RECOVERING LIFE HISTORY INFORMATION FROM ONTOGENETIC VARIATION IN THE CARBON AND OXYGEN ISOTOPIC COMPOSITION OF TOOTH ENAMEL FROM MAMMALS

The primitive condition of diphyodonty, or possession of deciduous teeth that are later replaced by a permanent, adult dentition, in mammals provides a unique opportunity to recover ecological information at different stages of an individual’s life. Unlike bone, which is remobilized over time, enamel, once fully mineralized, is metabolically inert. As such, its stable isotope composition is fixed, retaining diet and other ecological information from the time of its formation. Analysis of the carbon and oxygen isotope composition of enamel that formed during three important stages of an individual’s life (Stage I: in utero; Stage II: post-birth and pre-weaning; and Stage III: post-weaning) can be used to infer changes in feeding and habitat preferences of an individual through its ontogeny. Correlation between these ecological and life history traits may provide new insight into at least the early stages in the development of extant and extinct mammal species.

To examine the utility of applying this proxy to questions of life history in fossil mammals, we analyzed the carbon and oxygen isotope composition of multiple teeth along the lower tooth row of three steers raised on controlled diets. Through Stages I and II of their development, these steers were maintained on a consistently C3 diet. Stable isotope analysis of deciduous teeth and portions of the first molars that developed during these stages found a slight drop in δ¹³C values (<1.0‰) and a small, but significant increase in δ¹⁸O values as individual’s adopted a milk-based diet shortly after they were born. Subsequently, during Stage III, individuals were switched from a C3 to C4 diet. Carbon isotope values for enamel from permanent teeth formed during this stage were initially consistent with values for enamel formed during Stage I before undergoing a rapid and steady increase (~10.0‰) following the diet switch. Trends observed in enamel stable isotope values for these steers reflect diet and physiological changes that accompanied Stages I-III in the ontogeny of these individuals. Thus, the onset and duration of these three stages in extinct taxa can potentially be determined by comparing enamel δ¹³C and δ¹⁸O values from multiple tooth types sampled from fossil specimens.