Frozen Fauna of the Mammoth Steppe: The Story of Blue Babe

by Guthrie, R. Dale

  To understand how a gallop works we look at it in four beats or phases (fig. 5.2), one of which discloses the anatomical importance of humps. A galloping animal hurtles through the air unsupported after it pushes from the hind legs (phase 1) and after it pushes from its fore feet (phase 3). The other phases are the steps on the ground of the forelegs (phase 2) and between both stretched hind legs (phase 4). So the complete four-phase cycle of a gallop can be measured and classified by the footfalls or tracks. Phase 2 is where the hump becomes important. Normally the forelegs do not land simultaneously; rather, one hits the ground, taking the whole weight of the animal, and pushes backward at the same time that the second foreleg is reaching forward and making contact. That second leg then carries the animal’s weight and pushes backward, whereupon the entire body is projected forward through the air. The hind legs repeat this left-right sequence, again propelling the animal through space until it lands on its first fore foot (called the lead foot).

  Fig. 5.2. Four phases of the gallop. I divide the gallop into four units relating to suspension and distance traveled. The first phase marks the shift from hind legs to forelegs. European and American bison differ, as the latter seems not to leave the ground during this phase. The second phase is the distance gained as one fore foot steps ahead of the other. Animals vary considerably in this phase, but bison excel, taking an extremely wide foreleg step. Their tall hump is especially important in phase two. The third phase marks the shift from forelegs to hind legs. Most gallopers leave the ground during this phase, even a one-ton bull bison. The fourth phase marks the length of step from one hind leg to the other. This is especially short in bison; related to this, bison hindquarters are rather low and their hams relatively small.

  Fig. 5.3. Cantilevers and gallops. During phase two and phase three, one end of the quadruped is suspended by the other via a cantilever system of muscles, tendons, and bony struts. Bison have accentuated phase two more than phase four; consequently, the forequarters used in phase two are more exaggerated than the hindquarters.

  This shifting suspension of a galloping animal between anterior and posterior ends emphasizes the importance of the cantilever system of tall neural spines and tendon cables (fig. 5.3). Unlike some other large mammals, bison do not leave the ground in phase 1. Rather, the forelegs carry the center of gravity for an exceptionally long time (extended phase 2). Since the hindquarters are suspended out in space for a longer time than the forequarters, the main cantilever struts would logically be located in the shoulder area. Indeed that is where they are in most ungulates; bison are but an extreme case.

  For a large mammal the story of an efficient phase 2 of the gallop includes the shoulder hump. Freed from the restricted arc of the hind limb, the forelimb can swing not from the elbow as it does in a trot, but from the highest crest of the scapula. Thus a galloper needs to maximize the stride length of the forelimb, that is, phase 2. Among African antelope species, the alcelaphines (wildebeest, topi, hartebeest, blesbok, etc.) are all high humped. These species are smooth, efficient canterers and gallopers and seldom, if ever, trot. Bison are a bovine version of an alcelaphine. Wildebeest and bison can maintain their rocking, distance-eating canter for hours and days. Today they are here in the thousands; tomorrow they are gone to some distant pasture. This mobility is, I believe, the ecological significance of humps.

  Fortunately, Swiezynski (1962) has done a careful study of bison musculature that helps us understand how bison humps work. The high neural spines near the anterior portion of the thorax are the major origins for the splenius muscle, which inserts on the head and neck. These spines are also the origin of the powerful extrinsic muscle of the pectoral limb, the rhomboideus. The rhomboideus pars thoracalis et pars cervicalis attach to the dorsal portion of the tall scapula, acting as a powerful lever to pull it forward during the backstroke of the foreleg (fig. 5.4). These muscles are especially important during the gallop. Roskosz and Empel (1961) argue that these specialized muscles of the hump provide agility and flexibility, enabling bison to jump a 2 m obstacle from a standing start, but they also conclude that (p. 69) “the powerful muscles of the withers are decisive factors concerning the growth of the spinous processes. These muscles are connected with the characteristic pose of the head, manner of feeding, the great weight of the body, and the strongly developed anterior part of the European bison. The specific gait and great activity of this animal is not without importance.”

