Determining Dima’s age from tooth eruption pattern is more ambiguous, but Dima’s teeth do not contradict an estimated age of about four months. We do not know the age of dental eruption of mammoths; we can only assume it was roughly analogous to that of elephants. The second deciduous premolar is the first tooth to erupt, as the first deciduous premolar has been lost in proboscian evolution. In Dima’s case this second deciduous premolar was in use, and the third deciduous premolar was beginning to show wear. These two teeth provided a modest-sized occlusal surface and undoubtedly prompted the original age estimate of 7 to 8 months. Several researchers in this study assumed that a mammoth in its first year does not find much of its own food, but rather lives mainly on milk. This is the case for elephants, but it may not have been true for young mammoths. Both the first and second teeth are high crowned and have complex occlusal surfaces. These teeth are worn out in the first two or three years of life. Most high-quality herbage available during the growing season would have been digestible by a young mammoth. Young mammoths nearing the end of their first growing season may necessarily have been eating quite a bit on their own, and one might expect to find some food throughout Dima’s gastrointestinal tract.
That Dima’s gut was empty of suitable food is, I think, a clear indication that death was not instantaneous. Autumn, however, is not a season of starvation; forage would have been at its maximum volume or biomass. Also, how do we explain the mud, plant detritus, and hair found in Dima’s gastrointestinal tract? I agree with Ukraintseva that these features suggest a trapped animal. Studies of animals trapped in tar pits and other sticky substrate have shown that adults of larger species, such as bison and mammoths, are seldom entrapped. Adults are strong enough to pull themselves out (Akersten, pers. comm.). Young animals are apparently the most common victims. If a young mammoth were trapped in organically rich, water-saturated silt, its struggles may have pulled it even deeper. Comparable struggles have been seen and filmed in African springs and witnessed by various researchers (e.g., Gary Haynes, pers. comm.). Mothers do try to help their young and sometimes succeed, but not always. A mother’s presence, however, prevents predation.
A lost-mother scenario seems unwarranted in Dima’s case. The end of the growing season would have seen mammoth cows in their best condition, an unlikely time to die. We know indirectly that Dima’s mother was probably not in poor shape because, among elephant cows, only those in relatively good condition can become pregnant and have young. Among proboscidians, it is the young which have the highest mortality; Laws, Parker, and Johnstone (1975) estimate about a 30% mortality during the first year among African elephants. The death of a mammoth in its first year would probably not have been unusual either.
We can imagine that it was not uncommon for a young mammoth to die, while the mother lived on to old age. Cow mammoths had few predators, and surely any predators capable of taking an adult mammoth would have killed this tiny one-meter-tall baby as well. Vereshchagin’s proposal that the predators could have been human hunters, although possible, is not very convincing. Babies of Dima’s size do not run away from their dead mothers when hunters cull elephant cows today. The social bond to the mother elephant is strong (Laws, Parker, and Johnstone 1975). Also, I know of no evidence that human hunters were above 60° latitude north 40,000 years ago, if indeed Dima dates from that time.
It is not Dima’s death which we have to explain; there were probably hundreds of thousands of woolly mammoth calves dying every year across the Pleistocene northern steppes. It is Dima’s preservation which is unique. The reflex reaction of most people to fossilized animals, especially Pleistocene mummies, is to try to account for their death. The more appropriate issue is usually one of preservation. An example is Ivan Sanderson’s (1960) article about mammoths frozen with buttercups in their mouths. Sanderson’s emphasis was on death, focusing on change in climate, but he started at the wrong end. The interesting point is not that a mammoth died midway through its last bite. That, no doubt, happened to millions of unfortunate mammoths. The real puzzle is how a few mammoths became permanently preserved.
Even Vereshchagin, in his discussion of the Selerikan horse mummy (1977), conflates death and preservation in discussing the two main episodes of mammoth mummification: 10,000–12,000 yr. B.P. and 33,000–45,000 yr B.P. Instead of seeing mummies from these periods as products of unique depositional circumstances, he comments that these are two main times of die-off due to climatic change.
