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

Ami Fadhillah Amir Abdul Nasir, Christofer J. Clemente, Melissa L Wynn and Robbie S Wilson

In studies of animal performance, it’s commonly assumed that a prey animal should run as fast as it can when trying to escape a predator. Yet running too fast can compromise motor control and step accuracy. A recent mathematical model (Wheatley et al. 2015) shows that these mistakes cause prey to slow down, slip, or even fall, increasing its probability of capture.

We tested Wheatley et al.’s model using wild-caught northern quolls (Dasyurus hallucatus), which are squirrel-sized semi-arboreal (tree-living) marsupials. We wanted to understand how fast quolls moved across beams (i.e. branches) of differing widths, and how they traded off the benefits of higher speeds against the risk of slipping or falling when doing so.

We found that mistakes were very costly, with slips and missteps decreasing speed by around 50%; they were more likely to happen when the quolls ran faster and on the narrower beam. To avoid making mistakes, quolls therefore voluntarily reduced their running speed on narrower beams.

Our data provide support for the assumptions and predictions of Wheatley et al.’s model and suggest that animals do not simply run as fast as they can when escaping predators, but consider the situation and substrate when selecting movement speeds. Most previous studies have assumed that prey should run at top speeds during escape, so we hope our work can be used to understand how organisms select speeds in nature.

Read the article in full.

Image caption: Northen quoll.

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