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Examining the use of fecal pellet morphometry to differentiate age classes in Sonoran pronghorn

Woodruff, Susannah P., Johnson, Timothy R., Waits, Lisette P.
Wildlife biology 2016 v.22 no.5 pp. 217-227
Antilocapra americana, DNA, adults, age structure, animal age, demographic statistics, fawns, feces, genotyping, locomotion, managers, models, monitoring, morphometry, mortality, pellets, population growth, probability, regression analysis, wildlife, yearlings, Arizona
Wildlife managers require knowledge of population demographics, yet for low-density, wide-ranging species procuring demographic information is challenging. While accurate abundance estimates can be costly and difficult to obtain, recruitment and survival trends can be used as an alternative indicator of a population's trajectory. Physical capture has been the traditional practice for obtaining these demographic parameters, yet capture-related stress can lead to reduced levels of fitness, impaired locomotion, or even mortality for some species. Thus, noninvasive sampling methods may provide an alternative to physical capture. Population monitoring of endangered Sonoran pronghorn Antilocapra americana sonoriensis is critical for assessing the success of recovery efforts, and monitoring annual survival and recruitment by age class would provide information on the trajectory of population growth. We measured noninvasively collected Sonoran pronghorn fecal pellets collected post-fawning in Arizona, USA and matched to known age animals using fecal DNA genotyping to determine the feasibility of distinguishing age class by pellet dimensions. Based on cross-validation with logistic regression predictive models, we estimated a 98% probability of correct classification of fawn versus yearling and fawn versus adult using pellet width as a single explanatory variable. We could not, however, distinguish between yearling and adult. We additionally evaluated our ability to classify age class of fecal pellets by visual assessment only, and this approach was unreliable. Thus, we recommend measuring pellets for more accurate age classification. This measurement method is simple, relatively inexpensive, and shows potential for use in wild populations of pronghorn to discriminate fawns from other age classes. When combined with individual identification using fecal DNA, this approach could provide better knowledge of recruitment and age-specific survival for this and other species.