A dental dilemma: bear teeth reveal a spatial gradient in age of primiparity, but temporal trends muddled when bears grew fast.

Abstract

Age of primiparity (AOP) in American black bears (Ursus americanus) is driven by nutrition. We estimated AOP from patterns of cementum annuli in female bear teeth (more than 16 000), which were collected from hunters in Minnesota, USA, over five decades. AOP was delayed in northern Minnesota and hastened southward, matching the increasing food availability. In the core of the bear range, AOP remained stable at 5.3 years, whereas along the range edge, AOP increased from 4.5 years in the 1970s to 5.0 years in the 2010s. We posed five hypotheses to explain this enigmatic trend; (i) change in tooth readers, (ii) declining foods, (iii) increased density-dependent effects, (iv) hunters killing fast-growing juveniles, and (v) limits to the technique used. We rejected the first four hypotheses by comparing tooth readers and exploiting data on bear foods, age-specific bear masses, population trends and observed AOPs of radio-collared bears in each region. We concluded that food increased along the range edge, spurring faster-growing bears, which had harder to interpret cementum annuli. We previously developed and tested this technique on slower-growing bears. Teeth from harvested bears are a valuable source of reproductive information, but the technique may fail when employed beyond the tested limits of the bears' nutrition.