Most of Dima’s body might have sunk rather quickly as he struggled in deep mud. The vacuum created by attempts to pull a large object from silty-clay mud is quite strong; I have twice seen horses stuck in organic muds and had to work most of the day to pull each out, even with the help of other horses. During a recent hunting season, my partner sank a meter deep in soft organic mud while he was backpacking out a heavy moose quarter. Even with the pack removed he could not get out of the mud by himself, nor could I pull him loose with a rope. We had to lay a scaffolding of poles around him to pull him free. Such deep, organic mud is not everywhere common, but it does occur.
Vereshchagin rejected entrapment because the region today lacks deep thermokarst cracks like those proposed by Dubrovo. Water-saturated soils in that area also do not now provide conditions for mud entrapment. However, as Ukraintseva and others argued strongly, today’s conditions are quite different from those in which Dima lived. I have proposed that the complete ground cover and stable soils we know today were not characteristic of the Pleistocene landscape (Guthrie 1982). We can draw this conclusion from the mass transport of the sediments in which Dima was found. There seems to have been a basically dry landscape, with bare soil commonly showing through the vegetation cover, and a few wet spots in polygonal ground where unsorted sediments with a high organic content occurred along poorly drained benches.
In my opinion, the evidence surrounding Dima points to mud entrapment, but we have to explain why a trapped animal survived several days without being molested by predators. As long as Dima was alive, his mother probably would not have left (fig. 1.7). By the time Dima died, he may have been so deeply mired in the mud that predators could not see or smell him. Mud in the respiratory system and distended bronchi indicate Dima died when he no longer had the strength to keep his head above the surface. Once under the mud, the body would have been protected from scavengers. An empty gastrointestinal tract would produce few visceral gases; however, enzyme activity would have been sufficient to slightly rotate they body and swing the abdomen up and the heavy head down—the position in which the frozen mammoth was found.
An animal exerting itself in cold mud would have depleted its fat stores rapidly. And Dima may have been a rather sickly young animal in the first place, as the parasite load suggests. Perhaps Dima lacked the normal strength required to overcome the tenacity of soggy, deep mud.
If Dima had fallen into a small pond and drowned, visceral gases, even if Dima had had an empty stomach, would have bloated the body and floated it to the surface where it could be smelled and dragged to shore by predator-scavengers. Nor is snow a sufficient insulator to serve as a protective cover for the odor of a dead mammoth. Many mammals are good at scavenging under snow, but, as stated earlier, Dima’s gut contents allow us to rule out winter as a time of death.
Fig. 1.7. Postulated burial and preservation. Dima seems to have been a young animal, a baby of the year. Apparently it got stuck in wet silt late in summer or early autumn. Struggling to get out, it sank farther, probably over a period of several days (its body fat was depleted). Under the stress of entrapment, it ate plant detritus and silt. Finally, it sank deeply enough to die from inhaling the mud (silt particles in the trachea and lung alveoli). It rapidly sank beneath the surface (no sign of scavenging). The head being slightly heavier, it sank deeper than the torso and was frozen beneath the surface that winter. Segregation ice formed in the water-saturated soil surrounding the mummy. Dima then was covered by mass wastage of soil the following spring.
The presence of hair on Dima’s trunk, tail, and feet, and the better histological detail preserved in these structures, can be explained if we remember that the peripheral appendages cool faster than the body core. Cool mud would have quickly removed heat from the appendages but not from the more massive abdomen, where heat from enzyme action would have maintained body temperatures longer. This in turn would cause hairs on the torso to come loose from their roots, but mud would have held the hair in place. Movement by ice segregation or by the miner would leave that hair in the mud. Such hair is not as readily apparent as one might suppose. I think this phenomenon of hair actually present near the carcass but lost in the mud can explain Riabchun’s quandry—that Dima must have decomposed before burial to explain the absence of hair.
Mud entrapment would not only account for Dima’s debilitated condition and features of his preservation but also for the strange gastrointestinal tract contents. Trapped animals usually die from the stresses of being trapped and from starvation over a period that may last for days. In their delirium of anxiety, trapped animals exhibit strange displacement behavior.
