Preferred gait and walk–run transition speeds in ostriches measured using GPS-IMU sensors

1. What they did

Most of what we know about how animals choose between walking and running comes from treadmills, where the researcher sets a steady speed. In nature, animals speed up, slow down and change gait for many reasons. The authors wanted to know whether ostriches moving freely choose the same speeds that laboratory studies suggest are the most energy-efficient.

• The birds. Seven female African ostriches, hand-raised and 18 months old, living as a flock in a 165 × 120 m grassy paddock.
• The sensors. Each bird carried a small box on their back, strapped over the body’s centre of mass. It held a Global Positioning System (GPS) receiver and an inertial measurement unit (IMU), which records acceleration and rotation hundreds of times a second. Together they weighed less than 0.5% of body mass.
• The recording. The birds roamed freely for 2.5 hours around midday. For about 24 minutes in that time they were chased in short bouts by a quad bike, never closer than 20 m and with rests, so that they would run near top speed.
• Telling walking from running. The authors developed a new method: in walking, the body moves more forwards and backwards than up and down; in running, the up and down movement is larger. They checked this against laboratory measurements of the same birds on force platforms.
• Comparisons. They compared the speeds the birds chose with published energy costs for ostriches and with models of walking, and with what is known for humans and horses.

2. What they found

Steps recorded

• 10,997 walking steps, 21,657 running steps, 926 walk–run transitions and 890 run–walk transitions: about 18 minutes of walking and 16 minutes of running per bird.
• The new method agreed with an energy-based method for 83.5% of walking steps and 89.9% of running steps.

Walking

• Ostriches prefer to walk remarkably slowly, in a narrow range centred on about 1.0 m per second (median 1.03 m/s).
• This matches the speed at which walking costs ostriches least energy, and is consistent with field estimates of walking speed in wild ostriches.

Running

• Running speeds were spread widely, around a median of 4.35 m/s, with a spread nine times greater than for walking.
• This fits earlier findings that ostriches are economical runners, with an energy cost that stays nearly flat across running speeds. Horses, by contrast, have a narrow best speed within each gait.
• The fastest speeds measured were about 11.3 m/s (25.3 mph), with a top stride frequency of 1.95 strides per second.

Changing gait

• Birds switched from walking to running at about 2.05 m/s and from running back to walking at about 1.77 m/s. This small but consistent difference (called hysteresis) was statistically significant.
• Stride length and stride frequency changed abruptly at the switch, which had not been seen in ostriches before.
• Relative to leg length, ostriches switch to running at slower speeds than humans do, and well below the mechanical limit at which a walk must turn into a run. Their switch speeds match those of other ground-running birds.

3. What they concluded

• Ostriches walk very slowly, in a narrow speed range that keeps the energy cost of walking low.
• When running, they use a broad range of speeds, consistent with running being relatively cheap for them across speeds.
• The speed at which they start to run is better explained by saving energy than by a mechanical limit.
• The springy, specialized lower legs of ostriches, a heritage going back to bipedal theropod dinosaurs, probably explain their early switch to running and wide range of running speeds.
• Overall, laboratory measures of gait and energy predict how ostriches choose to move on natural ground. However, like humans, ostriches switch gait at different speeds depending on whether they are speeding up or slowing down, and steady-speed energy costs cannot explain this.
• Collar-mounted loggers are probably unsuitable for gait analysis in birds, because the body pitches during movement.

6. Other technologies, skills or innovations

• Back-mounted GPS and inertial sensing loggers (Royal Veterinary College, third generation) with a GPS base station, GPS sampled 5 times a second and accelerometers and gyroscopes 300 times a second.
• A new, automatic way to classify each step as walking, running or a transition from sensor data alone, consistent across individuals and needing no hand-tuning.
• Laboratory force platforms and high-speed video to check the method.

7. Implications for conservation (from the paper)

• The authors suggest that tracking wild ostriches with GPS and inertial sensors over long periods would help link laboratory findings to how ostriches move while foraging, migrating and escaping predators in the wild.