To ear tag or to passive integrated transponder tag? A comparison of mark loss rates for 8 rodent species

Capture-mark-recapture methods are commonly used in studies of wildlife to estimate demographic rates and behavioral monitoring. Two important assumptions underlie associated mark-recapture and other similar long-term monitoring: markings are not lost and the markings remain readable for the duration of the study. A number of methods have historically been used to mark animals including subcutaneous passive integrated transponder (PIT) tags, ear/fin tags, toe clipping, and others. We aimed to evaluate 2 different common small mammal tagging methods (PIT tags and dual metal ear tags) in 8 rodent species from 5 genera (Chaetodipus hispidus, Cynomys ludovicianus, C. parvidens, Microtus longicaudus, M. mogollonensis, M. ochrogaster, Neotoma mexicana, and Peromyscus sonoriensis). Our goal was to evaluate which methods would provide long-lasting marks (in relation to the lifespan of the species). We predicted that tag loss rates would differ by species and tagging method as ear size, skin thickness, and behavior differ among these species. We used multistate hidden Markov mark–recapture models to compare the rates of tag loss between PIT and ear tags. Our data suggested that tagging both ears had significantly lower annual tag loss rates compared to PIT tagging for P. sonoriensis. Conversely, PIT tagging had significantly lower annual tag loss rates compared to tagging both ears for M. longicaudus. We failed to detect any significant difference in annual tag loss between the 2 methods for the other 6 species. We did detect a difference in tag loss rates for at least 1 tag type between the short-term (first interval after capture) and long-term; P. sonoriensis and C. hispidus were more likely to lose an ear tag that first interval while N. mexicanus had the opposite trend. Five species were more likely to lose a PIT tag the first interval following application than later intervals. Additionally, we failed to detect a significant difference between tag loss rates in male versus female P. sonoriensis and C. ludovicianus. Results herein may assist wildlife biologists in minimizing tag losses in future studies of mammals, including imperiled species and taxa associated with zoonotic diseases of human health concern.