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

by Guthrie, R. Dale

  Fig. 7.2. Plant fragments compacted in molar infundibula. Histological structure allows even tiny pieces to be identified to plant group. Decay is so limited in frozen soils that plant fragments taken from Pleistocene-aged Alaskan fossil teeth can be identified.

  There are some frozen feces of Alaskan Pleistocenen large mammals, but these are very uncommon. Studying diets via plants in fossil teeth was much more promising, and I was especially fortunate to be working with fossils preserved in frozen ground because plant fragments in the teeth were usually well preserved. This is not the case among most fossils from temperate and tropical climates, where it is usual for plant parts trapped between the cusps to be thoroughly decomposed.

  Relying on fossil epidermal fragments in teeth was, however, quite different than sampling feces; I needed a modern analogue to tell me how well the material I took from fossil teeth reflected the actual diets of the animals on the Mammoth Steppe. In 1985–86, while on sabbatical in Africa, I was able to collect tooth samples from a number of large mammal skulls, including animals that had died naturally and ones that had been culled by wildlife managers.

  I sampled strict grazers, like the Alcelaphines (wildebeest, blesbok, etc.), mixed feeders (for example, springbok and impala), and browsers (such as eland and kudu). Fortunately, I was able to sample in areas where the diets of these species and the plant fragments found in the molars had already been studied. Correspondence between the natural diets of these species and the plant fragments found in the molars was almost complete. This information from African analogues allows us to say that the plants we find in the teeth of Pleistocene large mammal fossils probably indicate their usual diets. I report on this study in greater detail elsewhere, but the general conclusion is that grass fragments were the predominant food found in the teeth of steppe bison, horses, and woolly rhinos. Data summarized in table 7.1, based on plant material taken from 44 steppe bison, are especially relevant to questions about Blue Babe’s diet.

  Histological studies of bison known to be eating a grass diet show that their feces contain only 80–90% grass fragments. There are two reasons for this. First, a big-bite grazer is not selective enough to eat only grass; dicot forbs and moss grow in between grass stems and are ingested with the grass. Although more toxic than grass parts, small amounts of these dicot forbs from diverse species actually cause little metabolic harm. In fact, small doses of them can add greatly to overall nutrition because dicots normally are higher in protein than are grasses, which is why dicots are so well protected with pharmacologically active defense compounds. Second, woody plants such as willows and some other shrubs can be quite digestible, even for grazers, if they are eaten in the spring. These shrubs’ new growth tissue is fairly high in protein and low in toxic defense compounds.

  Table 7.1 Plant Cuticle Fragments Taken from First Molar of 44 Fossil Steppe Bison from Interior Alaska

  A second dietary issue is rate and pattern of toothwear. Bison vary tremendously from area to area and through time in the rate at which they wear their teeth (Reher 1974, 1977; Frison and Reher 1970; Haynes 1984). The lower Ml crown height (enamel height of the metaconid) has been chosen to quantify differences in wear. It is the first permanent cheek tooth to erupt and thus wears out first, giving the most dramatic index of tooth wear.

  Frison and Reher (1970) calculated wear rate for Holocene bison from the late Pleistocene Vore Site to be 3.5–3.8 mm per year. They point out that tooth wear rate changes with geological time among plains bison. Haynes (1984), however, observed that plains bison teeth always wear faster than those of extant bison in Wood Buffalo National Park located at the border of Alberta and the Northwest Territories.

  Haynes found Wood Buffalo Park bison have unusually low rates of tooth wear, almost half that of plains bison (Haynes 1984). The northern Canadian bison averaged 1.7 mm per year of enamel attrition on the lower Ml, using the same measurement of metaconid height as Frison and Reher did. Additionally, the occlusal surface of their teeth is different. It wears in a deeply furrowed pattern, creating a “steepling” effect. Most young bovines have this pattern, but with age it is smoothed into low ridges. This steepling is characteristic of an unabrasive diet and is common among browsers such as moose (Alces), which eat vegetation that is very low in phytoliths.

