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

by Guthrie, R. Dale

  This “B. latifrons–B. antiquus” line exhibits decreased horn size and increased doming of the frontals. Also the orbitals are less telescoped outward. The horn cores extend outward in more of a straight line from the central axis of the skull. There is less hook in the horns, and the bases are thick in comparison to northern bison. Wilson (1975) separates these northern and southern “types” in relation to the Holocene bison, a point to which I return later.

  In northern Eurasia, in Alaska, and in the Yukon Territory, the large B. priscus decreases in size as it continues through to the Holocene. It passes through forms Sher (1971) has called B. priscus diminutus–B. bison athabascae in Siberia; in Alaska, Skinner and Kaisen (1947) have called the forms B. crassicornis–B. preoccidentalis–B. occidentalis–B. bison athabascae. Whatever names one uses to label the change, there seems to be a diminution of body size in the far north, starting about or slightly earlier than 13,000–12,000 B.P.

  Europe is more of a puzzle. Vereshchagin (1959) proposes that in the Caucasus, B. priscus decreased in size toward the small living B. bonasus of Europe well before the Holocene. The fate of central and northern European B. priscus is unclear; whether it was replaced by a smaller bison from elsewhere (Kowalski 1967a) or whether it experienced a size decrease (Degerbol and Iverson 1945; Gromova 1965) is unknown. The living European bison, B. bonasus, seems to be an evolutionary regression toward the woodland-edge cattle adaptation. Like cattle, B. bonasus even marks its territory (Zablocki 1967).

  Thus during the Holocene (and perhaps earlier in some areas), we have a picture of Eurasian bison rapidly decreasing in body size. There was also a decrease in numbers and in distributional extent (Vereshchagin 1959).

  The picture in North America, however, is different. There Holocene bison were becoming more abundant while also decreasing in body size. At the end of the Wisconsinan, evidence shows that the northern small form of B. priscus (called B. occidentalis) reinvaded the Great Plains and came in contact with the line of B. latifrons (called B. antiquus) from the south (Wilson 1975). The northern forms, having decreased in size independent from the more southern forms, were then not very dissimilar in body size at the time of their early Holocene contact, although they differed somewhat in cranial and horn characters (fig. 7.8).

  Fig. 7.8. Ancestors of plains bison. Two late Pleistocene-Holocene bison groups are given a variety of names because their biological relationships are as yet unclear. The northern form is referred to as B. occidentalis and the southern form B. antiquus. One, or both, of these gave rise to the living plains bison, B. bison.

  Wilson (1975, and elsewhere), in a study of bison from well-dated archaeological sites, has presented a convincing model of the southern movement of the northern Bison occidentalis displacing B. antiquus. Additionally, he found several sites in which there were mixtures of the occidentalis and antiquus skull patterns (fig. 7.8) in various combinations. He interpreted this as strong evidence for genetic remixing of the two lines.

  The only thread of information running contrary to this interpretation of B. priscus–B. occidentalis is the data about hump shape (chapter 5). All of the fossil bison humps from Alaska and the Yukon Territory have a definite B. priscus dorsal arc, which shows a definite concavity in the dorsal contour in the region of T12–T13 neural spines; B. bonasus shows a similar contour. However, both B. bison bison and B. bison athabascae show a rather straight dorsal contour (van Zyll de Jong 1986). The nominal species, B. antiquus and B. occidentalis, show a slight concavity like B. priscus, but the anterior thoracic neural spines are very similar to B. bison (fig. 7.9), so the most appropriate specific designation is unclear.

  All this controversy over bison nomenclature seems unnecessary. If we had referred all late Pleistocene and living bison to one species as we have done with caribou or African buffalo, as some have suggested (Bohlken 1967; van Zyll de Jong 1986), bison phylogeny would not have clouded the important issues of bison paleobiology and biogeography. Many things, such as horn-size reduction, are obviously parallel adaptations, and multivariate techniques used to show nearest neighbor groups must incorporate biological-paleontological information into their models to give us much real insight.

