Energy-Storing Tendons Give Wallabies Their Bounce

Biological Strategy

Tamar wallaby

AskNature Team

Image: Pauline Bernfeld / Unsplash / Free non-commercial use

Store Energy

Once a living system captures energy or transforms one energy form into another, it must frequently save that energy for future use. But energy is difficult to store in some forms. So living systems need strategies to either use energy quickly, or to convert it from forms that are difficult to store (such as electrical or kinetic) to more storable forms. For example, grasshoppers store energy as potential energy in an elastic material in their tendons. When they need to jump, that energy converts into kinetic energy, providing the force needed to escape predators.

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Move in/on Solids

To obtain needed resources or escape predators, some living systems must move on solid substances, some must move within them, and others must do both. Solids vary in their form; they can be soft or porous like leaves, sand, skin, and snow, or hard like rock, ice, or tree bark. Movement can involve a whole living system, such as an ostrich running across the ground or an earthworm burrowing through the soil. It can also involve just part of a living system, such as a mosquito poking its mouthparts into skin. Solids vary in smoothness, stickiness, moisture content, density, etc, each of which presents different challenges. As a result, living systems have adaptations to meet one, and sometimes multiple, challenges. For example, some insects must be able to hold onto both rough and slippery leaf surfaces due to the diversity in their environment.

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Mammals

Class Mammalia (“breast”): Bats, cats, whales, horses, humans

Mammals make up less than 1% of all animals on earth, but they include some of the most well-known species. We know first-hand some of the characteristics that make mammals unique, like having hair, being able to sweat, and producing milk through mammary glands. Another critical shared feature is a set of highly-specialized teeth. Unlike sharks or alligators, for example, whose teeth are generally all the same size and shape, mammals have differently shaped teeth in different areas of the jaws to target specific foods or foraging strategies.

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Tendons in hind limbs use elastic recoil to boost energy efficiency.

Although most terrestrial animals that run, hop, or trot across the ground need to spend more metabolic energy to go faster, the hopping tammar wallaby can go faster with little or no increases in energetic cost. Furthermore, a female tammar wallaby can carry the heavy load of the infant joey in her pouch without increasing her cost of locomotion. These remarkable feats are likely due to the storage and recovery of elastic energy by the large springy tendons in the wallaby’s hind legs. During the leaping, aerial phase of the hop cycle, the wallaby’s forward movement represents kinetic energy and the gravitational pull back to the ground is a form of potential energy. These energies transform into elastic strain energy of stretching tendons when the foot hits the ground. That energy can then be recovered in the elastic recoil of those tendons that helps propel the wallaby back off the ground. As much as 90% of the energy stored in the tendon can be recovered for such reuse. The key to this energy recovery is that muscles attached to the tendons are stiff enough so that their length changes little as they generate force. If the muscles changed in length a lot, they might absorb and dissipate the tendon’s elastic energy, making it unavailable to power the next hop.

The faster the wallaby goes and the heavier the load, the more elastic energy gets stored and recovered, hence the cost of locomotion can be unchanged with speed or load over a normal range of speeds. The use of tendons and elastic energy is also found in many other large animals that run (such as horses and turkeys), but to a much less dramatic extent in terms of energy savings as those observed in kangaroos and wallabies. It is as yet unclear exactly why these macropods experience such high savings in energy compared with other animals. The general strategy of elastic energy storage as a means of increasing locomotor efficiency is also observed in a variety of swimming animals, from squid to dolphins.

The use of elastic energy storage could be considered in the human design of all sorts of moving structures to increase energy efficiency. “Spring loaded locomotion” has been used in the design of the pogo stick and some prosthetic legs.

Image: Michelle Bartsch /

From Island Beach.

Last Updated July 1, 2020

References

“Moderate to large macropodids can increase their speed while hopping with little or no increase in energy expenditure. This has been interpreted by some workers as resulting from elastic energy savings in their hindlimb tendons. For this to occur, the muscle fibers must transmit force to their tendons with little or no length change. To test whether this is the case, we made in vivo measurements of muscle fiber length change and tendon force in the lateral gastrocnemius (LG) and plantaris (PL) muscles of tammar wallabies Macropus eugenii as they hopped at different speeds on a treadmill. Muscle fiber length changes were less than +/-0.5 mm in the plantaris and +/-2.2 mm in the lateral gastrocnemius, representing less than 2 % of total fiber length in the plantaris and less than 6 % in the lateral gastrocnemius, with respect to resting length. The length changes of the plantaris fibers suggest that this occurred by means of elastic extension of attached cross-bridges. Much of the length change in the lateral gastrocnemius fibers occurred at low force early in the stance phase, with generally isometric behavior at higher forces. Fiber length changes did not vary significantly with increased hopping speed in either muscle (P>0.05), despite a 1. 6-fold increase in muscle-tendon force between speeds of 2.5 and 6.0 m s-1. Length changes of the PL fibers were only 7+/-4 % and of the LG fibers 34+/-12 % (mean +/- S.D., N=170) of the stretch calculated for their tendons, resulting in little net work by either muscle (plantaris 0.01+/-0.03 J; gastrocnemius -0.04+/-0.30 J; mean +/- s.d. ). In contrast, elastic strain energy stored in the tendons increased with increasing speed and averaged 20-fold greater than the shortening work performed by the two muscles. These results show that an increasing amount of strain energy stored within the hindlimb tendons is usefully recovered at faster steady hopping speeds, without being dissipated by increased stretch of the muscles’ fibers. This finding supports the view that tendon elastic saving of energy is an important mechanism by which this species is able to hop at faster speeds with little or no increase in metabolic energy expenditure.” (Biewener et al. 1998:1681)

Quills of the North American porcupine penetrate tissue easily due to stress concentration at the barbs that likely stretches or tears tissue fibers locally at the interface of the quill.