Combined Field and Clinical Methods Clarify Mortality Causes and Survival Patterns of Pacific Martens

Quantifying the demography of wildlife populations is imperative to evaluating population trends. Understanding patterns of influential demographic parameters often requires investigation of mechanisms influencing demography, which can be difficult to determine for cryptic species. We radio-tracked Pacific martens (Martes caurina), a small-bodied and elusive mesocarnivore, to estimate survival, determine causes of mortality, and assess co-occurring pathological conditions. We tracked 18 female and 33 male martens for an average (+/- SD, range) of 16.35 months (+/- 8.7, 1-41) from 2009-2013 and 2015-2017 in northern California. Annual survival rates were 0.81 (95% CI = 0.66-0.95) and 0.68 (95% CI = 0.57-0.79) for female and male martens, respectively. We documented 16 marten mortalities, 13 of which were suspected predation events. Seven mortalities were empirically linked to predation via clinical forensic evidence, with bobcats (Lynx rufus; n = 5) representing the majority of predator species identified. The proximate cause of mortality for 1 marten was attributed to toxicosis from anticoagulant rodenticide exposure, which has not been previously reported in free-ranging martens. Our findings suggest the timing of marten mortality aligned with reproductive periods that incurred increased energetic requirements, which may result in increased predation risk. We propose continued pairing of field and clinical assessments to better substantiate mortality sources of wildlife, elucidate underlying pathology, and identify novel stressors.