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

by Guthrie, R. Dale

  Fig. 3.19. The relationship of prey density, pride size, and male ornamentation. Anatomical ornamentation of males is exaggerated with increasing polygyny. A few males fertilize most of the females in a given population. This principle seems to explain some geographic differences in lion coat color. Male lions, often brothers, will share territory and females, thus creating a formidable defense against lone male contenders. In areas with large prides (eastern and southern Africa), pride males have large manes and there is a high frequency of dark-maned males. Where prides are small or dispersed during the dry season (Kalahari and Tsavo Park), manes are smaller and few males have large, dark manes. Pride size seems to relate to prey abundance and density. The less dramatic ornamentation and smaller prides of northern Pleistocene lions were probably related to prey density. Males in small prides must assist females in the hunt, and gaudy ornamentation can be a considerable debit.

  Paleolithic drawings of European Pleistocene lions show males had a slight ventral and dorsal raff on the neck, different from females but not nearly so exaggerated as the mane worn by males today in the larger prides of African lions. We know from reconstructions of the northern Eurasian habitats in which Pleistocene lions lived that they hunted in open landscapes not unlike that of African savannas (Guthrie 1982). These northern lions probably had very large territories. We know that at its best, the Mammoth Steppe was not nearly as productive as most African or Asian savannas, and since the territories would have been large to support even a small pride, probably little energy was exerted on territory defense. Van Orsdol, Hanby and Bygott (1985) have shown that African lion range size is negatively correlated with prey abundance during the lean season. They also showed that pride size and lion density are correlated with the abundance of prey during the lean season (figs. 3.20 and 3.21). Bertram (1973) had earlier suggested that pride size and density were regulated by food availability. The large ruff of an African male lion is an encumbrance in hunting (Schaller 1972); selective pressures against a ruff probably derive from greater pressure on male lions to participate in hunting. These later pressures would have been even greater among lions living in a small pride or as a mated pair. The small raff of Pleistocene males we see in Paleolithic art would have been a compromise between the social advantages of a large raff and its encumbrance in hunting. The slim ventral and dorsal mane on Beringian males served as an artificial enlargement of areas wielding critical weaponry (neck) and as an indicator of maturity, while still being somewhat inconspicuous so that the lion did not stand out like a haystack when attempting to get within attack distance of potential prey. This modest mane still allowed a male to display his sex and age to his peers.

  Fig. 3.20. Pride size is affected by the amount of available prey. Both small and large lion prides can occur with large numbers of prey, but only small prides occur when prey numbers are low. The data shown here are plotted from Van Orsdol et al. (1985) for African lion. They found that lean season biomass was most highly correlated with pride size. I use this well-known phenomenon to argue that northern lions probably did not live in large prides.

  Obviously, the African lion is only a rough analogue to the Alaskan one, as many life history features would have been different. African lions are very aseasonal in their reproduction (Bertram 1975), the males have short tenure (Bertram 1975; Smuts 1976), and they occur at the high density of 10 to 15 per 100 km2 (Smuts 1976). But for other features African lions may serve as an informative analogue.

  In areas of Africa where prides are quite small, male lions today have small, noncontrasting manes. Both in the heavy bush of Tsavo Park (Schaller 1972) where there is low game density and in the Kalahari (pers. obs.) where prides disperse during the dry season for a more solitary life, lions have small tan manes.

  Fig. 3.21. Lion density is highly correlated with prey density. There is, of course, a lower threshold of prey abundance on which lions cannot exist. It appears in this plot, taken from data in Van Orsdol et al. (1985), that lion density reaches zero at the same time as prey density. The single point on the plot which suggests this is based on prey and lion density in Kalahari-Gemsbok Park, which is an unusual situation. Although the entire park was used in density calculations, virtually all the park’s large mammals occur in the valleys of two dry rivers where boreholes have been drilled. Thus the functional density of lions and prey is actually greater than overall averages indicate. The point at which prey density will no longer support lions is difficult to derive, but it exists.

