Northern quolls, small carnivorous marsupials native to northern Australia, have been wiped out in many parts of their original range. Predation by cats, foxes and dogs is partly to blame. That's why it's important to understand which factors determine quolls' ability to escape predators, says Robbie Wilson, a performance biologist at the University of Queensland, Australia. And that may involve some counterintuitive ideas. Quolls might be expected to run as fast as they can when trying to escape capture. ‘But this ignores the fact that there are huge costs for running at your fastest speeds', says Wilson. ‘It's likely that an animal running at top speed will not be able to turn or manoeuver as quickly.’ To explore these trade-offs, Wilson teamed up with Melissa Wynn, Christofer Clemente and Ami Fadhillah Amir Abdul Nasir. ‘We wanted to see how running speed constrains an animal's ability to make rapid turns, which are likely to be very important for escaping predators, and what morphological attributes determine how well an animal can turn’, Wilson explains (p. 433).
When studying an endangered species, the first challenge is finding enough animals. Luckily, Groote Eylandt, an island off Australia's northeastern coast, remains a quoll stronghold. Using baited traps to capture 66 quolls, the team then put the marsupials through their paces in a specially designed racetrack consisting of two moveable channels joined with one of three removable angle structures (45, 90 and 135 deg) that allowed the team to challenge the animals' manoeuvrability. Then they chased quolls along the track with a polystyrene block and filmed their actions from above using a high-speed digital camera to explore how running speed affected the animals' chances of crashing when running around a corner.
As the team expected, quolls crashed more frequently when they approached a corner at high speed and when running around tighter corners. Then, knowing that higher running speeds increased the likelihood of the animal slipping, the team reasoned that quolls should choose to run more slowly when approaching a sharp corner, but do they slow down when negotiating tight turns in practice? It turns out that they do. When the team analysed footage of the running animals, they found that average turning speed decreased as the magnitude of the turn angle increased, and when negotiating 135 deg turns, quolls decreased their running speeds to around a third of their straight-line speeds. Finally, the team wondered how an animal's build may affect its agility. ‘We expected that larger animals would find it harder to get around tighter corners’, says Wilson, because they have to overcome greater angular momentum. But to their surprise, the team found this was not the case. Instead, they found that foot length was the strongest predictor of manoeuvrability. ‘It appears that it's the foot size of the animals that constrains their ability to negotiate a corner successfully’, Wilson explains. ‘The larger the foot size, the greater their grip.’
The team concludes that an animal's running speed constrains its ability to make quick, rapid turns. ‘This means that an animal should modify its speed when running away from a predator, optimising its combination of speed and manoeuvrability to maximise its chances of success’, says Wilson. In other words, the quolls that are most likely to survive attacks by introduced predators are those that master the optimal compromise between speed and agility.