When dealing with prehistoric extinctions there is rarely enough information to establish exactly what factors caused a particular loss. Because establishing an exact cause of an extinction is often impossible, scientists try to look for patterns that might help with identifying a mechinism. If for example, it can be demonstrated that a large number of species died out at a certain time, it may help to look at what distinguishes this time period from those immediately preceding and following it. If some creatures died out during a certain time period when others didn't, it may be useful to examine differences among habitats. For example, among the questions one can ask: Did all the losses occur in marine environments? Did they occur in both marine and terrestrial environments? Did they primarily affect certain groups, or species with certain adaptations?

Fisher (1996) examined tusks of mammoths and mastodons in cross-section in order to determine the spacing of the "rings" of dentine that are laid down during the life of the animal.

Just as the width of tree rings varies with factors like temperature and rainfall, so tusk rings vary with an elephant's physiological status. Elephants that have been starved for long periods will show reduced ring widths because growth slows down; healthy ones will have broader bands.

Interestingly, in female elephants birth events are also recorded. Typically in mammals, the mother's calcium metabolism alters during pregnancy, because of the developing infant's rapid skeletal development. In elephants, tusk growth essentially stops during pregnancy; this is reflected as a "gap" in the growth record of the rings.

In examining a large series of mammoth tusks from the end of the Pleistocene in North America, Fisher found that ring spacing was consistent with excellent nutritional status. In other words, despite changes in vegetation inferred by climate change theorists, the elephants were obviously able to find enough fodder.

But he also found something else: the tusks of females showed an unusual number of birth events. It appears that, although they were well fed, they were having an abnormally large number of offspring. However, the implication of that is not what you might think--it seems to mean that these Pleistocene elephants were breeding so often because they were losing their young at an unusual rate. The ring record is consistent with the female mammoths having a birth about once every four years--which is what is seen in modern African elephants that are subject to constant hunting.

Thus, a possible conclusion from this evidence is that human hunting was going on at such a rapid rate that the mammoths were trying to replace losses by speeding up the number of births. It may have been even grimmer: the infants that were produced may have quickly succumbed, rather than having grown up, because of the constant harassment of hunters. In the end, the impact of hunting would have been so great that populations began to plummet toward eventual extinction.

How do we know how old those fossil tusks are?

Radiocarbon dating (also known as carbon-14 dating) is the chief, but not the only, method of isotopic dating used by archeologists and paleontologists to determine the age of samples they are interested in. Although radiocarbon dating is a well-proven technique, it has several limitations. One is that contamination can easily occur; for example, "foreign" carbon introduced into a sample may raise or lower the "real" age estimate. Another is that the amount of radiocarbon formed in the upper atmosphere has definitely varied through time. Finally, with existing technology the method is suitable only for dating samples whose real age falls within about the last 40,000 years or so. Samples older than that have too little radiocarbon left to permit accurate age determination.