EFFECT OF AGE-CLASS ON THE SURVIVORSHIP AND SPATIAL MOVEMENT PATTERNS OF HEADSTARTED TEXAS HORNED LIZARDS SOFT-RELEASED INTO AN URBAN POPULATION

Abstract

Texas horned lizards (Phrynosoma cornutum) have experienced population declines and localized extirpations throughout their historic range, largely due to anthropogenic habitat loss. Consequently, there is broad interest in reinforcing persisting natural and urban populations, as well as reintroducing the species into previously occupied environments. To this end several headstart programs have been established that temporarily rear young wild-sourced or captive-bred hatchlings in captivity before their release into the wild. To understand how the age of headstarted lizards affects reintroduction success, we implemented a 2-year radio-telemetry study (April 2023 – October 2024) at an urban wildlife reserve on Tinker Air Force Base, Oklahoma. We compared the spatial movement patterns and survivorship of 2 soft-released cohorts of headstarted lizards, raised either to juvenile or adult age, to resident lizards of the corresponding age class. The headstarted adult cohort (n = 16) was smaller on average and exhibited significantly lower daily movement distances, smaller home range sizes, and lower survivorship compared to resident adults (n = 19). However, despite these differences, they successfully underwent brumation, attempted reproduction, and maintained body weight post-release. In contrast, we did not observe significant differences in spatial movement metrics between the headstarted juvenile cohort (n = 8), and the resident juveniles (n = 9). Although we did not detect a difference in survivorship between the headstarted and resident juveniles, the statistical power of our survival analysis was limited by small sample size. The headstarted juveniles were initially much larger than the resident juveniles due to accelerated growth during headstarting, though this difference diminished by the end of the season. Low survivorship was observed across both headstarted and resident age classes, with nearly all recorded mortalities being attributed to predation. Notably, nearly one-third involved unusual decapitations, indicating that nearby domestic cats may be responsible. Our findings highlight the necessity of post-release monitoring, and suggest that headstarting Texas horned lizards to juvenile age before reintroduction may serve as a worthwhile supplement to the current method of releasing headstarted hatchlings before brumation. For threatened urban populations, future reintroduction efforts should be coupled with predator management strategies to mitigate mortality risk.