Approximating the Age of Eurasian Otter (lutra Lutra) Cubs from Camera Trap Footage Using Proportional Body Measurements

Eurasian otters Lutra lutra are protected in Europe and vulnerable due to low reproductive rates. The ability to age cubs may be important to identify and protect breeding places and target seasonal surveys, yet little data exists. We developed a novel method to measure cubs relative to their mother using on-screen pixels on camera trap footage. We tested this on domestic ferrets Mustela furo of known length, then applied it to footage from an otter natal den to estimate relative growth curves of three litters from emergence (c. 2 months old) to beyond 7 months old. We tested inter-observer consistency, then developed some rule-of-thumb age-stage characteristics for cubs (< 3 months, 3–6 months and 6–9 months). Absolute error relative to known tail to body length ratios of domestic ferrets was typically < 10