Anyone who does cross-country running knows how uneven the real world is, and moving animals have to constantly negotiate bumps and obstacles in their surroundings. But how does size affect the strategy that animals use when encountering objects that impede progress, and which selective pressures have driven the strategies that they use for negotiating these obstacles? Monica Daley from the Royal Veterinary College, UK, explains that an animal's posture could dramatically affect how it manoeuvres across uneven terrain. Small animals that stand with a crouched posture could use a completely different strategy from large upright animals, and Daley wondered which physical factors have driven differently sized animals to select the strategy that works best for them. Explaining that ground-dwelling birds span a mind-boggling range of sizes from heavy weight ostriches down to dainty crouched quail, Daley and her student Aleksandra Birn-Jeffrey decided to find out more about the strategies that birds ranging in size from 0.2 kg up to 117 kg use when running over a step (p. 3786).
However, sourcing many of the animals in the UK, some of which are native to the USA, was trickier than Daley had anticipated. She also recalls that working with the ostriches was particularly challenging. ‘Ola [Birn-Jeffrey] hand-raised these ostriches for 2 years so that they were imprinted on humans so that they could be handled easily and weren't as dangerous’, she says.
Birn-Jeffrey and Yvonne Blum then filmed the birds as they ran over steps that ranged in size from 0.1 to 0.5 times the length of their leg, and also measured the forces exerted by the birds to find out how their bodies moved as they crossed the obstacle. However, after 4 years of bird rearing, and data collection and analysis, Birn-Jeffrey discovered that all of the birds, no matter how large or whether their posture was crouched or upright, negotiated the obstacle using the same strategy. Daley admits that she was surprised. ‘I thought that we would find that the small animals would crouch and extend the leg to buffer terrain variations and maintain a relatively smooth body motion’, she says, adding that she had expected the larger birds to vault up on to the obstacle and down the other side. Instead, all of the birds appeared to launch themselves up on to the obstacle and crouch slightly on top of it before extending their legs as they descended back down. ‘It's a compromise strategy in a sense’, says Daley.
Meanwhile, Daley was also investigating the physical factors – known as task-level priorities – that influence how birds negotiate uneven terrain in collaboration with Jonathan Hurst, Christian Hubicki and Daniel Renjewski from Oregon State University, USA. The engineers built a computer model to simulate the running birds, and Daley says, ‘We used the model to say, here are two alternative “task level priorities”, one of them being to prioritise stability – to try to return to a steady gait as quickly as possible – or alternatively to minimise energy cost’. Simulating heavy birds with long legs and lighter birds with shorter legs, the team found that instead of evolving to prioritise stability when running over an obstacle, the birds' movements were determined by the need to move economically and avoid injury. ‘What you find is that the model does something very similar to birds when you minimise energy cost and control the leg posture at the start of stance to avoid excessive leg loading’, says Daley, who hopes that she and Hurst can now begin to apply the lessons learned from running birds to build stable bipedal robots.