Last summer, Ashley Wall and Dean Williams fit 19 horned lizards with small radio-transmitters and located them every day. Each transmitter sends a unique signal to a hand-held receiver, allowing researchers to re-find tagged lizards. After weeks of tracking, we found that individuals had small home ranges and rarely crossed roads, indicating that their movement is very limited compared to populations in larger, connected habitats. Characteristics of the urban environment, like roads, can inhibit movement, which in turn prevents reproduction.
We also obtained DNA samples from every lizard we found (22 in Kenedy and 98 in Karnes City). By analyzing the DNA of each sample, we found low genetic diversity in both towns. Genetic diversity is important because it allows populations to survive and adapt to changing conditions (like an outbreak of disease). We then compared individuals in Kenedy to those in Karnes City and found large genetic differences between the towns, suggesting that lizard populations in the two towns have been isolated from each other for a long time. We even found large genetic differences within the towns. In Karnes City, groups of horned lizards are different east and west of Panna Maria and north and south of Calvert Avenue. In other words, these roads have prevented lizards on either side from moving and reproducing with each other so that the lizards on either side of the road have diverged genetically. We think this reduced movement of lizards in town may mean that when lizards disappear from a neighborhood they are unlikely to move back from areas that still contain horned lizards.
This summer we are censusing areas we searched last year and when we find horned lizards we are obtaining a DNA sample and inserting a small barcode under the skin which gives each lizard a unique number that can be detected with a small scanner placed above the lizard. This marking method is permanent so we can now start getting estimates of survival from one year to the next and also see if individual lizards stay in the same place from one year to the next. Visit the “TCU Horny Toad Project” on facebook and post any observations and pictures of horny toads you have from Karnes County.