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