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

by Guthrie, R. Dale

  The vertebrae of this steppe bison from the Utica Mine could be ordered in sequence by using the recent bison vertebrae as a comparison. In addition, we were able to cross-check our identifications and vertebral ordering because the articulations showed many unique patterns and asymmetry which had to match exactly. No two articulation facets of the anterior and posterior zygopophyses were alike in this regard, so our identifications were certain.

  Noting specific features of these different vertebrae help us reconstruct Blue Babe’s hump. Cervical vertebrae of steppe bison and the two subspecies of modern American bison differed mostly in the steppe bisons’ greater robustness of muscle and ligament attachments, attributable, I think, to their massive horns which increased the weight of the head. The last cervical vertebra, C7, however, was quite different in shape as well as size. This cervical vertebra is incorporated in the thoracic series and can be thought of as thoracic in shape and function. It is immediately identifiable in any jumble of vertebrae because it has no demifacets for rib attachments, yet it has a very long neural spine, like the thoracic vertebrae.

  The shape of steppe bison vertebra C7 is very different from that of a living American plains bison. In steppe bison the C7 is tightly arched posteriorly, that is, it has a convex surface all along its anterior face. The C7 neural spines of plains bison rise from the base in the same posterior bend but reverse themselves halfway in an anterior bend (concave on the anterior face). Also the anterior-posterior width of the neural spine base is greater in steppe bison than in plains bison. From this wide base the spine tapers gradually to a narrow distal end. American plains bisons’ C7 spine keeps much of its anterior-posterior width instead of narrowing to a point. Unfortunately, few terminal portions of C7 are present among fossils, as this part is quite edible and is usually scavenged.

  In contrast, the C7 neural spine of the Canadian wood bison more closely resembles that of the steppe bison, but is not so wide at the base. Wood bison are in between plains bison and steppe bison, but more akin to the latter. One specimen of wood bison, NMC 32628, had a longer C7 spine (390 mm) than plains bison (355 mm); the other wood bison, NMC 45632, had a much smaller neural spine (330 mm). The C7 of European bison measured by Roskosz (1962) seems smaller than that of both American bison and steppe bison. Roskosz pictured only the C7 of female bison. These were straight in profile and tapered in width to the distal tip, like those of steppe bison, but were not arched posteriorly.

  The first thoracic vertebrae, the Tl, evidences another difference. Two subspecies of living American bison have a more smoothly curved concave anterior border to their neural spines, whereas the neural spine anterior border of steppe bison is convex on the proximal half and slightly concave only on the distal half. The Tl neural spine of wood bison is longer than any of the other neural spines. The wood bison neural spine lengths of the Tl were 470 mm and 485 mm. The Tl length of the American plains bison was 400 mm and the steppe bison’s was 415 mm. These measurements and those that follow are from the dorsal-anterior point on the neural arch to the distal end of the neural spine, along the anterior face.

  Thoracic neural spines were more convex on their anterior borders among steppe bison than among either modern American bison subspecies. Both wood bison and plains bison spines almost always started from a concave sweep as they rose from their bases and arched more convexly on their distal half. Comparing neural spine lengths within individuals, both plains bison and wood bison had longer Tl and T2 spines than any other vertebrae. They differed subtly in pattern in that wood bison tended to have longer absolute length than plains bison. I had only one mature male plains bison for comparison, so perhaps some individuals reach lengths comparable to wood bison, but in general a higher forward portion of the hump is characteristic of wood bison (Geist and Karsten 1977).

  A striking difference between Alaskan steppe bison and the above specimens is that in steppe bison, the longest neural spine is T5. As can be seen in figure 5.8, the thoracic hump of steppe bison is located farther back than in wood or plains bison. For example, in contrasting the two, the wood bison had longer neural spines from Tl through about T4; the steppe bison had larger spines between T5 and T12.