  Sinclair (1977) found that the majority of African bull buffalo were killed along rivers where lions had some cover, allowing the lions to approach within attack range. Since Blue Babe was found in the lower part of a valley and there were small sticks and twigs in among the silt matrix, it is possible that the Alaskan lions also used valley bottom shrubs for cover before attacking.

  For that same reason—requiring cover for their attack—lions are mainly nocturnal hunters (Schaller 1972). Lions are not long-distance coursers, but rather rely on their ability to accelerate faster than their prey in a short chase. A great amount of muscle distally in the limbs gives lions their acceleration, but this heavy musculature is metabolically costly and severely limits the lion’s distance once full speed is attained. The opposite is true for ungulates; their anatomy emphasizes efficiency once maximum speed has been attained, with some sacrifice of initial acceleration rate.

  An ungulate tries to avoid capture by feline predators by maintaining escape distance—distance sufficient that the lion tires before overtaking the ungulate. For an attack to succeed, the lion must slip fairly close, to within the ungulate’s escape distance, before giving chase.

  Ungulates are at their greatest disadvantage when lions can gain proximity under cover of darkness. In Chobe, Botswana, with Petrie Viljoen I followed (in a vehicle) a pride of lions for several nights. Our sample of those hunts was consistent with his larger experience that lions have a much higher success rate on cloudy, windy nights. On one such night a pride made multiple kills of tsessebe from a single stalk. Tsessebe are one of the fastest antelopes and normally are difficult for lions to approach and catch.

  Taking this African-Alaskan analogy to its logical conclusion, it is likely Blue Babe was ambushed and killed in shrubs along the valley bottom of Pearl Creek by a small group of lions during the lengthening dark of early winter.

  Postmortem Scavenging Activity at the Carcass

  After lions abandoned the frozen bison it appears that scavengers arrived and continued to visit the carcass until spring (fig. 3.22). Today any large mammal carcass that lies out all winter is visited by a variety of large and small predators and scavengers. Many of these species, such as wolves (Canis lupus), foxes (both white, Alopex lagopus, and red, Vulpes vulpes), and wolverines (Gulo gulo) occur as fossils from late Pleistocene deposits in the Fairbanks area, and there is no reason to presume they were not part of the local fauna 36,000 years ago when the bison died. Although few avian bones have been found in Alaskan Pleistocene deposits, some scavenger birds, such as ravens (Corvus corax), which are today ubiquitous in the Arctic and subarctic, as well as magpies (Pica pica), which now occur discontinuously across the Holarctic, were probably present during much of the Pleistocene.

  Fig. 3.22. Blue Babe’s carcass in late winter. The manner in which bones and tendons were chewed indicates Blue Babe had been used by avian and mammalian scavengers. Scavenging would have continued all winter. The frozen head and lower-leg skin kept scavengers from using these portions. Other parts were heavily used; most of the vertebrae, femora, and pelvic girdle were either consumed at or carried from the site.

  The mummy’s general appearance shows scavenging took place, but the carcass was not completely utilized. This is not surprising as such a large frozen carcass presents quite a challenge to small scavengers. When we screened the silt around Blue Babe we found many bone fragments, probably from mammal feces deposited by scavengers. (Today scavengers commonly mark a carcass wit
h feces accompanied by secretions from anal glands.)

  There were other signs of scavenging. Several vertebrae were missing; neural processes of other vertebrae had been chewed. Easily eaten bones such as the pelvic girdle were completely missing. The hard, outer bony cortex is thinnest on innominate bones of the pelvic girdle and vertebrae, so these bones are usually among the first that scavengers eat. The scapulae were found near the carcass, both with chewed dorsal margins. These margins are made of soft cartilage, and my past experience with recently scavenged carcasses affirms that these soft edges are a choice target.

