Determining kangaroo age from lens protein content

Abstract

Kangaroos can be aged by external morphometrics only when young (pre-weaning). Indices of molar progression are used to estimate the age of older kangaroos, but that method rests on a number of tenuous assumptions. This study explored the use of the eye lens as an alternative method for determining age. Body mass, foot size, arm length and wet mass of the lens were measured in 40 red (Macropus rufus), 476 western grey (M. fuliginosus) and 57 eastern grey (M. giganteus) kangaroos, ranging in age from 3 days to ~20 years. Total protein contents were determined for 556 lenses from these animals. Body mass and limb dimensions increase with age, at different rates for males and females, but the wet mass and protein content of lenses are independent of sex. Furthermore, the lens data approach their asymptote later than any of the other measurements, making them more reliable for aging older animals. The relationship between total protein contents (in milligrams) and age (in years estimated from molar index) can be described with a single equation for all three species over the whole age range from newborn to adult.