  A comparison of neural spines of vertebrae T4, T5, and T6 (fig. 5.9) highlights thoracic hump placement. Those of the plains bison (T4, T5, and T6) are short relative to other vertebrae. These same neural spines are longer in the wood bison. Among steppe bison these neutral spines are longer than in either living American bison species, and they are set differently, being bent posteriorly in a more acute angle. A similar pattern can be seen in T8, T9, and T10 (fig. 5.10). Neural spines of steppe bison are also broader anterior-posteriorly than the others. Posterior to T10, neural spines among all of the bison are rather similar, including those of the lumbar vertebrae. Among both female and male European bison pictured by Roskosz (1962), the thoracic vertebrae had concave anterior surfaces to their neural spines, more like those of American plains bison.

  Fig. 5.9. Lengths and angles of the fourth, fifth, and sixth thoracic vertebrae. These are longest in fossil steppe bison, and they emerge from the centrum at a lower angle, bending more posteriorly.

  Fig. 5.10. The neural spines of thoracic vertebrae eight, nine, and ten. These are located at the posterior “stop” or drop-off of the hump of steppe bison, B. priscus, and, as a consequence, differ from their homologues in wood bison, B. b. athabascae, and plains bison, B. b. bison. Those of the steppe bison are broader (anterior-posteriorly) and longer.

  All this means that the hump shape produced by thoracic vertebrae of Pleistocene Alaskan specimens differed considerably from that of living plains and wood bison. Wood bison have a high hump that is straighter in profile from the saddle of the back (lumbar area) forward to the neck. In wood bison, the forward edge of the hump drops abruptly, almost vertically. In plains bison, the hump is high in front but drops less abruptly to the neck. Overall the plains bison has a slightly concave profile to the highest point of the hump.

  As reconstructed from Alaskan fossils, steppe bison had a more gently ascending hump from the front which reached a high point well posterior to that of both American bison subspecies. Posterior to the scapula, the hump arched upward then dropped rather abruptly in a convex arch. The steppe bison’s hump was more similar to that of the living European bison, B. bonasus, but the former’s hump was much more exaggerated in height and contour.

  Although he did not have access to Alaskan fossil material, Poplin (1984) examined bones of American (B. bison) and European (B. bonasus) bison, comparing the dorsal contour of the neural spines. Poplin did not distinguish the two subspecies of American bison in his work. However, I wanted to use the subspecies’ differences because I felt the geographic proximity of wood bison to Alaskan Pleistocene bison might be significant. Poplin compared many individual bison, both females and males; a diagrammatic summary of his findings is presented in figure 5.11. Poplin was primarily concerned with testing the idea that European cave artists drew bison with a dorsal contour different from that of now living bison, not because of artistic style but because of a biological difference in their model. Fortunately, he was able to find two almost complete specimens of European steppe bison, B. priscus, the same species that, on the basis of other evidence, I have argued lived in Alaska, the species to which Blue Babe belonged.

  Fig. 5.11. Male bison dorsal skeletal contours. When Poplin (1984) compared dorsal contours of vertebrae from different bison groups, he was able to show there were differences in hump shape. Humps of living bison and extinct European steppe bison (B. priscus) have diagnostic shapes. The dashed base-line contour is that of an American female bison; solid-line comparisons are different males. The lower three are fossil steppe bison. (After Poplin 1984)

  In length of neural spines, Poplin found that among American bison the first thoracic, Tl, is normally the longest. Among European bison it is T2, and for European Pleistocene steppe bison it is T3. Remember that Alaskan Pleistocene
steppe bison had the longest neural spine of T5, even farther posterior than European steppe bison. More important, Poplin was able to show that, unlike living bison, Pleistocene steppe bison had a dorsal hump placed posterior to the shoulder and, furthermore, that the hump’s posterior edge was strikingly convex. As I have shown, this pattern is identical to that of steppe bison from Alaska.

  Functional Significance of Steppe Bison Humps

  As our comparison showed, the biggest difference in hump shape is between the humps of the steppe bison and the American plains bison. The crest of the American plains bison hump is located in front of the scapula, while the steppe bison’s crest is behind the scapula; a possible difference in function may be the humps’ mechanical relationship to the nucal ligament. Let me propose a new theory.

