Did climate change kill off North America’s mastodons at the end of the last ice age? Or did humans wipe them out? A new study by paleontologist Daniel Fisher of the University of Michigan at Ann Arbor bolsters the latter theory.
Fisher examined the tusks of eight mastodons, both males and females, dating from 12,000 to 10,000 years old. Tusks, says Fisher, grow in layers like tree rings, as seen in this longitudinal section. When the animals are in better condition they can add more tusk material within a fixed time, so the intervals get thicker. When the animals are stressed, these increments get thinner, he says.
Fisher found precipitous drops in tusk growth in males when the animals were 10 to 13 years old. He suspects that it represents the first stage of sexual maturation--a time when modern-day, sexually maturing male elephants are kicked out of their matriarchal family and forced to live alone. It is a very stressful time for them, and they deteriorate. Fisher also believes he pinpointed the age at which females began calving, which places heavy demands on calcium and phosphate stores, slowing the growth of tusks. Moreover, he found that over the course of 2,000 years the age of sexual maturation in the mastodons decreased by about three years--from age 13 to age 10.
If climate change was the principal stress on the mastodons, Fisher would have expected to find just the opposite trend. Under environmental duress, with less food available, the animals would take longer to reach full size and sexual maturity. This is seen in living elephants, who will delay maturation until the age of 20 or 22 when exposed to environmental deterioration, says Fisher.
In contrast, earlier sexual maturity makes sense if hunting stressed the animals. Over time, hunting would, through natural selection, produce mastodon populations that could breed at younger ages. Such populations would have a better chance of reaching sexual maturity and reproducing before being hunted down. If levels of predation are increasing, says Fisher, survival is a much chancier matter. It becomes a better bet to reproduce a little earlier.