Inter-stride variability triggers gait transitions in mammals and birds

1. What they did

As animals speed up, they switch gaits, for example from a walk to a trot. The most common explanation is that they switch to save energy. But it is hard to see how an animal could sense, from moment to moment, that a new gait would be cheaper. The authors tested a second idea: that animals change gait when their steps become irregular and unsteady, and switch to regain a steady rhythm.

They set out two competing hypotheses. If gaits change to reduce variability, then stride-to-stride variation should be highest just before the switch and drop after it. If gaits change to save energy, then the speed at which animals choose to switch should match the speed at which switching becomes cheaper, and energy use should always drop after the switch.

• New experiments. Virginia opossums and tufted capuchins were trained for about six weeks to walk and trot on a treadmill inside a clear chamber that measured their oxygen use. The speed was raised every 15 seconds to find where each animal switched from a walk to a trot, filmed from the side. For each speed, 30 strides were chosen at random and the variation in stride duration was calculated.
• Earlier data. Data for domestic dogs (northern breeds, hounds and retrievers) came from an earlier study by one of the authors.
• Published data. They added six more species from eight published studies: brush-tailed bettong, Australian water rat, American mink, North American river otter, Svalbard rock ptarmigan and common ostrich. Because raw data are rarely published, they rebuilt data points from reported averages and figures and resampled them.
• Comparisons. For each species they compared the energy cost of moving a kilogram one metre (the cost of transport) and the variation in stride duration, before and after the gait change.

In birds, which do not trot, they looked at the switch from walking to “grounded running”, a running style with no moment when both feet are off the ground.

2. What they found

Energy

• In opossums, capuchins, bettongs and water rats, switching from a walk to a trot or bound significantly lowered the energy cost.
• In domestic dogs, ptarmigan, ostriches and river otters, the energy cost did not change significantly after the switch.
• In capuchins, opossums and dogs, the speed at which switching would save energy was significantly lower than the speed at which the animals actually switched. Mink and ostriches also kept walking past the energy-saving point, while ptarmigan switched before it.
• For river otters, bettongs and water rats, the energy-saving speed did fall within the range of speeds where they switched.
• Of the nine species, only the bettong and the water rat met both energy predictions.

Stride variability

• In all species, variation in stride duration changed with speed, and in almost all species it peaked near the speed where the animals switched gaits.
• In every species except the Australian water rat, variation dropped significantly after the switch.
• In capuchins, opossums and dogs, the speed with the most variation did not differ significantly from the switching speed. In mink, ptarmigan, ostriches, otters and bettongs, the peak in variation fell within the switching range far more often than expected by chance.
• The water rat was the exception: its peak in variation fell within the switching range less often than expected.
• The pattern held across birds and mammals, across body sizes from 0.67 to 114 kg, and across walk to trot and walk to bound transitions.
• Data from wild ostriches, from another study, also showed a large spike in stride variation just before grounded running began.

3. What they concluded

• Energy savings do not reliably predict when mammals and birds change gait.
• Instead, animals seem to switch gait when their strides become too variable, moving into a steadier, more rhythmic and more predictable way of moving. This may lower the risk of limbs striking each other, tripping or falling.
• Saving energy is an important benefit of changing gait, but it is probably secondary. The trigger is likely the loss of steadiness. Irregular strides may also cost more energy, which links the two ideas.
• Birds and mammals have probably each evolved ways of sensing stride steadiness, and this is worth further study.
• Limitations given by the authors: rebuilding published data rests on assumptions that are hard to check; almost all data come from treadmills, not natural settings; and only the switch from walking to a faster gait was studied, not the trot to gallop or the trot to walk change.

6. Other technologies, skills or innovations

• Treadmill inside a clear Plexiglas metabolic chamber, with open flow respirometry (Field Metabolic System, Sable Systems International) measuring oxygen use and carbon dioxide output.
• Side-view video to measure the share of each stride a foot is on the ground (duty factor) and to confirm the gait near the switching speed.
• A method for rebuilding and resampling data from published studies (DataThief III to read values from figures, simulated data points and jack-knife resampling in MATLAB).