  The low-slung character of the American bison’s head is, I believe, derived from its specialized adaptation to the thin sward of the American shortgrass plains. Not only is the grass normally low in growth form after being grazed, but it readily regrows into a very short “lawn” pattern in an especially thin sward. To get sufficient quantities of grass, American bison must spend many hours grazing with their heads almost touching the ground. There is a dearth of midheight forage.

  As a direct product of this adaptation, the entire anatomy of the American plains bison has located the normal position of the head lower than in other bovids (like a lawn mower set low to the ground). This changes the optimum mechanical arrangement of the nucal ligament for head raising and lowering to a more forward emphasis on the hump, its point of origin (fig. 5.12). Additionally, the lower head position changes the optimum mechanical angle for the neck ligament to a more forward or anterior location. Thus the hump of the American plans bison may be part of a network of adaptations to a lowered-head position as a lawn grazer, quite unlike the environmental adaptations of other large bovines. If that is correct, their hump would be an evolutionarily derived trait from a more steppe bison–like hump.

  Fig. 5.12. Arrangement of nucal ligaments in living European (B. bonasus) and American bison (B. bison). Normal head carriage is lower in American bison, necessitating a more vertical alignment of the neural spines and a more forward location of the longest neural spines.

  Steppe bison seem to have held their heads higher, at rest, than do American plains bison—more like fossil and recent European bison. This can be seen in Paleolithic art. Steppe bison probably did not occur in vast enough herds to promote a lawn system of grazophilic grass species, as do wildebeest and plains bison. Rather, modest densities on a relatively cold-arid grassland would have necessitated considerable mobility. The larger shoulder hump apparently enhanced the ability of these large creatures to move rapidly across the landscape. Not only did Pleistocene bison have to move great distances to find fresh ranges and water, but they also had to run away from predators. This necessitates seeing predators before they come dangerously close.

  On the generally open Mammoth Steppe landscape, approaching lions could be seen at considerable distances by a bison with head raised to a position of modest height. Among small herds of steppe bison, predation alert would have been a selective pressure countering a permanently lowered-head arrangement. Given some advantage to a higher resting head position, the location of the steppe bison’s hump crest behind the scapula may be the optimum pattern to maintain both speed and galloping efficiency of this large and powerful bison.

  Fig. 5.13. The dorsal contour of steppe bison from Paleolithic art. Numerous drawings of steppe bison in Paleolithic art show an abrupt “stop” just posterior to the hump which seems to have been a darker color as well. A distinct neck mane is shown which is also dark. This mane is not present in American plains bison (B. bison), but there are slight traces of it in European bison (B. bonasus). These bison are drawn from (a) Marcenac, (b) Etchberriko-Karbia, (c) Santimimine, (d) La Pasiega, (e) Le Portel, (f) Pindel, (g) El Castillo, (h) Le Tuc d’Audoubert, and (i) Niaux.

  Despite the absence of Blue Babe’s hump, we have successfully reconstructed it indirectly. Given that Blue Babe’s hump is similar in structure to the hump of Pleistocene European bison, it is likely that the pelage of Blue Babe’s dorsal line was also more similar to European steppe bison than to any other bison, European Paleolithic art provides many drawings of steppe bison (fig. 5.13). In these drawings the stop in the posterior thorax is emphasized by a contrasting, dark-colored hair patch; the dorsal neck hackles also form a hump and are of a contrasting dark color. These appear as two humps.

  Working on the ethology of extinct species may strike most ethologists as a strange enterprise, but most ethological studies rely heavily on the literature, that is, on others’ earlier observations. In many ways, Paleolithic works of art are very old references, just recently uncovered. While they do not always tell us exactly what we want to know, they provide a wealth of information, some of it obviously incorrect, as indeed in the case of older scientific literature. But we can see by their work that Paleolithic artists were empirically oriented, drawing mainly from observation rather than copying some stylized icon form.