  In figure 7.10 I have summarized this brief history in relation to body size, geography, and chronology. I have emphasized body size because it is conveniently pictured, but size is correlated with a number of other traits such as frontal sinus development, horn size, and many horn characteristics, such as ventral rugae or flutes on the horn core. I have tried to be both abstract and conservative. There are a few “outliers” which do not fit this portrayal and several controversies involving placement of the hatched lines. I think, overall, it is an accurate summary.

  Ancestral interaction of these groupings is, however, a different matter. The most logical route is to derive a later from an earlier one in the same region; we know this does not always happen, but it is the usual case. Therefore my phylogenetic connections are vertical unless there is some evidence to believe otherwise (fig. 7.10).

  Fig. 7.9. Hump shape in American bison. Bison humps can be seen in these mounted skeletons of B. bison (upper left), Smithsonian, National Museum of Natural History; B. latifrons (upper right), Idaho State Museum; B. antiquus ~ occidentalis (lower left), Nebraska State Museum; B. antiquus ~ occidentalis (lower right), American Museum of Natural History. Though the fossil specimens are not identical to those of recent B. bison, they all have a more forward hump than the Eurasian steppe bison, B. priscus (compare with fig. 5.8). (Drawn from photos)

  In previous chapters I have shown that Blue Babe is virtually identical to Pleistocene bison in Siberia, known as steppe bison, B. priscus. The Alaskan Pleistocene mummy is also closely related to European Pleistocene steppe bison. Steppe bison ranged from Alaska to western Europe, where they were important figures in Paleolithic art. The Blue Babe mummy allows us to compare the easternmost steppe bison with its counterpart two continents away in Europe. Although B. priscus seems rather conservative compared to many large mammals, the species did change some, both geographically and over time. Blue Babe also allows us to better compare northern steppe bison with their more southern relatives in the rest of North America.

  Fig. 7.10. Schematic pattern of bison body size changes throughout the Quaternary. The course of bison evolution is not simple, but it appears that earlier forms were small (there are some exceptions) and that bison increased in size in the mid-Pleistocene. There was a late Pleistocene reduction in size among all bison, and this accelerated during the Holocene. Horn size followed the same pattern.

  While different from his Pleistocene contemporaries in Europe and his later relatives in North America, Blue Babe resembles both. In basic color, in pelage length and in hump shape he seems more closely connected to European B. priscus, yet, his tail length is more like living American bison. In almost all traits he is more similar to B. bison athabascae, the northern subspecies of American bison, than to the more southern subspecies, B. bison bison.

  These characteristics, both in time and from west to east, seem to follow general social-anatomical patterns first seen by Geist (1971a, 1971b) in ungulates. They include shortening of the tail, concentration of social display into the forward part of the body, and a richer, more contrasting pelage pattern. These trends apparently reversed during the European Holocene, as the earlier European B. priscus is much more dramatically adorned than its Holocene descendant, European B. bonasus.

  8

  THE MAMMOTH STEPPE

  The Boreal Forest Today

  Today, by any judgment, the Fairbanks area is not good bison habitat; yet in the rich Pleistocene fossil deposits of interior Alaska, bison are the most common large mammal. As I write these lines, over a meter of snow covers our yard, and until April this undrifted snow will blanket most of the interior of Alaska. In record years snow may reach a meter and a half, or more. With effort, moose can extract their long legs and wade through such deep snow, but movement in t
hese circumstances is very difficult for bison. Moose now feed easily on willow trees near our house. Moose are browsers, and they have a wide selection of twigs above the snow. Bison have less choice; they have to get under the snow to reach grasses. The energetics of reaching grass buried under a meter of snow would not balance the investment, and bison would not survive such a winter.

  Bison can tolerate deep snows for a short time, but their limit for snow depth for a sustained period is 60 to 70 cm (depending on the size of individual bison) (fig. 8.1). Across interior Alaska, ridges above treeline, where snow is windblown and re-sorted into hard pack, provide winter range for dall sheep. Like bison, sheep cannot regularly walk or feed in deep snow. But bison cannot survive on winter ranges of mountain sheep because the ridgetops lack the proper kinds and quantity of vegetation to sustain bison.

