No abstract

Documenting the transition from infant nursing to an adult diet can shed light on the reproductive strategies of mammals, including the enigmatic Diprotodon and other megafauna that once roamed Queensland. Concentrations of the trace element strontium in primate teeth have been used as a proxy to estimate this transition. An influential model posits that strontium levels (relative to calcium) should be low during initial nursing due to limited strontium in milk, increase with the introduction of solid foods that contain higher amounts of strontium, peak at the cessation of suckling, and slowly decline as the gut begins discriminating against strontium in favour of calcium. This study tests this model by assessing trace elements in 13 human and non-human primate first molars (M1) with laser-ablation, inductively coupled plasma mass spectrometry. Only 54% of M1s had a peak in strontium immediately after the cessation of suckling, and none of these showed a subsequent decline in strontium. Alternative approaches are needed for inferring the weaning ages and life histories of ancient marsupials.

Objectives: Intra-tooth patterns of trace elements barium (Ba) and strontium (Sr) have been used to infer human and nonhuman primate nursing histories, including australopithecine and Neanderthal juveniles. Here, we contrast the two elemental models in first molars (M1s) of four wild baboons and explore the assumptions that underlie each. Materials and Methods: Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was employed to create comprehensive calcium-normalized barium and strontium (Ba/Ca, Sr/Ca) maps of M1 enamel and dentine at 35 micron resolution. Results: Postnatal Ba/Ca values were typically high, peaking ~0.5 years of age and then decreasing throughout M1 crown formation; all four individuals showed minimal Ba/Ca values between ~1.2-1.8 years, consistent with field reports of the cessation of suckling. Enamel Sr/Ca did not support patterns of previous LA-ICP-MS spot sampling as the enamel rarely showed discrete Sr/Ca secretory zonation. Increases in Sr/Ca appeared in coronal dentine beginning ~0.3 years, with varied peak value ages (~0.7-2.7 years) and no evidence of a predicted postweaning decline.Discussion: Inferences of baboon weaning ages from initial Ba/Ca minima are more congruent with behavioral observations than Sr/Ca maxima; this is consistent with studies of captive macaques of known weaning ages. Elemental variation is more apparent in the coronal dentine than the enamel of these baboons, which may relate to its more rapid mineralization and protection from the oral environment. Inferences of nursing histories from enamel Sr/Ca patterns alone should be reconsidered, and elevated values of Ba/Ca and Sr/Ca in teeth formed after weaning require further study.