When humans and other animals walk, jog or run, most fall into a comfortable pace that they can sustain for lengthy periods. Monica Daley, from the Royal Veterinary College (RVC), UK, explains that animals appear to conserve energy when moving, selecting the most efficient style of movement – gait – that matches the speed at which they travel. However, most of the studies that have investigated why animals switch from one gait to another have analysed the movements of animals on laboratory treadmills. ‘We wanted to explore whether we could start trying to bridge the gap from laboratory work to field-based study’, says Daley, who is keen to understand how animals select their movements in their natural environments. However, instead of choosing to analyse the movements of a diminutive species with an even temperament, Daley, Anthony Channon and undergraduate Grant Nolan opted to investigate the movements of a flock of ostriches.
Despite the bird's fearsome reputation, Ola Birn-Jeffery had become mother to a flock of 12 ostrich chicks during her PhD at RVC, and Daley recalls how local ostrich farmer, Scott Dyason, had trained Birn-Jeffery in ostrich wrangling before she took charge of the youngsters. However, the ostriches weren't always as cooperative as Daley would have liked. Although they were content for Channon and Nolan to strap boxes containing the GPS tracker and accelerometers to their backs and to saunter around the enclosure, the team could never persuade them to go faster than a gentle trot, even though they can easily outrun a human. Eventually, Channon and Nolan resorted to chasing the ostriches on quad bikes to get them to shift into top gear.
Having finally recorded the ostriches roaming around their paddock over their full range of speeds, Daley was faced with the task of analysing 30,000 strides. And when the server carrying the colossal data set crashed, the project was in serious jeopardy until Jade Hall painstakingly pieced the information back together. Despite the setbacks, Daley eventually identified that the switch from a walk to a run was characterised by a specific shift in the accelerometer signals, which allowed her to reliably identify the transition and specify exactly how the birds were moving at all times.
Analysing the movements, she noticed that all of the birds walked over a very narrow range of slow speeds around 1 m s−1, which was surprising, because humans (another biped) walk over a much wider range of speeds. In contrast, the birds’ running speed range was immensely broad, 1.4 m s−1– 11.3 m s−1. Daley explains that most animals have a narrow range of speeds over which they walk or run and these speeds occur when the energetic cost of the movement is lowest. However, the energy cost of running remains remarkably constant over a wide range of speeds for the ostriches, and Daley says, ‘Ostriches are exceptionally well specialised for economic running’.
In addition, Daley analysed the speeds at which the birds shifted up into a run or broke into a walk as they slowed, and she was intrigued to see that they were different. ‘If the animals are choosing gaits solely based on energy costs, you wouldn't expect this gait transition hysteresis’, says Daley, who suspects that other factors contribute to the animals’ choice of movement as they speed up and slow down. However, she admits that she is pleased that the measurements of the ostriches’ preferred speeds as they roam around agree well with those that have been collected in the laboratory.