Life histories of female American mastodons (Mammut americanum): Evidence from tusk morphology, stable isotope records, and growth increments.

Abstract

Proboscidean tusks (hypertrophic incisors) are excellent sources of paleoecological data because they grow continuously by accretion. This allows for tracking of changes in morphology, stable isotope composition, and growth rate throughout an individual’s life. Tusks can provide evidence of sex, age, and season and cause of death, as well as the timing of life events (e.g., maturation). Here, analyses of tusk morphology, stable isotope composition (oxygen, carbon, and nitrogen), and growth increments are used to characterize the lives and circumstances surrounding the deaths of female American mastodons (Mammut americanum) from the latest Pleistocene in North America. Female mastodons are less common in the fossil record than males, but the recent discovery of an assemblage of female mastodons at the Bothwell Site in Indiana substantially increased the number of female mastodons available for study. Tusks from this new site, and other localities, are used to investigate mastodon sexual dimorphism, social structure, and life history. Chapter 2 presents an improved method for identifying the sex of a mastodon using only tusk measurements. Chapter 3 documents similarities in the character of African elephant (Loxodonta africana, Loxodonta cyclotis) and mastodon tusk dimorphism that can be used to infer similar social structures and behavior in Loxodonta and Mammut. Chapter 4 presents evidence that indicates multiple, temporally discrete mortality events led to formation of the female mastodon assemblage at Bothwell. Evidence from tusks suggests that the site represents a Paleoindian meat cache. Chapter 5 presents long-term tusk growth records for two female mastodons. Periodic variations in annual growth rate are proposed as evidence of pregnancy and lactation. The analyses presented here have value for interpreting other mastodon sites as well as for understanding how mastodons responded to climate change and human hunting, factors commonly cited as causal hypotheses for extinction. The results of this study do not provide support for a specific hypothesis, but evidence from growth increment and stable isotope analyses indicates that these mastodons did not experience nutritional stress at death. This suggests that, although the late Pleistocene environment was changing, environmental change did not unduly stress these mastodons.