Australia: The Land Where Time Began
Kangaroos - the hopping gait
A study on a red kangaroo, in which it was trained to use a tread mill after getting used to wearing a mask, found that while the animal was walking the oxygen consumption increased steeply with speed until it reached 10 km/h, at which point it started hopping. From the time it started hopping the oxygen consumption did not increase up to a speed of 35 km/h (Dawson & Taylor, 1973). This explains how the early explorers were astonished at how easily kangaroos could outrun their dogs, especially after being chased for about 2 hours by fox hounds. Gould (1863) reported that ". . . it was also plain that he was still fresh, as, quite at the end of the run, he went over the top of a very high hill, which a tired kangaroo will never attempt to do." This chase is reported to have taken 2 hours to cover a distance of about 29 km, an easy jog for an animal that has been clocked at 40 km/h.
A similarly sized quadrupedal animal, such as the foxhounds in Gould's report, oxygen consumption increases linearly with increasing speed. The foxhounds that chased the kangaroo would have consumed double the amount of oxygen as the kangaroo. When walking slowly, in which they use their tails as a 5th leg as they move their large hind legs forward, kangaroos' walking is much less efficient than that of the dogs. The efficiency of hopping also applies to the pademelon and the tammar wallaby, but with the smaller macropods such as the quokka, the brush-tailed bettong, Bettongia panicilliata, and the long-nosed potoroo, all weighing 2.5 kg or less, locomotion is more in line with that of quadrupeds (Baudinette, 1969), increasing speed being accompanied by increasing oxygen consumption, though the rate of increase is less than in similar sized quadrupeds (Webster & Dawson, 2003). The long-nosed potoroo, weighing about 1 kg, prefers a bounding gait involving all 4 legs until it needs to move at high speed, when it switches to hopping (Baudinette, 1993). The smaller species are less ungainly than kangaroos when moving slowly, not needing to use their tails to move their hind feet forward.
Macropods increase their speed by increasing the length of their stride, not the frequency of hops. The stride length is dependant on leg length, the smaller species attaining their maximum speed by having a higher hopping frequency than the larger species. The stride frequency is 3.5 strides/s at all speeds, the stride length of from 0.3 m to 1.8 m for the brush-tailed bettong (Webster & Dawson, 2003). The red kangaroo has a hopping frequency of 2.5 strides/s and a stride length of 0.8 - 4.0 m (Dawson & Taylor, 1973). For the tammar wallaby the figures are intermediate, a frequency of 3.5 strides/s and a stride length of 0.8-2.4 m (Baudinette et al, 1987).
The kangaroo uses the calf muscles, gastrocnemius and plantaris muscles in hopping. The large calf muscles, the main muscles used in lifting the body off the ground and extending the foot forward, begin on the underside of the femur, swelling into 2 large muscle masses that narrow into the powerful tendon attached to the heel. The smaller plantaris muscle, attaches to the femur, its tendon passing around the heel to attach to the sole of the foot, mainly to the 4th digit, the great toe the kangaroo uses to drive the hops. The hind feet are fully extended while the animal is off the ground, at the end of a stride, the legs flex quickly as they touch the ground, stretching both muscles and their tendons to their fullest extent. The length of the muscles doesn't change much, as a result of the arrangement of the muscle fibres in a pennate manner, though the tendons, that can be 200 mm long in a large kangaroo, are stretched by 11 mm. As the tendons shorten in the next jump, the elastic energy stored in the stretched tendons is released rapidly.
The economy of locomotion achieved by the kangaroo is enabled by the elasticity of the tendons and the leg geometry, the maximum efficiency being achieved at speeds above 10 km/h. The amount of energy stored in the tendons increases as the landing force of the legs increases, the energy being proportional to the square of the force acting on it. With increasing speed, the landing force increases, resulting in an increasing amount of energy being stored in the stretched tendon. It has been calculated that the total energy required for a kangaroo's hop was 24 Watts/kg at 10 km/h and by 22 km/h it had increased to 36 Watts/kg (Alexander & Vernon, 1975). The actual cost of hopping at both speeds has been found to be 20 Watts/kg (Dawson & Taylor, 1973), the elastic energy stored at each jump is 4 and 16 Watts/kg respectively, that can be regarded a considerable saving.
