Verification of Natural Marking for Individual Identification Using a Duplex Marking Approach in Ijima's Sea Snakes, Emydocephalus ijimae (Reptilia: Elapidae).

Abstract

The mark and recapture method for free-ranging animals provides valuable information in ecological studies. Recently, natural marking has become more frequently used for individual identification, but it almost inevitably induces problems associated with the corroboration of individual specificity and the persistence of the given markings. We employed a duplex natural marking approach to resolve this problem over a four-year field study of a banded hydrophine sea snake and tested the effectiveness of this approach in corroborating the accuracy of individual identification. We conducted monthly field surveys in southwestern Japanese waters and photographed the patterns of the last five bands on each captured sea snake. We converted the band patterns into profile codes with five sections (one section corresponding to each band), according to the scale configurations involved in the bands. We considered the bilateral band patterns as a duplex set of natural markings for individual identification and checked their accuracy mutually. We looked at 593 photos of recorded snakes and recognized 179 unique profile codes on both the left and right sides, 96 of which were recorded more than once on both side. A particular code for the left side was always accompanied by a particular code on the right side in the same combination. It is certain that the 593 recorded snakes consisted of 179 snakes and their recaptures. The perfect correspondence between the left and right side profile codes throughout the four years showed the high individual uniqueness and persistence of each pattern. This study also showed that the duplex natural marking approach is effective in verifying the accurate individual identification. The duplex natural marking approach can be applied to various animals to justify the usage of a given natural marker for individual identification, without the aid of combined artificial markings. The duplex method itself can be a combination of the first five bands and the next five bands on the same side in a single photo, or a combination of some patterns on the head and those on the body.