  On large mammal carcasses I have watched in past winters, the initial flurry of scavenger activity seems actually to decrease accessibility for subsequent scavenging. Snow on and around the carcass is trampled into a dense icy floor that covers many otherwise edible parts. Then, because the carcass is not insulated by snow and is exposed to the full effect of winter air temperatures, it freezes even harder.

  On Blue Babe we found “feather brushes” of tendonous fibers attached to a number of bones, such as the vertebrae, which indicates avian scavenging. Mammalian carnivores use their carnassials to scissor these muscle parts completely off. Birds leave strands of this tough connective tissue behind as they peck away at the meat. Few bones were cleaned completely in the way a large felid will do, using the rough surface of its tongue like a rasp. The absence of these characteristically cleaned bones further supports our interpretation that the lion(s) quit the carcass as it froze. One wonders about the consequences of sticking a wet tongue on a −40° F(−40° C) bison tibia.

  Some chewing marks were found on the bison’s lower lip, but in general the head remained unscavenged. Red muscle was attached to the occipital part of the skull. The face and lower jaw muscles were protected by thick hide.

  Once we had cleaned and split Blue Babe’s skin (in preparation for the taxidermy work), we spread it out for mapping. At this point it became apparent that the bison had been opened from the dorsal surface, just to the left of the midline. To gain entrance to moose, most wolves and bears open the ano-genital area or the thinner posterior-ventral abdominal skin. I learned, however, from Gary Haynes (pers. comm.) that bison in Wood Buffalo Park normally go down on their abdomen when killed by wolves and are customarily eaten from the dorsal side downward. This behavior may explain why the legs of the mummy were so well preserved. When skinned and eaten from the dorsal surface, a bison’s legs are covered by a blanket of skin draping down from both sides. In fact, Blue Babe’s front legs were found in a collected position, so it is likely they were contracted under his body during all the scavenging.

  The lower parts of the limbs (distally from the calcanium and the olecronon process) were essentially unscavenged (fig. 3.23). The sternum and its articulated ribs likewise remained in place. The uneaten ventral ends of the ribs (averaging around 12 cm in length) were still attached to the sternum and ventral skin, in their bed of oxidized fat. Skin was folded over this ventral part, making it less accessible, especially if one imagines the hide frozen before the carcass was thoroughly eaten. Once the skin had been peeled down the sides, the carcass would have frozen in that condition, forming a gigantic saucer-shaped block. Further trampling would have cemented this large disc to the ground, making the entire carcass difficult for carnivores to detach, overturn, and eat from the other side. The following bones of Blue Babe remained with the carcass (see also fig. 3.24): skull, with horn sheaths, and mandible; anterior six cervical vertebrae; last three thoracic vertebrae; first three lumbar vertebrae; last three caudal vertebrae; left and right scapulae; all the bones of the pectoral limb; all the bones of the pelvic limb, minus femora; most of the sternum; and numerous rib fragments, particularly those parts attached to the sternum.

  Fig. 3.23. A relatively unscavenged lower front leg. Blue Babe’s preservation suggests that the front legs were wedged in under the frozen skin of the torso and unavailable to scavengers.

  Fig. 3.24. Bones found with Blue Babe’s carcass. Some of the missing bones were probably dragged away from the carcass. Others, like the pelvic girdle, may have been eaten.

  Postdepositional Changes

  The bison mummy, while well preserved for its age, underwent certain changes during its 36,000 years underground. While it is not easy to separate condition at burial from postdepositional diagenetic factors, in Blue Babe’s case some clearly identifiable changes occurred well after burial.

  The most obvious change is in the color of the bones, which had acquired the staining of surrounding sediments—a mixture of oxides and organic browns. The hair, however, seems to have retained its original color despite the fact that the hair of other Alaskan mummies has either been bleached by the soil or stained (or both). Mummified legs of caribou, moose and mammoths from Pleistocene Alaskan deposits have hair of a peculiar reddish pink color, perhaps a result of a bleach-staining processes, or more likely a differential bleaching of the melanins and eumelanins. Other specimens, such as Blue Babe, show no such modification: blacks are glossy, and reddish tans still look like fresh hair.

