Estimating age of wild eastern grey kangaroos through molar progression

Age is an essential attribute in studies of animal development, survival and reproduction. Here we evaluate the age estimation technique of molar progression devised for kangaroos in 1965. We used 71 wild eastern grey kangaroos (Macropus giganteus) that were first captured and aged as pouch young at Wilsons Promontory National Park, Victoria, and subsequently found dead between the ages of 1 and 14 years. We expected that the original equation, derived from captive kangaroos in Queensland, would not estimate age correctly due to differences in diet and/or clinal variation in skull morphology. We found no difference in rate of molar progression between males (n = 44) and females (n = 27). Our overall regression of age on molar index (MI) was log10 (age, days) = 0.284 (MI) + 2.511, r2 = 0.97. The slope of this equation was indistinguishable from that of the original one, meaning that molar progression in the wild was equivalent to that originally developed on captive kangaroos, despite likely differences in diet and morphology. The age of an animal affects every aspect of its biology. Almost 60 years ago, a researcher devised a way to estimate the age of captive kangaroos from their teeth, but the results have never been confirmed. We followed wild kangaroos through their whole lives and found that the estimates closely matched their true ages, so this method can now be used with confidence in kangaroo field studies. Photograph by Wendy J. King.