  Haynes (1984) attributes this pattern and slow rate of tooth wear to the diet of the Wood Buffalo National Park animals, which consists mainly of sedges growing there in dry pans of Pleistocene lake beds. Unlike grasses, sedges have less biomass in opaline phytoliths. These bison therefore consume fewer abrasive particles per volume of forage. Haynes proposed that amounts of phytoliths in the diet and the amount of dust on leaf surfaces are the main factors in tooth wear. The differences between the teeth of bison Haynes studied and those of the Great Plains bison offer the opportunity to assess teeth of fossil bison in light of these ecological differences.

  There has been considerable controversy over the nature of Pleistocene vegetation in Beringia (which I discuss in detail in chapter 9). Several botanists (Cwynar and Ritchie 1980; Colinvaux 1980) have argued that Beringian vegetation was more alpinelike mesic tundra, not very different from today’s high-arctic tundras. These present-day tundras are dominated by sedges with few grasses, and the grasses that do grow there tend to be more mesic adapted. If Beringian vegetation was more tundralike, one would expect fossil Alaskan bison, particularly those living during the interstadials when conditions were even more mesic, to be mainly sedge eaters and would show tooth wear rates and patterns similar to those studied by Haynes.

  We can figure how rapidly Blue Babe was wearing his teeth. The metaconid height was 16.2 mm. Haynes uses 41.71 mm as the crown height for a newly erupted lower Ml in a 3-year-old animal. Assuming Blue Babe was 8 years old at death, we can calculate rate of tooth wear to be 5.1 mm per year. This rate is much more rapid than Haynes found among his Canadian bison; in fact it is among the more rapid rates of tooth wear observed in bison anywhere. Reher (1974) calculated that the average enamel height of an 8.5-year-old bison, at the 10,000-year-old Casper Site in Wyoming where the tooth wear rate was considered rapid, to be 20.5 mm (in contrast to the 16.2 mm of Blue Babe).

  Fig. 7.3. Bison diets as indicated by tooth wear. Teeth are a rough indicator of diet. Sedge-eating bison in Wood Buffalo Park have peculiar teeth: they wear down at less than half the normal rate and show a pattern of high steepled cusps like teeth of a browser. Steppe bison teeth are worn more like those of grass-eating bison on the Great Plains.

  The wear pattern on the mummy’s cheek-tooth occlusal surface is more similar to that of living plains bison and quite unlike that of the Wood Buffalo Park bison (fig. 7.3). This suggests Blue Babe’s diet was more like that of open plains bison, which are obligate grazers.

  Age tables of Pleistocene Alaskan bison made by Skinner and Kaisen (1947) on the basis of mandibular tooth wear clearly show wear classes and attritional survivorship curves similar to those from plains bison and dramatically unlike the sedge-eating Wood Buffalo Park bison. From a sample of 1,322 individual Alaskan Pleistocene bison, Skinner and Kaisen showed that the majority, that is, those dying in the senility mode of the curve, had almost exhausted or had exhausted the entire crown of Ml. In their sample, 14–16 years represents a very old animal (Skinner and Kaisen 1947), judged by horn sheath annuli; this corresponds well with the rate of tooth wear in Blue Babe (8 years for half-worn Ml). Skinner and Kaisen’s first age-class (S-4) identified an extreme degree of crown wear with an almost completely worn Ml. This age class contained more individuals than any other (40% of the 1,322 mandibles).

  Examining the same hundreds of mandibles studied by Skinner and Kaisen, I also found flatter patterns of tooth wear than those Haynes observed among his nongrazing bison; rather, the steppe bison molars were similar to those of Blue Babe and Holocene Great Plains bison. I did not find teeth with extreme furrowing or steepling characteristic of the Wood Buffalo Park bison. These data suggest a grassy arid
habitat in Beringia.

  In addition to studying bison cheek teeth to determine patterns and rates of wear, the shape and size of the nipping incisors and the premaxillary plate against which they “occlude” can tell us something about steppe bison ecology. There are considerable differences in these characters among different bison groups. Like cattle, European bison have narrow premaxillary widths and American plains bison have the broadest, while steppe bison have intermediate widths. The question is why.