  6

  STEPPE BISON ETHOLOGY

  Social Organs and the Art of Spying on Ice Age Behavior

  Many aspects of an animal’s appearance are “social” in function; the wattles, stripes, manes, horns, and color patterns shown in an illustrated field guide, for example, are usually social organs. Social organs distinguish the sex, age, and social stature of the bearer; they are flaunted in displays of aggression, submission, sexual attraction, or social bonding. Of course, much of a species’ social behavior is shaped by environmental adaptations that also affect anatomy, and thus appearance is an interacting composite of social garb and environmental adaptations. Too brilliant a coat can be dangerous, so while an individual concerns himself with winning stature and females he must also worry about predation. Likewise, features that make an individual stronger among peers (large horns, muscles, and heavy bones) may also make him slow and less agile in defending against predators. Social anatomy and survival have to strike an evolutionary compromise, but the balance of this compromise should theoretically vary with environment.

  Estes (1974) outlined the interaction of behavior and environment among African bovids in a gradient from woodland edge to savanna and fully exposed plains. I retailored these (fig. 6.1) to describe evolutionary trends in northern bovines (Guthrie 1980). I think that Pleistocene bison living in an open steppe experienced increases in agonistic behavior, nonlethal fighting apparatuses and techniques, group size, class hierarchies, and gaudy social display organs. In addition to the above changes, homeland fidelity probably declined and was replaced by more migratory or nomadic tendencies. Feeding in a steppe environment became more unselective, and wintertime diets included a high percentage of fiber.

  Animals living on the Mammoth Steppe experienced a highly seasonal environment: the quality, abundance, and digestibility of their food changed markedly throughout the year. Monocotyledons and nonwoody forbs were relatively rich and abundant during the short summer, but each fall, as growth stopped, these plants moved most nutrients to their roots, leaving poorer quality dead tissue above ground. Pleistocene bison depending on this low-nutrient winter forage required a large rumen to incubate quantities of the high-fiber, poor-quality food. Severe northern winters and attendant mortality would have kept bovine populations well below summer carrying capacity, thus reducing competition for resources during the growth season. Abundance and quality of food in the growing season gave each animal the resources to attain its full genetic potential for horn and body size. I have proposed (Guthrie 1980, 1982, 1984a, 1984b) that such strong seasonality shifts selection optima and favors a larger body size.

  Fig. 6.1. Woodland versus steppe adaptations. Early bovines originally were adapted to wooded habitats. The expansion of open grasslands created a potentially rich niche, but the move to more seasonally variable grasslands involved new demands and produced different adaptation
s. Behavioral and morphological traits characteristic of woodland and plains bovines are summarized in this illustration.

  The larger group size characteristic of plains ungulates (Estes 1974) generally precludes a linear social hierarchy. Linear hierarchies are common in feral cattle (Schloeth 1961; Frazer 1968) as long as the cattle are in small groups. Hierarchies in large dairy herds of fifty or more often include such subgroupings as “triads” (Brantas 1968). Both American bison (McHugh 1958; Lott 1974) and European bison (Jaczewski 1958) have rank hierarchies, but the former tend toward a class ranking system, particularly when studied in more natural situations in large parks (Shult 1972; Petersburg 1973). Holocene bison of Europe inhabited parkland openings in a forested landscape and probably experienced selection pressures for homogeneity of color and reduced display organs. The reverse is true of American plains bison on expanding Holocene grasslands. These grasslands had no counterpart in Holocene Eurasia. American plains bison developed extreme display paraphernalia, weapon morphology (short stubby upward-oriented horns), and fighting styles (head clashing). Along with wildebeest, American bison are at the farthest end of Estes’ (1974) spectrum of plains-adapted bovines that live in large herds (fig. 6.1).

  Migratory species living in highly seasonal plains environments sometimes do not form reproductive territories; instead, males display themselves, and outside the brief reproductive period, mature bulls form separate herds. Sexual dimorphism is most exaggerated among these animals. During the breeding season, males join the female bands where they seek out and “tend” females in estrus, fending off an array of contenders, then move on to the next estrus female. Intense selection pressures favor males sporting the most extreme display paraphernalia. These social organs are used for only a brief period, but without them bulls are reproductively neutered (Petersburg 1973).