The high efficiency of the kangaroo's hopping gait was believed to be achieved by special characteristics of the calf muscles and tendons, not found in non-hopping mammals, but this was shown to be incorrect (Proske, 1980), in the muscles and tendons there was no difference between these structures in kangaroos and other mammals, with the leg muscles changing by a negligible amount during hopping, as a consequence of their pennate structure, whereas the tendons changed by a substantial amount. The muscles maintain the tendon tension, an essential component of the mechanism, functioning in a manner similar to a shock absorber. It had also been suggested by some that an oxygen debt accumulated, with the accumulation of lactic acid by anaerobic respiration, that would be repaid later as in athletes. But it was found that lactate levels didn't increase in limb muscles during hopping at increasing speeds (Baudinette et al., 1992), leaving the elastic energy of the tendons as the most likely explanation of the economical locomotion of hopping.
2 other aspects of hopping were studied in the tammar wallaby, they set out to discover if the heart rate and respiratory rate were phase locked with hopping frequency (Baudinette et al., 1987). All the large leg muscles in the hind legs of both the tammar wallaby and the kangaroo are in an active state simultaneously when the animal lands. This is not the case in quadrupeds, where the muscles in 1 leg relax as those in the other leg contract. It had been suggested that simultaneous contraction of the all the muscles would affect the blood return to the heart, resulting in the heart rate being phase locked with hopping frequency. However, it was found that the heart rate increases 1.8 times faster than hopping frequency as speed increased. In the tammar wallaby it was found that the the animal inhales as it leaves the ground and exhales as it lands, resulting is a 1:1 phase locking between the breathing cycle and hopping. This probably explains the lighter, less muscular diaphragm of the tammar wallaby compared with other animals of its approximate size. It is believed the mass of the gut is thrown forward against the diaphragm at each landing, causing the air to be expelled piston fashion.
The economical advantages of hopping are realised most by large animals, above 5 kg, below which there is less advantage, mechanically and physiologically. It has been suggested that the hopping mechanism may have originally evolved in small bipedal animals as a means of making rapid vertical jumps, energetically expensive, but would provide a means of gaining very rapid acceleration, as would be advantageous when avoiding predators. Hopping may have evolved in the early macropods as a better means of escape from predators on the floor of the forests where it first arose (Baudinette, 1991). As it has been retained in all modern macropods, even the tree kangaroos that remain predominantly bipedal, though the gait has disadvantages for tree climbing. It may be a feature that is difficult or impossible to reverse. Whether or not is is reversible, it has proven to be a very useful preadaptation for the later macropods as the forests declined and the grasslands and open woodlands expanded with the progressive aridification of the continent. It allowed the larger macropods, above about 5 kg body mass, to radiate rapidly as the forests opened up, and is probably the reason most of the macropodines have a weight of 5 kg or more. It has been suggested that this efficient form of locomotion may be a reason for the decline of the smaller species, the advantages favouring the larger species, especially when needing to escape from introduced placental predators (Tyndale-Biscoe, 2005).
Both species of grey kangaroo are smaller than they were prior to the arrival of humans. This has led to the suggestion that there may be a limit to the advantage of large size once humans were the predators (Flannery, 1994). The red kangaroos did not get smaller. It has been suggested that the large kangaroos of the present may be at the optimum size for maximising the advantage gained from hopping, possibly because the large size made them slower, or the efficiency of the hopping may have been less in the larger extinct animals. The large Sthenurines may also have been too slow to escape human hunters (Tyndale-Briscoe, 2005).
Tyndale-Biscoe, Hugh, 2005, Life of Marsupials, CSIRO Publishing.
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