  Generally speaking, grazers have very broad incisor batteries and premaxillary widths, and those of browsers are quite narrow. Actually, the story behind these differences is that selective feeders must have a more refined “forcep” because they choose to eat very specific foliage, high in energy and nutrients, but often adjacent to other plants or plant parts which are to be avoided because of their antiherbivory defenses. Some grazers are also quite selective, like roan and sable antelopes (Hippotragus); these have pointed snouts and comparatively narrow premaxillary widths. So only certain kinds of grazers have these traits exaggerated. When one examines grazers more carefully, the anatomically wide nippers are the ones that graze on a sward of shortgrasses. More important, these species are large herd animals which often regraze the same area several times during the same growth season. Many grasses are adapted to this kind of repeated grazing and respond by shifting away from a tall-growth form and sexual reproduction to a more prostrate growth form and vegetative reproduction by tillers. This low-growth vegetation makes grazing not only difficult for a large grazer but reduces the “take” of each bite. In response, the “lawn grazers” have expanded the width of their biting apparatus—the premaxillary plate and incisor battery—not to be less selective but to increase the volume intake of each bite such that these regrazed lawn growth forms can be exploited. This is not to say that wildebeest or plains bison always or regularly needed this broad nipper, just that they have met with those requirements sufficiently frequently in the past to be able to exploit and survive those situations when they occur.

  Whatever the character of the Mammoth Steppe rangeland across northern Eurasia, the mammoth fauna probably never were so numerous that they resorted to lawn-forming kinds of grazing behavior. This is not true of American plains bison. We know from early records that herds of millions of bison did regraze. Likewise, the kinds of grasses growing on the shortgrass plains were adapted to lawn formation. Some of the grasses, like the ones eaten by wildebeest in Africa, are cultivated today as commercial lawn species because these grasses are already adapted to “mowing.”

  Although the premaxillary and incisor width of Blue Babe and other northern steppe bison, B. priscus, is not as wide as that of the American plains bison, B. bison, it is quite wide (figs. 7.4 and 7.5). These bison are very unlike the living European wisent, B. bonasus, which has the narrow nipping arrangement of a more selective grazer indicative of either thicker grasses or a more eclectic diet, and a diet higher in dicots.

  We can say that Blue Babe and steppe bison in general were grazers; that these bison chose bites from a thin to modest sward but were not well adapted to using and reusing law-forming grass species to the extent of modern bison on the American Great Plains. We have assumed that the steppe bison did little browsing because for tens of thousands of years during the full glacials there was little browse. There were few trees on the mammoth steppe. The anatomy of steppe bison incisors and premaxillaries supports this assumption. During the interstade, when some woody vegetation reinvaded the Mammoth Steppe, bison probably utilized the leafy parts of dicots, especially during the summer, as a dietary supplement.

  I found compacted plant matter in Blue Babe’s first molars. Cheek teeth of large herbivores have small infundibular pits between the cusps which become permanently filled with plant fragments from the food being chewed. Leaf and stem cuticles are very resistant to both chemical destruction and decomposition, as they form the outer protective coating or seal of the plant against parasites or infection. Because of this durability they preserve quite well. Also, they form an imprint or “fingerprint” of the underlying epidermal cells which tend to be very characteristic for each plant group. So, not only do epidermal fragments preserve well, but they are very informative as to which plants were eaten. Many studies have used epidermal fragments from fecal pellets or stomach contents to reconstruct diets of grazing mammalian herbivores.

  Fig. 7.4. Incisor width and diet. Among grazers the width of lower incisors (the lower canine is incorporated into this row in incisiform pattern) is associated with adaptation to grazing on short grasses. The incisor row is widest in American plains bison (left) and intermediate in steppe bison (middle). European bison have the narrowest incisor row (right).

  Fig. 7.5. Premaxillary width and diet. The “breadth of bite” among ungulates is a general indication of dietary selectivity; species with narrow mouths can be very selective about each bite. Browsers are normally more selective and hence have narrower biting structures than grazers. Bite width among grazers also corresponds to the volume of sward regularly available in one bite. Animals that feed on very short, previously grazed “lawns” tend to have wider mouths. Premaxillary bones which underly the biting pad are widest in American bison (left), intermediate in steppe bison (middle), and narrowest in European bison (right).