  As I said earlier, the second phase of the gallop is one of the most important for bison. The tall scapula hinges from its apex, allowing a wide stretch between forelegs, even for such a heavy creature as a bison. But the spines of the anterior thoracic vertebrae are tall in all bison species, and so are those near the posterior end of the thorax, but these latter are proportionally and absolutely much larger in the steppe bison.

  Unfortunately, we need the muscles, tendons, and ligaments to say for certain how the steppe bison’s unique hump functioned, and we do not have those because they were removed from the Blue Babe specimen by predators and scavengers. We can, however, make a good guess. Perhaps the rhomboideus extended more posteriorly; we know from the shape and size of the anterior neural spines that it had a different shape in steppe bison. But another muscle, the spinalis, is a series of bellies that originate on the distal ends of each thoracic vertebrae and that travel posteriorly, inserting toward the lumbar vertebrae. These bellies act in synchrony to arch the back convexly (dorsally), and because of their size and origination height they provide a powerful mechanism by which the back can add push to the hind limbs for greater distance during the first beat or phase of the gallop.

  This same enlarged spinalis muscle originating from a higher angle would also have given the males more power in fighting, as the back comes from a collected convex to an extended concave position when the head is forced upward during pushing bouts. There seems to have been a difference in fighting mode between America bison and the large-horned Pleistocene steppe bison (Guthrie 1980, and reviewed elsewhere in this book). So in addition to its differences in locomotion the steppe bison may have had a muscle-lever system which accompanied a different adaptation in fighting strategy and style.

  Fig. 5.4. Thoracic muscles of European bison. Some shoulder muscles used in the suspension process are portrayed in this anatomical dissection redrawn from Swiezynski (1962). The rhomboideus and splenius pull the scapula forward, rotating the foreleg backward (the main strength of the bison’s gait). The spinalis pulls the neural spines backward, using that leverage to arch the vertebral column in phase one, as well as supporting the rear quarters in phase two.

  The long neural spines of a bison’s hump are also important as girders or struts for the attachment of the elastic neck ligament, or nucal ligament. This is a thick elastic yellow cable which allows energy to be stored as the head is lowered by gravity (for example, when the animal grazes). Stretch ligaments make it less costly to return the head to its natural position (fig. 5.5). The nucal ligament originates from the tips, or distal ends, of the neural spines and inserts onto the back, or occiput, of the skull. But this ligament may have another function.

  Fig. 5.5. Role of stretch ligament in gallop of European bison. The neural spines in the hump act as girders to which the nucal ligaments attach. The tall spines provide leverage as the head bobs up and down during the gallop. During phase two the head is lowered by gravity and by the pull of the muscles attached to the rearward-moving foreleg, providing a counterweight to the suspended hindlegs. The nucal ligament aids in raising the head, decreasing the expenditure of energy otherwise required.

  To my knowledge, no one has mentioned the nucal ligament’s importance in galloping. A galloping animal alternately lifts its head up and down (a novice horse rider must learn to move the taut rein in this rhythm). The down thrust of the galloping animal’s head comes during phase 2 (fig. 5.5); it seems to act as a counterweight in the suspension of the cantilevered rear end. The tall trusses of the hump incr
ease that mechanical advantage. The nucal ligament seems to be the main cable of energy transmission in that process. The lowering of the head requires little energy expenditure because it uses gravity. And as we have seen, raising the head is greatly assisted by the energy storage capacity of the nucal stretch ligament. The upward spring of the head aids in the vertical lift of the forelegs during phase 4 (fig. 5.5).

  A familiar analogue to this energy storage process in stretch ligaments is the pogo stick, where one bounces along on a stick with springs. The spring stores the energy of the gravitational descent and converts it to a vertical ascent, making movement much more energetically efficient than that performed on an unsprung stick. A heavy window or garage door sprung by a counterweight is similar. The energy of gravity used to lower the structure is stored, allowing one to easily raise the garage door. Thus the tall hump of a bison, acting as the origin of the nucal stretch ligament, may increase galloping efficiency by allowing the heavy low-slung head to be employed as a counterweight lift during phase 2 and recovered to normal position at reduced metabolic expense.

