Evidence against the use of fecal pellet size for age determination in European wild rabbits

Techniques for estimating the age of wild animals are crucial to many aspects of the study of population biology. Accurate estimates of the proportion of different age classes in wild rabbit populations would be very useful, and the possibility that it could be obtained from the pellet size holds great appeal. However, this suggestion has created controversy in the literature as this technique has not been validated. This study involved assessment of whether threshold fecal pellet diameters could be used to differentiate adult and juvenile rabbits. The proportion of adults in four wild rabbit populations living in semi-natural conditions was compared with the proportion of animal pellets greater than threshold diameters of 6 mm and 4 mm. Our results suggest that inferring a relationship between the proportion of pellets >6 mm diameter and the proportion of adults in a population is not applicable to European wild rabbits, and that the use of this method could produce erroneous interpretations. The use of a 4 mm pellet diameter threshold appeared to produce even more inaccurate results. Studies that use this technique should include validation, as the results can vary greatly among individuals and populations.