Identification of Individual Texas Horned Lizards (Phrynosoma cornutum) Using Genotypes and Ventral Spot Patterns

Identifying individuals within a species is vital for monitoring population dynamics and determining appropriate conservation efforts. Traditional methods for marking individual lizards include toe-clipping, branding, tattooing, and passive integrated transponder (PIT) tags. However, some of these methods can potentially cause stress, affect performance and survival, and raise concerns about the ethical treatment of animals. We conducted a long-term study on the urban ecology of Texas horned lizards living in two small towns in south Texas, USA. Our study was in the unique position of possessing a dataset of individuals that were PIT tagged, genotyped, and photographed, which allowed us to validate genotyping and natural markings for individual identification. We calculated our genotyping error rate by comparing genotypes of recaptured individuals identified by PIT tags. Our mean error rate per allele was 0.0016, our mean error rate per multilocus genotype was 0.032, and we had high power to identify individuals. We used HotSpotter software to match photographs of individuals identified by PIT tags and genotyping. HotSpotter successfully matched photographs of the same individual 94% of the time. This could be increased to almost 100% by looking at the top 10 picture matches by eye to validate the matching. Additionally, individual spot patterns were unique and stable across years. Using pictures of ventral spots is an easy way to identify individuals, avoids potential rare infection or mortality, and is inexpensive relative to PIT tags and genotyping.