Until recently, I did not fully appreciate fossil teeth. Their significance for identifying species and narrowing down the general diet of extinct animals was obvious, but I didn’t understand that teeth also hold intricate records of an individual animal’s life. Tiny pits and scratches on enamel can reveal what a creature was eating around the time it died; oxygen isotopes preserved inside teeth can be used to make inferences about climate, body temperature, and habitat; and the way teeth respond to stress and strain can allow paleontologists to identify the feeding habits of long-dead species.
Prehistoric teeth can also be confusing. Like modern elephants, American mastodon had large tusks that were actually modified upper incisor teeth. The tusks of these recently-extinct behemoths – known as Mammut americanum to paleontologists – are ivory records of their lives, but isolated tusks have presented scientists with a persistent problem.
American mastodon tusks were obviously quite different from the incisors in our own mouths. For one thing, they kept growing throughout the life of the individual animal, changing shape and size as they did so. Tusks of Mammut were also sexually dimorphic, meaning that the tusks in the adult animals were significantly and consistently different in males and females. Two sexes, two adult forms.
The trouble is that the tusks of young male mastodons look like those of adult females. Even though there were only two adult tusk forms, the tusks of juvenile males passed through a stage in which they looked like tusks from mature females. This has frustrated attempts to determine what an isolated tusk means for the life history of an individual animal. In order to correctly identify the sex of an animal from a tusk, you also have to know its age, without which the details of the animal’s life history are confounded.
In previous studies, for example, University of Michigan paleontologist Dan Fisher has found that signs of pregnancy in female mastodons can be detected by studying the details of the dentin that makes up the majority of their tusks. So too can details of musth – a spike in testosterone associated with mating and aggressive behavior – be seen in the tusks of male mastodons. But these patterns can only be properly interpreted when the sex of the animal the tusk belonged to is known. If the tusk of a young male is confused for that of an older female, the evidence embedded within the tusk will be misinterpreted. In turn, confused findings about misattributed tusks can affect hypotheses about why the American mastodon disappeared, as Fisher has used deep-tusk records from North American proboscideans to argue that they show similar birth intervals to elephants under hunting pressure and were therefore being turned into mammoth steaks by prehistoric humans.
Paleontologist Kathlyn Smith recently worked with Fisher to find a way around the problem of ambiguous mastodon tusks, and a detailed account of their attempt has just been published in Paleobiology. What they were looking for was some kind of distinguishable pattern in tusks of both male and female mastodons that could be used to separate the sexes without any other information from the rest of the skeleton. Twenty one mastodon tusks collected in the Great Lakes region of the United States were investigated in the effort to detect these trends.
The twenty one tusks used in the study already had male or female sex assignments based upon skeletal evidence, dental evidence, or the correlation between tusk circumference and the depth of the pulp cavity. Rather than accept these at face value, however, Smith and Fisher wanted to see if these assignments held true by looking at the details of the tusks themselves using principal components analysis. The purpose of this technique was to cut through variation within the sample to see if certain aspects of the tusks were truly related to one another. The data included the circumference of the tusk at several points, in addition to the depth of the pulp cavity and other features. Since the tip of the tusk was the oldest, and the tusk portion closest to the skull was the youngest, measurements from different portions of the tooth could be used as indicators of tusk anatomy at different points in the a
Smith and Fisher found that the depth of the pulp cavity could be a useful indicator for investigating the sex of some mastodons. Even though tusks grew longer and added to their circumference throughout life, the pulp cavity inside the tusk only changed at a few key times – the pulp cavity grew deeper in young animals, stayed at that depth for some time, and then became shallower in old adults. A tusk with a large circumference relative to the depth of the pulp cavity, for example, came from an older mastodon, while a tusk with a larger pulp cavity compared to tusk circumference represents a young individual. This means that pulp cavity depth can be used as a rough estimate for age, and, compared to the length and circumference of the tusks, may help distinguish between tusks belonging to males or females. Still, this variable would mainly distinguish between the tusks of adults and would not have enough resolution to tell the difference between a young male and an adult female.
Measurements from the circumference of the tusk at various points provided a more refined look at age. Paired with pulp cavity depth, the measurements of tusk circumference allowed the approximate ages of the mastodons to be plotted. These were in accord with what had previously been suggested on the basis of other bones from the same animals. This means that paleontologists can estimate age of a mastodon on the basis of tusk anatomy alone and, in turn, these findings can be used to more accurately determine if a relatively short and thin tusk comes from a young male or an adult female.
Paleontologists can’t simply take the measurements and throw them into the computer, though. Detailed knowledge of each tusk is needed to weed out factors that could trip up age assignments. Smith and Fisher mention a pair of tusks from two different female mastodons informally called North Java and Powers. Both tusks came from females of similar age and generally corresponded in overall shape, but the North Java mastodon had worn down about 50 centimeters from the tip of her tusk, obliterating about six years worth of growth. This would make the North Java female seem younger than she actually was in the analysis, and so Smith and Fisher had to correct for the damage the tusk suffered in order to accurately approximate the animal’s age.
Despite such potential confounding factors, though, the results obtained by Smith and Fisher were consistent with the sex assignments that had been proposed on the basis of other teeth and bones found with many of the mastodon specimens in the study. When analyzed carefully, tusks alone can be used to distinguish between males and females across a range of ages. This might prove to be very useful for paleontologists studying the last days of the mastodons and mammoths. If paleontologists are studying a bonebed with several isolated or disassociated tusks, Smith and Fisher suggest, the scientists can still identify the ages and sexes of the animals those teeth belonged to and determine whether the assemblage is consistent with a herd or instead represents a group of bodies that accumulated in one place over a long period of time. The same techniques may also allow paleontologists to distinguish between male and female mammoths belonging to dwarfed island populations, as well, in which the change in size masks the differences between the sexes. Mammoths and mastodons have been extinct for thousands of years, but they have left us wonderful records of their Pleistocene lives inside their tusks.
