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

1. 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. 2015) 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. 2. We tested several key assumptions and
predictions of Wheatley et al.’s (2015) 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. 3. 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. 4. 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. 5. Our data provide support for
the assumptions and predictions of Wheatley et al.’s (2015) model of
optimal escape speeds, and suggest that animals optimise rather than
simply maximise 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 affect task success.