Female Mastodons at the Bothwell Site: Simultaneous or Successive Mortality Events?

The Bothwell site, discovered in northwestern Indiana in 2005, yielded a late Pleistocene mammalian assemblage. Over 300 skeletal elements, including at least 13 American mastodon (Mammut americanum) tusks, were recovered from silty, organic-rich sediment. Morphological analysis of complete to nearly complete tusks (n=9) indicates they all belong to females. A site with exclusively female mastodons is rare, and this study seeks to investigate this death assemblage by answering questions pertaining to: number of individuals, age distribution, possible life associations among individuals, and season, simultaneity, and cause(s) of death. To answer these questions, tusk morphology has been evaluated, as well as dentin growth histories (on annual and fortnightly scales) and isotopic compositions. Ages, based on the number of annual increments in tusks or by morphological comparison with tusks of known age, range from approximately 20 to 40 years, but most tusks appear to represent mastodons between approximately 20 and 30 years of age. The lack of juveniles suggests that the site does not represent a complete matriarchal family unit, but this does not eliminate the possibility of social relationships among individuals. Fortnightly growth rate profiles of the final years of life for four tusks show that individuals died in different seasons, indicating that the assemblage represents multiple mortality events. Serial analysis of carbonate δ18O corroborates growth-increment season of death for one tusk, and is being used to evaluate season of death for the other tusks. Cause of death has not been determined, but because there is no apparent decrease in growth rate at the end of life, and three deaths occurred when nutrients should have been readily available (spring and summer), death by nutritional stress is unlikely. Beyond answering the above questions, the methods employed in this study can be used to explore the life histories of mastodons at this site, with implications for recognition of reproductive events in tusk records.