  Bison were reintroduced to interior Alaska over forty years ago. These modern Alaskan bison are able to live in special habitats where winds, sweeping out of mountain passes, strip snow from broad river floodplains. Frequent summer winds and braiding streams produce grassy river bars and meadows, thus providing the combination of grasses and winter access that bison require. Such appropriate habitat is, however, quite limited, and bison have not colonized other habitats. Bison at Healy are fed supplemental brome hay during the winter; Delta Junction bison forage in the local barley fields, which are frequently left to overwinter unharvested, due to bad fall weather. Although Healy and Delta Junction bison now use some agricultural resources, the habitats in which they live can overwinter a few head of bison naturally. Smaller groups of bison overwinter without assistance on outwash plains of the Fairwell, Copper, and Chitna rivers.

  Fig. 8.1. Foot and leg adaptations to snow. Large mammals are not equally restricted by deep snow. When snow reaches the chest (this depends on leg length), an animal can no longer lift its legs above the snow surface, and this consumes a great deal of energy. Foot loading, the ratio of weight to surface area of the foot, is also important. Some animals are able to walk on the snow surface. Both leg length and foot loading are important in the degree of snow tolerance characteristic of each ungulate group. This plot shows that bison excel at neither; they are poorly adapted to deep snow. (Data after Telfer and Kelsall 1984.)

  Away from the mountains, the interior has very little winter wind, and as a consequence, large snowdrifts are never produced. Snow accumulates on fence posts in tall top hats that gradually bow toward the sun, as the low sun angle recrystallizes and sublimates the southern face of the snow column.

  Most ungulates in the far north are adapted to winter cold. Their major stress is dietary: finding sufficient quantities of digestible food. Northern ungulates have adapted to meager winter diets and usually survive so long as snow does not severely restrict access to food.

  Unlike bison, caribou are true snow deer. Broad fore feet allow caribou to dig down to lichen if the snow is not too deep or too hard packed. Bison lack adaptations to deep snow; they have relatively short legs (unlike moose) and very high foot-loading (unlike caribou), and bison hooves are not shaped for digging snow. Instead, bison use the sides of their heads to sweep away snow in a sweep-sweep-bite pattern which is not effective in deep soft snow or in packed snow.

  Access through snow is not their only problem; in much of interior Alaska there is little bison can eat. The understory and occasional meadows of the taiga forest and tundra contain little food for a large grazer like the bison. Caribou shun most plants beneath the snow; they select lichen which is low in calories but very easy to digest. Caribou can locate lichen by smell, even under deep snow. One might think a long-legged browser such as a moose would be able to eat anything in sight, but palatable and digestible twigs are not common in a mature boreal forest, and moose must roam about, looking for the right sorts of willows in stream bottoms or old burns. They drift through our yard all winter, clipping and reclipping the willows, garden trees, and shrubs.

  Many plants in the boreal forest and tundra are simply unpalatable or even toxic to large mammalian herbivores. Biologists have just begun to understand that that is part of a cycle peculiar to the north (fig. 8.2). Permanently frozen ground and cold soils tie up nutrients. Plants that can grow in these low-nutrient conditions tend to be tolerant of nutrient stress. They are adapted to rather slow growth and very slow rates of nutrient removal from the soil (Chapin 1980); they are conservative and cannot afford to keep reserves stored in underground roots. Thus, most of their biomass is above ground. Plants such as Labrador tea (Ledum) and spruce (Picea) cannot survive heavy browsing by herbivores because they lack sufficient underground resources to recover quickly. They use nutrients conservatively, allocating some resources to growth and others to toxic chemical defenses. Thus, we may find a six-inch diameter black spruce tree that is a hundred years old. This spruce survived because it produced toxic defenses—terpines and phenoloics that made it taste terrible to any passing herbivore venturing a bite. Producing the toxins was effective but costly.

