An animal escaping a predator may be tempted to simply run as fast as it can to escape with its life, but numerous observations suggest that this isn't the case. Indeed, both predator and prey rarely adopt their maximum speeds in the natural world. Quite how and why animals choose their speed remains difficult to understand. However, it is likely that animals have to balance the benefits of high speed with the costs, such as stumbling.

In order to measure how the risk of mistakes affects an animal's speed choice, Ami Nasir and her colleagues from The University of Queensland, Australia, brought wild-caught quolls – agile arboreal marsupials – into the lab. In their neat experiments, the team introduced the timid animals into a Perspex enclosure in which the only means of escape was along a balance beam, the width of which could be manipulated. The researchers videotaped the fleeing quolls, and afterwards analysed how fast they ran and how many mistakes they committed in the process.

Nasir and colleagues observed that the faster the quolls tried to sprint, the more often they slipped. On average, every mistake halved the speed at which the animals navigated the escape beam. But the nimble marsupials were not headless chickens – they ran more slowly across narrower beams. Furthermore, the calculating quolls were able to fine-tune their getaway and put on the brakes for the final two-thirds of the narrowest beam (they sustained a constant, optimal speed on wider beams, however).

The quolls were also able to learn from their mistakes over time. Each individual was tested 10 times on each beam width, and they got faster and faster on the wide and medium-sized bars. But practice did not make perfect for the narrowest beams, on which the quolls continued to struggle. Even with experience, slips cost the quolls speed during the trickiest task.

This smart study has clear relevance for the perilous, arboreal world of snakes and birds of prey in which the quolls live. But it also serves as a metaphor for the balancing act of speed and safety that may dictate how fast animals in general choose to move.

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