Birds on the run: what makes ostriches so fast? | www.scienceinschool.org
Birds on the run: what makes ostriches so fast?
Nina Schaller
What makes ostriches such fast runners? Nina Schaller has spent nearly a decade investigating.
Image courtesy of John
Carnemolla / iStockphoto
When admiring a soaring seagull or a diving penguin, we rarely consider that these feathered animals have something very rare in common with us: whereas most other animals move on four, six or more legs, birds and humans are the only true bipeds.
Evolution has solved the challenge of moving on two legs in two ways: humans are plantigrade (we place our entire foot on the ground when we walk or run), whereas birds are digitigrade (they walk on their toes, or digits).
Some avian species can run more quickly not only than humans run, but even faster than their flying counterparts fly. The fastest long-distance runner is the African ostrich (Struthio camelus). At a steady 60 km/h with top speeds exceeding 70 km/h, it could cover the 42 km Olympic marathon in 40 minutes rather than the two hours needed by a human. This remarkable combination of speed and endurance allows the ostrich to cover great distances to find fresh grazing pasture or to outdistance hungry hyenas.
Nina and one of her full-
grown co-workers, Tiffy
Image courtesy of Gisela Löffler
for Bild der Wissenschaft
Scientists have long explored the challenges of terrestrial locomotion, particularly the running abilities of dogs and racehorses. However, studies on avian locomotor modes have typically explored flight dynamics while paying less attention to cursorial species (those that are specialised for running).
10-week-old ostrich chicks
in their new home
Image courtesy of Jürgen Gass
After finishing my degree in biology in 2002, I volunteered at Frankfurt Zoo in Germany, where I became fascinated by the ostrich’s racing ability and decided to investigate it. The hypothesis of my PhD research was that the ostrich locomotor system transmits power to the ground with a high degree of efficiency, maximising energetic output (speed and endurance) while minimising energy demands (muscular and metabolic work).
To test my idea, I decided to study both form and function of the ostrich locomotor system. Using dissection, I explored ostrich anatomy, searching for specialised limb structures that might reduce the metabolic cost of locomotion. Simultaneously, I studied the biomechanics of live ostriches: how physical forces acted on their anatomy when they moved.
To enable close observation of natural motion sequences, I hand-raised three ostriches in a large outdoor enclosure and, over four years, habituated them both to me and their experimental racetrack. Mutual trust was crucial: a kick from an ostrich can kill a lion.
Maximising speed: long, light legs
In a running animal, higher speeds are achieved by increasing both the length and frequency of steps. Longer legs can swing further, and if the leg’s muscle mass is located proximally (close to the body), the leg can then swing faster, in the same way that moving the adjustable weight of a metronome closer to the pivot increases the tempo.
Figure 1: Linear leg segment lengths; n indicates number of specimens examined.
Swinging portion of the leg:
Thigh bone (held horizontally): dark blue
Shin bone: pale blue
Image courtesy of the Maungatautari Ecological Island Trust; image source: Wikimedia Commons
The greater roadrunner, Geococcyx californianus
Image courtesy of Back1a5h; image source: Wikimedia Commons
Cassowary feet
Image courtesy of Bjørn Christian Tørrissen; image source: Wikimedia Commons
To investigate this principle, I compared leg segment lengths ( Figure 1 ) and muscle mass distribution of fast-running, ground-dwelling bird species. Of all cursorial birds, the ostrich possesses the longest legs relative to its size and has the longest step length when running: 5 m. In addition, to a greater degree than other bird species, it has the majority of its leg musculature located very high on the thigh bone and hip, whereas the lower swinging elements of its leg are comparatively light, moved by long, mass-reducing tendons (Figure 2). This arrangement optimises the ostrich leg for high-velocity locomotion, giving it both a long step length and a high step frequency.
Figure 2: Comparative anatomy of human and ostrich legs: in birds, the bone between ankle and toes, the tarsometatarsus, is much longer than in humans, and serves as a functional equivalent to our upright shin bone. The bird’s ankle joint is at our knee level, which explains why a bird appears to flex its ‘knee’ backwards. Its actual knee joint, hidden under plumage, is permanently flexed and connects to the hip joint through a short, horizontal thigh bone. Red lines connect anatomically equivalent joints; green lines connect functionally equivalent joints.
In the ostrich, muscle mass is concentrated at the top of the leg, while long tendons prevail in the lower regions. Shaded areas show major muscle mass distribution, blue lines indicate location of major tendons. Click on image to enlarge
Image courtesy of Nina Schaller
Figure 3: Experiment
Image courtesy of Nina Schaller
Maximising endurance: stable joints
A wide range of joint motion allows humans to climb trees or ballet dance, but this flexibility has a cost. When we run, muscle power is used for propulsion but also to prevent sideways joint movement, thereby increasing our energy requirements over a given distance. I suspected that ostriches had a more efficient approach.
Unlike energy-consuming muscles and their tendons, ligaments can act as a ‘joint corset’, limiting sideways motion without consuming energy. To demonstrate that this mechanism was present, I filmed my running ostriches from various angles to record the range of motion of their legs. I then repeated these measurements with an intact dead ostrich, and finally with a dissected ostrich leg that had had all muscles and tendons removed, leaving only the skeleton and joint ligaments. The range of sideways motion in live and dead ostrich specimens was nearly identical. In contrast, a similar comparison in humans would reveal a huge difference in sideways motion range, especially at the hip joint, which is stabilised by muscle action. My measurements showed that ligaments are the main elements that guide an ostrich leg through the stride, allowing muscle power to be devoted almost exclusively to forward propulsion.
When manipulating the dissected ostrich legs, I made a further new discovery. When trying to flex the ankle joint, I had to overcome some resistance – an unexpected finding in a lifeless limb devoid of muscles. When I released the joint, it snapped back to an extended position, suggesting that ligaments were passively keeping the bird’s leg extended. To test this theory, I exerted pressure from above on the standing, dissected leg until the ankle joint collapsed into a flexed position (Figure 3). It required 14 kg of downward force — 28 kg of weight that an ostrich standing on two legs would not be required to actively support when walking or running. This experiment showed that saving metabolic energy by using ligaments as a passive leg-stabilising mechanism is an excellent locomotor endurance strategy.
Making ground contact
the body. Click on image to
enlarge
Image courtesy of Nina Schaller
We have seen that light limbs are a precondition for fast, efficient locomotion and that one way in which the ostrich achieves this is by concentrating the leg muscle mass close to the hip joint. A further strategy for reducing lower-leg mass involves specialised toe morphology and positioning. This can also be observed in other cursorial animals; modern horses, for example, have evolved from five-toed ancestors to gallop on the toenail of their middle toe – the hoof. The ostrich has undergone a similar evolution: whereas most birds have four toes and the majority of large flightless birds possess only three, the ostrich is unique among birds in walking on only two toes ( Figure 1 ). Furthermore, it is the only bird to walk on the tips of its toes.
I wondered how this, the largest and heaviest living bird, manages to balance and grip at high-speed on tiptoe. Since there is no established method for investigating toe function in live birds, I used a pressure plate, commonly used by orthopaedists to analyse pressure distribution in human feet. I trained my ostriches to walk and run over the plate, capturing high-resolution real-time data of ostrich ‘foot’ pressure during ground contact. This showed that the big toe supports the majority of the body mass while the smaller toe prevents the ostrich from losing its balance by acting as an outrigger, especially during slow walking.
Figure 5: Load-distribution