Assessing relative age and age structure in natural populations of Bolomys lasiurus (Rodentia: Sigmodontinae) in northeastern Brazil

Wear-induced changes in the crown topography of molariform teeth have been widely used to index relative age in rodents. To assess the consistency of molar-wear estimates in natural populations of a sigmodont rodent (Bolomys lasiurus), we investigated the association between molar wear and two other age-dependent craniodental characters: degree of exposure of molar roots from the alveoli and ossification of the basisphenoid-basioccipital suture. We compared magnitudes of Spearman correlation coefficients among states of these characters in samples cross-classified by vegetation, season, and locality, under the null hypothesis of high correlation in the absence of differential environmental effects on molar wear. A bootstrap procedure was used to derive empirical sampling distributions of the age-index correlations and, thus, to establish realistic confidence intervals. We then employed a multivariate procedure to reduce subjectivity of classifying combinations of indices of age into age classes. Using a principal component analysis of the correlation matrix of craniodental indices, variation expressed among indices was summarized as a multivariate age factor. An objective ordering of sets of indices of age (the various combinations of tooth-wear, molar-root exposure, and suture-ossification conditions observed among all samples) was provided by projecting individuals onto the first principal component. The method revealed age-frequency differences between wet- and dry-season samples from northeastern Brazil, ostensibly due to the occurrence of a reproductive peak at the beginning of the rainy season, and predicted a maximum life span for B. lasiurus of ca. 1-1.5 years in the wild.