  There is an increasing awareness in locomotor studies of the role of stretch tendons in energy storage (Alexander 1984; Alexander, Dimery and Ker 1985). These energy-storage devices make a gallop a comparatively energy efficient gait, allowing some animals to use it in long-distance travel.

  The Ecology of Humps

  High humps are the mark of open-plains grazing specialists. In areas where grasses grow thinly and the sward is short, forage is limited in volume and any one area can be quickly exhausted as rangeland. Likewise, quality forage is often scattered across the landscape in a mosaic—rains may come ten kilometers away. The phenological peak of plant quality also occurs at different altitudes and at different aspects of slope at different times of year. A plains-adapted social creature traveling with a herd must keep on the move. An animal adapted to fibrous forage cannot go very long without water, yet the highest quality of range is often quite distant from water and frequent trips must be made between the two. An efficient and fast gait is important on the plains.

  Humps are often part of another set of traits. As animals moved away from the forest where thick vegetation afforded protection from predators, many species resorted to sociality as a substitute “cover.” The presence of a hundred other animals dilutes a predator’s focus on one’s self. Forest ungulates are built for great speed over very short distances. Unlike bison, most forest ungulates emphasize phase 1 of the gallop; the rear is large and the front small. Forest antelope accelerate quickly to foil an attack and can disappear in dense habitat. Thus they do not require sustained speed over long distances. Ungulate biologists (Estes 1974; Geist 1974) have referred to this as the “duiker syndrome,” named after small forest- or brush-adapted African antelope that have large hams and high rumps, giving the body a downward tilt toward the shoulders, quite the opposite of wildebeest.

  One can actually see these morphological traits in horses bred for different purposes. The American quarter horse has been bred to run fast over short distances (a quarter of a mile or about 400 m). It is a breed favored by cattlemen who have to quickly cut-off a cow as it veers from the herd. Quarter horses are built like dragsters; they are large hamed and slope forward like duikers.

  Another part of the hump syndrome is an adaptation to grazing out in the open. Diets also changed as animals moved into more open country. Grasses have most living-tissue and energy storage organs beneath the ground. Grasses can treat their seasonal, above-ground photosynthetic tissue as disposable. As such, tissue above ground does not have to be so well defended from herbivores. Woody plants, on the other hand, usually produce toxic chemicals to protect their above-ground woody parts because these parts are more costly to grow. The open sun-drenched plains can have excellent forage part of the year when grasses are abundant and high in nutrient quality and calories. But forage on the plains is more seasonal than in shade-buffered woodlands. Seasonal plenty alternates with lean, inhospitable times, when the only food available to a herbivore is standing dead grasses which offer abundant empty calories, but lack other essential nutrients. A plains-adapted animal weathers this bleak period by having the ability to use poor food (many woodland species cannot survive on such undigestible material). Most northern plains species also rely on fat reserves gained during earlier seasons of plenty. To live year-round on the open plains a species must make full use of available resources. Mobility that is both rapid and efficient is a critical part of survival.

  American bison are like wildebeest in their adaptations to grasslands. American bison have large shoulder humps, and they frequently use a rocking canter. But because their body mass is bovine in scale, they have different problems in locomotion. Bison humps are unusually large, the largest of all ungulates in their size range or smaller. When one examines photos of galloping bison, the span between the fore feet in the second phase of the gallop is enormous. A high hump allows bison scapula to be quite tall; it is a feature of bison osteology that easily separates (fig. 5.6) bison from cattle (Bos) and buffalo (Bubalus, Syncerus) or even equids. As I pointed out earlier, a tall scapula increases the height of the arc through which the forelegs can travel and hence the potential length of stride, making a bison’s gallop exceptionally efficient (fig. 5.7).

  Fig. 5.6. Partial skeletons of bison, cattle, and horse. Note dramatic differences in neural spines.

  Fig. 5.7. Relative stride lengths of galloping horse and bison. Even though bison are short-legged, they travel about the same distance in phase two (distance between right and left fore feet) as horses, because their high shoulder raises the apex of the angle of foreleg rotation. In phase four (the span of hind feet), bison travel a shorter distance than horses.