As part of other natural history studies, I have necropsied several thousand carcasses of mammals caught in steel traps. These carcasses often had dirt in the gastrointestinal tract. Trapped and starving animals desperately eat or chew on anything in reach, often their own hair. I originally found the presence of hair perplexing, but realized that the trapped animal often pulls and chews at its own body when it is in a great deal of pain. All of the hair in Dima’s gastrointestinal tract had been broken, not shed like hair combed out during grooming. A scenario of entrapment in cold autumn mud can thus explain several things: (1) why the carcass was not scavenged (it was totally buried in wet mud); (2) why the carcass was preserved so completely (the cold mud curtailed maceration until the carcass was frozen in winter); (3) why the gastrointestinal tract was empty of suitable food (being trapped, Dima did not have access to normal diet); (4) why both silt and Dima’s own hair were present in the gastrointestinal tract (delirious displacement feeding and self-mutilation); and (5) why there were characteristic horizontal ice lenses in the silt surrounding the carcass.
Whatever the proximate cause of death, it is apparent Dima was embedded in water-saturated soils. The first freeze would have segregated ice from the silt. Fairbanks potters use this freezing action to dehydrate their clay slurry every autumn. The creamy mix is placed outdoors, and, as it slowly freezes, the water segregates into an icy block that the potter can then chip from the clay. Damp clay ready to work into ceramic ware remains. The ice lenses described surrounding Dima are of similar origin. The permafrost specialist, Riabchun, recognized these characteristic long, lateral lenses as segregated ice.
Unfortunately, different kinds of ground ice are not always easy to distinguish, but their identification is critical in the case of mummies because it can determine how the animal was preserved. Geologists (Watanabe 1969; Péwé 1975a) distinguish a number of categories (fig. 1.8) of ground ice:
Glaciers are sometimes buried by sediments and become “fossilized.” (Herz, remember, thought the ice around the Berezovka mammoth was glacier ice.)
Aufeis commonly occurs in small northern streams that freeze to the bottom early in winter. The stream continues to flow, building up ice on the surface, and by spring aufeis is often thick and massive. It is recognized as very clear, clean ice that is stratified horizontally. Aufeis can be buried by silt in spring and preserved.
Fig. 1.8. Quaternary ground ice. Ice found in frozen silts surrounding large mammal mummies can provide important clues as to the agents of burial and preservation. Misidentifications and misinterpretations of this ice have lead in the past to some exotic theories of mummy preservation. Different kinds of ice are difficult to portray in pen and ink, but they resemble these forms: aufeis—from stream overflow, which is buried by sediment the following spring; ice veins—which includes ice-wedge ice from ground contraction due to extreme cold; pond ice—which is buried the following spring by sediments; segregation ice—which forms in water-saturated sediments as they freeze. These latter are the most common form of ground ice associated with large mammal mummies.
Fig. 1.9. Ice Wedges.
Pond ice is found as solid, flat-topped ice that was buried during spring and insulated from summer thaw. It is recognized as a homogenous structure with pond inclusions.
Pingos are the main representative of subsurface ice intrusions. They are formed when liquid moisture is trapped between growing permafrost margins. They usually occur in the aftermath of thaw-lake senescence, when frost invades saturated, thawed ground where a thaw-lake has recently been.
Ice wedges occur when frozen ground contracts, leaving deep cracks that fill with water in spring and subsequently freeze. This process, repeated many times, results in massive, vertical, wedgeshaped curtains of ice in a polygonal pattern. They contain bubbles and dirt aligned vertically. These existed near both Dima and the Berezovka mammoths but not immediately around them. In the far north the tops of these active ice wedges are just under the insulating sod (fig. 1.9).
Ice veins are smaller versions of ice wedges. They are by definition quite thin and usually not wedge-shaped. They are also crack fills but occasionally are of more complex origin than ice wedges.
Segregated ice can occur in two primary forms:
Massive ice beds are horizontally oriented and developed in situ, usually within an open system of underground water movement or within a closed system of saturated or supersaturated soils. Segregation ice which forms these massive structures is rare.
