Damaged segments on fossils of male mastodons' tusks hint that the creatures engaged in fierce combat with each other during a specific time almost every year of their adult lives, a new study suggests. That behavior parallels the annual period of heightened aggression and hormone-fueled jousting for mates in modern bull elephants. Scientists call the yearly period musth.
"American mastodons were not just docile herbivores that whiled away their time in forests and meadows," says Daniel C. Fisher, a paleontologist at the University of Michigan in Ann Arbor. "They were very aggressive animals."
Mastodons roamed North America from 4 million to 10,000 years ago. When Fisher examined the 11,480-year-old remains of a male mastodon excavated in 1999 in Hyde Park, N.Y., he noticed regularly spaced rows of shallow pits along the undersides of the long, curved tusks. A microscopic look at a cross section of one tusk revealed that the zones of dentin underlying the external pits were also damaged, Fisher reported in Ottawa last week at the annual meeting of the Society of Vertebrate Paleontology.
The lesions seem to originate at the boundary between the dentin and the cementum, the hard outer layer of the tusk. The cells that form new ivory lie along that interface at the base of the tusk, he notes.
The tusk damage always appears in ivory that formed between midspring and early summer of each year after the mastodon reached the age of 20. The ratios of chemical isotopes enable scientists to identify the annual growth patterns in the tusks of mammoths and mastodons. …