Photographic identification in reptiles: a matter of scales

Photographic identification is a promising marking technique alternative to the toe-clipping, since it is completely
harmless, cheap, and it allows long time identification of individuals. Its application to ecological studies is mainly limited
by the time consuming to compare pictures within large datasets and the huge variation of ornamentation patterns among
different species, which prevent the possibility that a single algorithm can effectively work for more than few species. Scales
of Reptiles offer an effective alternative to ornamentations for computer aided identification procedures, since both shape and
size of scales are unique to each individual, thus acting as a fingerprint like ornamentation patterns do.We used the Interactive
Individual Identification System (I3S) software to assess whether different individuals of two species of European lizards
(Podarcis muralis and Lacerta bilineata) can be reliably photographically identified using the pattern of the intersections
among pectoral scales as fingerprints. We found that I3S was able to identify different individuals among two samples of
21 individuals for each species independently from the error associated to the ability of the operators in collecting pictures
and in digitizing the pattern of intersections among pectoral scales. In a database of 1043 images of P. muralis collected
between 2007 and 2008, the software recognized 98% of recaptures within each year, and 99% of the recaptures between
years. In addition, 99% and 96% of matches were ranked among the top five, and no more than 5 minutes were needed
for digitizing and processing each image. The lepidosis of reptiles is a reliable alternative to ornamentation patterns in
photographic identification of reptiles, which can be effectively analysed using the I3S software. This result represents a
significant improvement in photographic identification of reptiles since (i) this procedure can be easily extended to most
other species of reptiles, (ii) all kind of individuals within a species can be marked (i.e., young, subadults and adults) despite
the differences in ornamentation patterns, and (iii) it is the only technique for species, like the western green lizard, which
lack a clear ornamentation pattern.