  It was once thought that humps related to heavy heads or big horns (Koch 1932), but there seems to be very little correlation. The open-ground grazing niche is a better predictor of who is humped. Among bovines, bison are the most well adapted to life on open grasslands, and bison have by far the tallest humps. Cattle, in comparison, are small humped; they are woodland edge or parkland creatures. Even among cattle breeds with heavy heads and immense horns, for instance, Watusi cattle and Ankol cattle of Africa, the “withers,” as cattlemen call the hump, are low (Roskosz and Empel 1961); that is, the neural spines of the thoracic vertebrae are short. In America we do not think of cattle as adapted to the woodland edge, but associate them with cowboys on the open plains. But cattle on the plains must be fed hay for a big part of the year. Compared to bison, cattle are not well adapted to living wild in these habitats.

  Blue Babe’s Hump

  My first step in reconstructing Blue Babe’s hump was to assemble vertebrae from three American bison, in a long band down three pine boards, using one-inch (25mm) strips of clay about four inches (100 mm) wide. The ventral part of the centrum was pushed into the clay, with neural spines pointing dorsally (see fig. 5.8). Of course the board provided a base quite unlike the complex contour of a living bison, and the dorsal outline of neural spines it produced was not the same as that of a living bison. Still, this exercise allowed us to take precise measurements because all vertebrae were aligned on a common baseline (Poplin 1984). Other American and European bison, as discussed later, were also compared using this same baseline.

  Vertebral columns of European bison have been thoroughly studied by Polish mammalogists, so I compared illustrations and measurements from their work with fossil vertebrae from the Alaskan steppe bison. In addition, Poplin (1984) made a careful study of the dorsal outline of living bison species as well as the extinct European Pleistocene Bison priscus. Although these published studies were excellent treatments of neural spine shapes and sizes as well as vertebral musculature, I chose to examine three skeletons of American bison in order to make direct comparisons with fossil Alaskan specimens and to get a better feel for potential differences. Because Blue Babe was a mature bull, only bones from mature bulls were used. One was a large plains
bison (Bison bison bison), National Museum of Canada No. 45632, from Montana. The second was a wood bison (B. b. athabascae), National Museum of Canada No. 32628, from an introduced bison population in Elk Island Park, Alberta. The third, also a wood bison subspecies, National Museum of Canada was from Wood Buffalo National Park, taken before plains bison were introduced to that area (table 5.1).

  Fig. 5.8. Thoracic and lumbar vertebrae. Vertebrae are assembled on a flat base to illustrate differences in hump contour. The highest point of the steppe bison hump is far posterior to that of plains bison. Also, the neural spines of plains bison fall smoothly to the rear; in steppe bison this contour is more abrupt.

  Table 5.1 Bison Neural Spine Length

  Note: All measurements are taken from the posterio-dorsal rim of the neural canal in a straight line to the most distal part of the neural spine, to the nearest 0.25 cm.

  a. National Museum of Canada No. 45632 mature male.

  b. National Museum of Canada Wood Buffalo Park mature male.

  c. National Museum of Canada No. 32628 mature male.

  Several hundred vertebrae of Pleistocene Alaskan bison at the University of Alaska Museum were sorted for specimens with complete neural spines. These were ordered in categories of cervical, anterior thoracic, posterior thoracic, and lumbar. The individual shapes of these bison vertebrae seemed similar, but they varied greatly in size and robustness. Fortunately the collection contained an assortment of vertebrae from a single animal, University of Alaska Museum No. 1178, collected in 1956 at the Utica Mine on the Imnachuck River. We know this was an old bull, judging from its size and also from its arthritic asymmetry and the pearling around the centra. As an animal grows older the vertebrae exhibit arthritic osteoporosis and roughness at points of muscle origin and insertion and in edges of the vertebral centra. The Utica Mine bison bones were thus the right sex and age to allow for a good comparison with the vertebrae of the three mature bulls from Canada. Using vertebrae from a single steppe bison avoided the problem of assembling vertebrae from individuals of slightly different age and size.