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First published online June 7, 2004
Journal of Experimental Biology 207, i (2004)
Copyright © 2004 The Company of Biologists Limited
doi: 10.1242/jeb.01099
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EXCAVATION, AMPHISBAENIAN STYLE
kathryn{at}biologists.com
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Life's tough when you rarely surface from beneath the ground. Yet some creatures have opted for subterranean life styles and most are well equipped with powerful earth moving limbs to carve chambers and tunnels; that is except for amphisbaenians. This ancient line of limbless reptiles excavates complex networks of slender tunnels by burrowing head first. But little was known about their unique burrowing technique or physiology as most amphisbaenian species rarely appear above ground. Carlos Navas was fascinated by these enigmatic creatures and it was only when Brazilian amphisbaenians began rushing to the surface, driven from their subterranean homes by the rising water levels of the São José do Calçado Dam project, that he knew he would have his chance to investigate these unusual creatures. Rescued from the water, the amphisbaenians were sent to Navas's colleague at the Instituto Butantan, Carlos Jared, where he and his coworkers were curious to know how the slim reptiles excavate channels equipped only with their heads (p. 2433).
But few had ever kept the elusive Brazilian amphisbaenian alive in captivity; could they survive? Fortunately Jared had observed the animal's natural habitat, and built the amphisbaenians a comfortable terrarium, loosely packed with soil, covered with rocks and bark, and well supplied with fresh ground beef and cat food at the surface. The amphisbaenians thrived.
Curious to know how the animals shovelled aside soil, Navas and his colleagues built a slender terrarium, just wide enough for an amphisbaenian, to film the burrowing animal's descent. The animal's needed little encouragement to dig, and Navas watched the slender animals push their heads forward and lifting it upward to compress the soil into the tunnel's roof and extend it by a few centimetres. And when the team measured the force the animal's tiny heads applied, Navas was amazed; the slender amphibaenians' necks were able to produce 24 N of force, more then enough to lift a 2 kg mass!
Intrigued by the reptile's extraordinary weight lifting prowess, Navas and his colleagues began investigating the amphisbaenian's main digging muscle, the longissimus dorsi in the reptile's back. Knowing that the creature was capable of producing enormous forces, they suspected that the muscle would comprise powerful anaerobic glycolytic fibres, rather than aerobic muscles, which are weaker but would have the stamina needed to construct their extensive tunnel networks. But when they investigated the muscle they noticed that it was a dark pink near the head and became lighter towards the end; the longissimus dorsi appeared to contain myoglobin, a hallmark of aerobic muscle fibres. And when they looked at the fibre's composition, they identified several types of aerobic muscle fibre, but no powerful anaerobic fibres. The aerobic longissimus dorsi muscle could provide amphisbaenians with the stamina they need to dig, but how could the muscle provide enough soil-packing power?
Navas and his team began investigating the muscle's structure, and found that the amphisbaenian's muscle fibres were incredibly long, angled at up to 35°, and curved around the reptile's body, increasing the muscle's effective cross sectional area and power output. Amphisbaenians have found a way of increasing aerobic muscle's force production while maintaining stamina to keep the slender reptile burrowing for hours.
References
Navas, C. A., Antoniazzi, M. M., Carvalho, J. E., Chaui-Berlink,
J. G., James, R. S., Jared, C., Kohlsdorf, T., Silva, M. D. P. and Wilson, R.
S. (2004). Morphological and physiological specialization for
digging in amphisbaenians, an ancient lineage of fossorial vertebrates.
J. Exp. Biol. 207,2433
-2441.
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