  The posture of the mummy at the time of excavation may not have been identical to that before its burial. Blue Babe could have been contorted during burial or by ground movement after burial. Ice has peculiar characteristics that allow it to change shape (as in a glacier), and these are sometimes difficult to separate from forces affecting frozen mammals before they were buried.

  Most of Blue Babe’s broken bones, ribs, and zygopophyses, for example, were clearly broken by carnivores or scavengers, but two large bones do not appear to have been broken during the killing or scavenging process. The right tibia was broken with both pieces in place, the distal part still attached to the carcass. The bone does not appear to have been broken when fresh because (1) it would most likely have been dragged away from the carcass by a scavenger if it had been broken then, (2) the raw edges of the break show unscavenged white marrow, and (3) the break looks more shattered than is typical of carnivore breaks that occur in a clean sharp-edged spiral fracture.

  The same is true of the left mandible. It has a rough, jagged fracture near the symphyses, and evidence indicates that this too is a diagenetic change. Both mandibles were held in place by dehydrated muscle and skin and were permanently skewed to the right side. This degree of distortion cannot be achieved in a freshly killed animal by the weight of the head itself, nor even by the weight of a dozen centimeters of silt. It is because of this contortion that the left mandible was broken. I think both the tibia and mandible were broken by diagenetic postdepositional processes rather than by predators or scavengers.

  Since most of the hair was found in sediments immediately surrounding the bison and not attached to the skin (it is unlikely the hair would have been plucked out by scavengers; at least no known scavenger exhibits this behavior), I suspect the hair was still affixed to the hide when Blue Babe was buried but later slipped as part of postdepositional processes. This suggests that the skin of the buried bison may have thawed during the first or even subsequent summers, allowing residual enzymes to break down protein in the hair follicles.

  Fig. 3.25. Blue Babe’s burial. The lack of fly pupae cases and other evidence suggests Blue Babe was buried in early spring by silt moving down an adjacent side slope, probably from snowmelt water running over exposed soil. As this silt in suspension reached the more gentle slope of the valley floor, it was redeposited at sufficient depth to cover the bison which lay on still-frozen soil. The newly deposited silt insulated the frozen soil, preventing significant decomposition. Subsequent years added to this cover and eventually the bison lay in permanently frozen ground.

  In summary, we can conclude that Blue Babe died in early to mid winter, before his large reserves of fat were significantly reduced. He was killed and partially eaten by one or, more likely, two or three lions, which fed for several days until the carcass was frozen. Freezing slowed consumption by these lions, who then left the kill, allowing other scavengers to p
ick at the carcass throughout the winter. Indeed, the bison probably was scavenged by an array of mammalian and avian species, judging from bone fragments, feces, and a characteristic pattern of tendon connective tissue left by avian scavengers.

  At the close of winter, the bison carcass was buried by silt carried in rapidly moving snowmelt water (fig. 3.25). As the mummy lacked blowfly pupae cases and concentrations of scavenger beetles, burial must have occurred prior to the emergence of these insects in spring. The carcass probably thawed the first summer, but remained bedded on frozen ground and covered by cold silt. He was refrozen in subsequent winters and, as silt accumulated year after year, was gradually interred beneath the lower reaches of annual thaw within permafrost.

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  RECONSTRUCTING BLUE BABE’S APPEARANCE

  Vertebrate paleontologists work mostly with bones. Reconstructions of extinct animals rely on our anatomical understanding of modern animals to make analogous decisions concerning the form of an animal for which we may have only a handful of bones. Often times, however, a strong dose of fancy is also added. The fossil record rarely preserves details of external appearance. The skin and hair preserved in a mummy such as Blue Babe provide a unique view into the past.