  Fig. 8.2. The boreal forest cycle. Present vegetation in the far north is maintained by a cycle of summer moisture, low evaporation, and insulating mats of toxic vegetation that decompose slowly. This insulating layer raises the permafrost zone and limits the rate of annual frost removal. Restricted access to soil nutrients favors conservative plants with few subsurface reserves. These plants are well defended against herbivory by toxic compounds that also inhibit decomposition. Fires rarely burn deep enough to remove this insulating toxic mat; fires tend to interrupt rather than break the cycle.

  Even dead needles and leaves of these conservative plants are so toxic that decomposers leave them for decades until physical processes begin their breakdown. In a spruce forest or sedge meadow, this toxic plant litter accumulates, forming a thick, spongy mat that insulates the soil and inhibits summer thaw. Gradually permafrost creeps upward until it lies just under the mat; summer thaw does not drive very deep. With deeper soils frozen, only shallow soil nutrients are available and nutrients from dead plants recycle slowly because of slow decomposition. Plants living in these conditions must be able to extract nutrients from a shallow, nutrient-poor zone, and for this reason they have a shallow root system. This is why a fallen spruce tree’s roots look like a large suction cup or saucer popped off just beneath the surface; there are no tap roots. The conservative, rather toxic plants that are able to live and grow in these poor, shallow soils are generally not the kinds of plants large mammals can eat.

  It is possible that a tourist driving through Alaska, looking day after day at boreal forest, covering all the major roads in the state, and scanning thousands of square miles, may never see a large mammal. Moose are thinly distributed and are limited to habitats where some disturbance breaks this inhospitable substrate, such as a stream that deposits fresh nutrients along its banks every year and keeps permafrost at bay by the relative warmth of water running at nearly freezing temperatures all winter, resulting in well-drained banks and edible willow cover. Fire also plays an important role in creating moose habitat. Permafrost is lowered by a forest fire when the burn strips insulating tree cover. But fires do not usually burn the mossy soil insulation, and before long more conservative plants are growing up in the old burn.

  In winter, moose cruise the countryside looking for these polka dots of hospitable edibles: some tasty willows on a gravel bar or, two ridges over, a patch of willows growing in a fifteen-year-old burn. Between these there is little for moose to eat, but likewise there are no niches open for bison, elk, or horses, especially in the winter. Bison do not live here because there is poor to nonexistent summer range and no winter range at all.

  A large percentage of the plant mass in the north is poisonous to most large mammals. Even in the vast green summer landscape, there is little to eat for most large herbivores. Far to the south, say in the Mississippi Valley, vegetable resources are more substantial: tubers, nuts, thimble-sized or larger fruits, cereal seeds, and so on, but these are almost
nonexistent in the far north. Before Europeans came, northern natives ate mostly fish and mammals. Plants served as a garnish; plant resources supplied few energy or growth nutrients.

  The scattered habitats for bison, caribou, and sheep in the far north seem to be a relatively recent phenomenon, dating since the early Holocene. We find bones of these species in Pleistocene deposits many kilometers away from their present habitats. Pleistocene soils seem to have been more fertile over a broad area, favoring the kinds of plants bison and other grazers need. What would happen if northern soils were more fertile—if we were to dump a mixture of nitrogen, phosphorus, and potassium on today’s northern vegetation? Chapin (1980) performed this experiment and found that mosses and sedges were quickly replaced by grasses. Grasses and some classes of forbs take up nutrients quickly; they outpace and displace more slow-growing conservative plants. Increased soil nutrients changed the competitive balance, favoring plants with a different life strategy.

  These plants (mainly grasses but some forbs as well) can escalate their rate of nutrient uptake in accordance with the amount of nutrients available. Every year they generate new leaf tissue and renew root endings. Root systems of these less conservative plants are elaborate, enabling them to tap richer below-ground resources. Because they can rapidly extract soil resources, these plants keep large reserves in their extensive root systems. Changes above the soil surface thus affect them quite differently. Fire or grazing, for instance, can actually increase their competitiveness. Fire removes insulating litter and quickly recycles mineral nutrients. Unselective clipping of green tissue can eliminate adjacent plants that lack comparable recovery reserves, allowing the grazophilic plant to expand laterally by stolens and rhizomes.