Kinematics and Performance Capacity for the Concertina Locomotion of a Snake (Coluber constrictor)

1. What they did

When snakes and other limbless animals move through tunnels that are wider than their bodies, they often use concertina locomotion. They bend their body into curves that press against both walls, hold that part still as an anchor, and push or pull the rest of the body forward. Then they anchor again further on. This stop-start movement is known to use a lot of energy.

The authors wanted to know how tunnel width and speed affect how far and how fast snakes can move this way, and how the snakes change their movements to go faster.

• The snakes. Five black racers of similar size, kept warm at a body temperature of 30 °C during tests.
• The treadmill. A circular tunnel that turned like a treadmill, with friction tape on the walls. Foam strips narrowed the tunnel from 7 cm to 5 or 3 cm.
• Endurance trials. Each snake moved at 4.2, 4.9 and 5.6 cm per second in each tunnel width until it failed to keep up three times in quick succession.
• Burst speed trials. The treadmill speed was raised in steps of about 2 cm per second until the snake could no longer keep up. After about four hours of rest the snake was tested again at the fastest speed.
• Video analysis. From video filmed above the treadmill, they followed a paint mark near mid-body and measured eleven variables for each cycle of movement, including time spent still and moving, distance moved forward, speed, the number of anchor points against the walls, and the angle of the body to the walls. One snake was also filmed across a much wider range of speeds, from slow to its top speed.

2. What they found

Endurance and burst speed

• Endurance dropped sharply above the slowest speed. At 4.9 cm per second, snakes kept going for only 17 to 37% as long as at 4.2 cm per second.
• The longest average endurance was 80 minutes, at 4.2 cm per second in the 5 cm tunnel. One snake was stopped at 2 hours.
• In the narrowest (3 cm) tunnel, average endurance at 4.2 cm per second was only 24 minutes, significantly less than in the 5 or 7 cm tunnels. The 5 and 7 cm tunnels did not differ significantly.
• Tunnel width did not significantly affect burst speed. Average burst speeds were 13.5 to 16.8 cm per second, and the fastest single burst was 20.8 cm per second in the 7 cm tunnel.

How the snakes moved

• To go faster, snakes mainly made their movements more often (higher frequency) and moved faster during the moving phase. They did not significantly change how far they moved each cycle, or the share of time they spent still, across the endurance speeds.
• The share of each cycle spent anchored stayed at about 41% on average.
• Narrower tunnels changed body posture. The distance moved per cycle averaged 6.0 cm in the 3 cm tunnel, 8.5 cm in the 5 cm tunnel and 9.9 cm in the 7 cm tunnel.
• Snakes used more anchor points in narrower tunnels: on average about 11 at once in the 3 cm tunnel, 7 in the 5 cm tunnel and 6 in the 7 cm tunnel.
• The curves of the body met the walls at a smaller angle in narrower tunnels: on average 39°, 60° and 86° in the 3, 5 and 7 cm tunnels.
• Tunnel width did not affect speed while moving.
• In the single snake filmed over a wide range of speeds, distance per cycle did increase with speed, and at speeds above about 60% of burst speed the snake often had all of its anchor points in contact at once.

Compared with other movement

• Concertina locomotion was very slow. The same species reaches an average burst speed of 153 cm per second when moving by lateral undulation, reported in an earlier study.
• Black racers were no less effective at concertina movement than other limbless reptiles studied before.

3. What they concluded

• Tread speed and tunnel width affected different parts of the movement: speed mainly changed timing, and tunnel width mainly changed body posture.
• Changing frequency mattered more than changing the size of each movement for going faster.
• Because each cycle includes a stop, moving more often means more stopping and starting, which the authors expect to cost more energy and so reduce endurance.
• Big changes in posture did not always change performance: snakes in the 5 and 7 cm tunnels moved differently but had similar endurance, and burst speed did not change with width.
• Endurance and sprint ability do not necessarily go together.
• The authors call for studies of the side forces snakes apply to tunnel walls, and for comparisons among species with different body structure.

6. Other technologies, skills or innovations

• A circular motorized tunnel treadmill with adjustable width, friction tape on the walls and a variable-speed motor.
• Overhead video at 30 frames per second with a video timer, analysed frame by frame by following a paint mark on the snake’s back.