Relationships between morphometric variables and age for captive individuals may not accurately estimate the age of free-ranging juvenile koalas (Phascolarctos cinereus)
Tucker, GM, Clifton, ID & McKillup, SC 2012, Australian Journal of Zoology, vol. 60, no. 1, pp. 173-179.
The development and application of a model for determining the ages of free-ranging juvenile Queensland koalas based on morphometric variables suggests that similar models based on data obtained from captive animals may not be suitable for estimations of age of wild koalas.
Existing models for determining the age of juvenile Queensland koalas (Phascolarctos cinereus adustus) are based primarily on growth data obtained from captive individuals. To determine whether such models are suitable for predicting the ages of free-ranging koalas, the authors of this study identified the exact birth dates of two male P. c. adustus joeys on St Bees Island in central Queensland, with each observed either minutes or hours after birth. From these individuals, a total of seven unique head length measurements were obtained and used to create growth curves. As no significant differences were found between these two curves, they were taken to estimate the same relationship which was best fitted with a quadratic function for up to 637 days of age. This growth curve of age versus head length was then used as a point of comparison to assess the accuracy and precision of five other models. While four of the models gave either inaccurate or imprecise estimates of age for the two koalas, a model developed by Tobey et al. (2006) based on body mass emerged as the best predictor of age. The Tobey et al. (2006) model was then employed alongside the model developed in this study to estimate the ages of several other juvenile koalas from St Bees island for which growth measurements were obtained during two separate capture events. As a result, two dates of birth were estimated for each koala using each of the models. The model developed by Tobey et al. (2006) was found to be both less accurate and less precise than that developed in this study, as indicated by the greater difference between the two birth dates estimated for an individual by the former.
Despite being based on only seven data points from two individuals, the model for age versus head length developed here was likely highly accurate as the birth dates of the individuals that served as the basis for the model were known, rather than estimated as in other models discussed by the authors. Additionally, morphometric variables such as head length are considered to be better predictors of age than body mass because the latter variable can fluctuate due to factors not associated with age. It is, therefore, surprising that the model developed by Tobey et al. (2006), based on body mass, was the most suitable for predicting the ages of St Bees koalas among others assessed.
As a result of their findings, the authors argue that caution must be exercised when applying growth models based on captive populations to their free-ranging counterparts. The most reliable model for estimating the ages of individuals within a population will generally be one that is derived from that population itself.
Tobey, JR, Andrus, CH, Doyle, L, Thompson, VD & Bercovitch, FB 2006. Maternal effort and joey growth in koalas (Phascolarctos cinereus), Journal of Zoology, vol. 268, no. 1, pp. 423-431.
Summarised by Joanna Horsfall
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