  Blue Babe’s tooth contents were sent to a commercial histological laboratory (at the Department of Range Management, Colorado State University, Fort Collins, Colorado) for identification, where the cuticles mentioned earlier in this chapter were analyzed. Only about 20% of the cuticle fragments were identifiable, and then only to genus. Two grasses were the most common. Wheatgrass or Agropyron was one of them. There are several species of wheatgrass in Alaska today. They are somewhat “weedy,” that is, they are found on disturbed gravel bars and riverbanks or high alpine meadows. They all occur in rather dry habitats (Hultén 1968).

  The other kind of grass cuticle in Blue Babe’s teeth was Danthonia, a grass that is moderately common at mid latitudes in the plains states, but does not now occur in central Alaska. There are some specimens collected from the Matanuska Valley and Seward Peninsula, and it is possible that these are introduced from farther south. From Hultén’s (1968) map this genus seems to be especially widespread in the southern Yukon Territory.

  In addition to these grasses, a willow (Salix) twig tip was found among the cuticle remains, so the presence of grass and some dicots is consistent with the dietary portrayal of steppe bison predicted by its morphology. This would be especially true of the interstadial time, in which Blue Babe lived, when more willows would have been available.

  Interstadial Bison in Alaska

  Discussions about the ecology of large mammal species on the Mammoth Steppe have more or less been limited to glacial episodes (see articles and references in Hopkins et al. 1982). We know much less about Beringian mammals during interglacials, primarily because the depositional and preservational environment was better during glacials (Guthrie 1985a). More acid soils and complete vegetational mat characteristic of warmer and wetter peak interglacials left a peaty deposit relatively devoid of bones. However, some specimens, including Blue Babe, date from the last interstadial (Hopkins’s Boutellier Interval); these can tell us something about interstadial conditions.

  Strangely enough, most of the Mammoth Steppe fauna continued throughout the last interstadial (from about 60,000 to 28,000 years ago), in roughly the same proportions. Three species (bison, horse, and mammoth) dominate most faunal assemblages reported from that time (as an example see the radiocarbon dates listed from the articles in Hopkins et al. 1982). What is odd about the persistence of bison, horse, and mammoth during the interstadial is that this was a time when trees recolonized much of Beringia, approximately reaching their present distribution. Hamilton (1979) described spruce stumps 41,000 years old found on the Chandalar River, near the present northern limit of spruce. However, the presence of Mammoth Steppe fauna during the last inters
tadial suggests that the climate and landscape must have been rather dry, unlike that of today. The continued presence of bison, horse, and mammoth indicates an arid, grassy environment, with facies not too different from those occurring during glacials over a wide area in all kinds of topographic terrain. Yet tree species that now characterize the boreal forest were present (Schweger and Janssens 1980). Such a combination is strange to consider because it is not comparable to our experience with the present boreal forest or with an arid and treeless steppe. This picture reflects what Andersen found in the pollen around Blue Babe (Appendix B).

  Today, both the northern and altitudinal limits to the boreal forest are apparently controlled by warmth, that is, by degree days. Since the present amount of moisture throughout the north is so meager, we might assume it represents a minimum quantity, but in fact it can be reduced. The southern border of the boreal forest is controlled by aridity, at least in the southwestern part. Picture for a moment a combination of these factors: aridity controlling treeline from below, so to speak, and summer warmth controlling the upper reaches of treeline. We can better imagine the interstadial as a combination of warmth allowing trees to expand northward to their present altitudinal and latitudinal extent, but under more arid conditions. Trees would then grow only in well-watered locations, particularly along streams, like the way trees penetrated more arid southern grasslands (Wells 1970). To empty Beringia of trees, as occurred during glacial peaks, by cold alone would have required very acute changes. But increased aridity in conjunction with cold would be a severe challenge to trees. The interstadial climate then was an unusual mixture of warmth, with a little less than the same degree days as present, and considerably less moisture. This allowed woodlands to grow along valley bottoms, with restricted accumulation of peats; at the same time extensive grasslands marked better-drained environments and probably much of the broad lowlands away from stream arteries.