Feasibility and application of using Texas horned lizard scat to predict lizard size and age class

Texas horned lizards (Phrynosoma cornutum; hereafter THL) were once abundant throughout Texas; however, their distribution and abundance have decreased, and currently they are listed as a threatened species in the state. Detection of THL can be difficult because of their cryptic nature and sporadic distribution. Using indirect signs such as fecal pellets (i.e., scat) to determine occupancy is a widely practiced survey technique. However, it is unknown if scat size can be used to determine the age structure of THL. Our objectives were to determine 1) if THL scats were consistently produced and at consistent size in relation to lizard snout‐vent length (SVL), 2) if THL scats remain intact in the environment, and 3) if scat length and diameter could be used as an indicator of THL size, which could be used to determine population age structure. We tested the feasibility of using scat morphometrics to estimate age class ratios, body size, and population estimates for THL. Data collection was conducted in 3 phases, which occurred from 2001–2019. Scats were collected daily for 6 days from 6 hatchling, 10 juvenile, and 10 adult THL collected from southern Texas. Texas horned lizards were measured for snout‐to‐vent length (mm) and categorized as either hatchling, juvenile, or adults based on estimates from the literature. On average, THL produced one scat/day, typically in the morning. Scat length and width increased as lizard size (i.e., SVL) increased with each age class. Scat size of each age class of THL was consistent between days; thus, objective one was met. Scats were measured for length and width (mm) and placed in direct sunlight for 90 days, submerged in water for 2 days, and placed in typical summer weather conditions of southern Texas. Texas horned lizard scat did not shrink or swell in size or decay due to environmental conditions during the 90‐day experiment. Thus, THL scats do remain intact in the environment. We collected scat from 70 known‐size THL from south‐central Texas. Simple log‐log regression models demonstrated a linear relationship (r² = 0.89) between loge (SVL) and loge (scat length, mm). Slopes were similar among age classes, but intercepts differed as expected. We tagged individual THL to follow growth rates through time. Thirteen of 20 tagged females produced 178 hatchlings, of which 56, 23, 19, and 6 were recaptured during their hatchling, 2nd, 3rd, and 4th year, respectively. Models for growth rates of known‐age THL were not equal (ηˆ²ₚ = 0.73,F₆,₆₉ = 44.6, P < 0.0001) among age classes. Growth rates were faster in year 1 (0.101 ± 0.003 mm d⁻¹) and year 2 (0.099 ± 0.007 mm day⁻¹) and slowed (0.076 ± 0.009 mm day⁻¹) during their 3rd year. Scat from THL reliably can be used to estimate population age class structure, body size, and population estimates. However, our growth rate analyses suggest that THL appear to experience slower growth rates than during previous decades, and thus, THL require reevaluation as to what SVL size constitutes an adult THL.