Lenticular segregation is the most common form of segregation ice. It consists of horizontally oriented lenses of ice interspersed throughout the soil. Both of these segregated ice forms are horizontally aligned, elongated, and hold bubbles and dirt particles. My work with Blue Babe and descriptions of Dima and the Berezovka mammoth show that sediments surrounding mummies contain mainly segregated ice.
The physics of water segregation is still debated (Washburn 1980), but water is indeed drawn toward the freezing front. Additionally, contrary to one’s intuitive sense, there is still liquid moisture in frozen ground, and this liquid moisture may migrate. Thus, especially in silts, segregation ice lens formation can continue behind the freezing plane (Washburn 1980). For extreme segregation to occur, slow freezing is important. In a closed system, such as that of saturated soils well below surface and subject to slow freezing, the buildup of ice lenses desiccates the soil. This desiccation process is critical in understanding the diagenetic changes that affect frozen carcasses and result in the true frozen mummies.
It is this water withdrawal, in the form of segregated ice, which dehydrates a mummy in frozen ground. This process is misunderstood by Shilo and Titov, who assumed that Dima’s carcass would have to lie in the open air to be desiccated.
Is Mummy a Misnomer?
The word mummy has long been used to describe carcasses preserved in northern permafrost. Some have objected to this usage on the basis that preservation by freezing is unlike “real” mummification of an embalmed or dried corpse. However, frozen corpses, like Dima and Blue Babe, are indeed desiccated and fully deserve to be called mummies.
The process of frozen-ground desiccation is a fascinating story. Underground frost mummification should not be confused with freeze-drying, which occurs when a body is frozen and moisture is removed by sublimation, a process accelerated by a partial vacuum. The freeze-dried mummy is left in its original form, minus most of its moisture—a common technique used in food preservation and taxidermy studios. I have often freeze-dried items, sometimes inadvertently, during our long Alaskan winters, where the temperature seldom rises above freezing for eight months of the year. A bison or caribou foot left on the barn ledge, near where it was butchered in early winter, is freeze-dried to perfection by the following spring. Winter freeze-dried jerky can be made through this same process, by hanging meat strips up away f
rom carnivores and birds.
However, the desiccation of fossil mummies is quite different than freeze-drying. Moisture contained in a buried carcass is not released to the atmosphere but is crystallized in place, in ice lenses around the mummy. This process is more comparable to tightly wrapped food left too long in a freezer. When a stew is first frozen, it swells to a somewhat larger size, bulging the sealed plastic container. The longer it stays in the freezer, month after month, the more moisture begins to separate, forming ice crystals inside the container. The stew itself shrinks and desiccates. Year follows year, and the stew becomes more and more desiccated, as ice segregates from it. Eventually, the stew has become a shriveled, dehydrated block; unlike freeze-drying, in which the object theoretically retains its original form, the stew is shrunken in size and surrounded by a network of clear ice crystals. This segregated ice also occurs in the silt around frozen mummies. Soft tissue becomes mummified and shrunken down, looking like a desiccated mummy dried in the sun. These two processes of cold mummification and freeze-drying were not distinctly understood by people unfamiliar with long winters and the back corners of deep freezers.
Dima’s Burial
Once buried and cooled in the mud, which overlaid permanently frozen ground, Dima could have been preserved at “refrigerator” temperatures for a few weeks until it was completely frozen, without being totally macerated. As winter pulled heat from this mud, moisture separated from silt as segregated ice. The place where Dima lay was apparently covered the following spring by more silt and mud flushed from upslope when the little drainage brought a load of sediment from steeper slopes down to the gradual (7 to 8 degree) slope of the terrace. The flow slowed, and sediment was deposited on the terrace. This is not really alluvial deposition in a conventional sense. Removal and concentration of surface soil downslope during the spring following Dima’s death would explain why material around the carcass (and the gastrointestinal tract) contained immature pollen grains as well as mature seeds from autumn dispersal; new spring seeds would have remained attached to the green plants and not dispersed downslope, nor would they have been embedded in the plant detritus.
Frozen Fauna of the Mammoth Steppe: The Story of Blue Babe Page 3
