistakes increased the total time needed to traverse the entire beam, and each mistake decreased average escape speed by around 50%, representing a substantial cost for slips or trips. To circumvent the costs of these mistakes, quolls voluntarily reduced speeds in situations when they are more likely to make a mistake (i.e. narrower beams), thereby allowing them to decrease the total time it took to traverse the beam. Our data provide support for the assumptions and predictions of Wheatley et al.'s () model of optimal escape speeds, and suggest that animals optimize rather than simply maximize speeds when running along challenging substrates. Our work provides a foundation for understanding the movement behaviour of animals when their objective is to escape predatory attacks, and demonstrates that animals should select their escape strategy based on how both the speed of movement and motor control a