Flat-Tailed Horned Lizard Reproduction
A male Flat-Tailed Horned Lizard courting a female with a radio transmitter.
What Are We Studying?
The goal of this study is to examine reproduction in Flat-tailed Horned Lizards (Phrynosoma mcalli) by assessing reproductive condition and using radio telemetry to monitor movements and behaviors of gravid females during the egg-laying period. We are also investigating dispersal, potential barriers to gene flow, effective population size, and parentage using genomic analyses. Although we have been studying these amazing lizards since 2011, the current project began in 2017 and is still ongoing. Funded by the Department of Defense, Bureau of Reclamation, and the Flat-tailed Horned Lizard Interagency Coordinating Committee, our study sites are on the Barry M. Goldwater Bombing Range and Bureau of Reclamation land south and east of Yuma, Arizona. In-depth knowledge of reproductive ecology is critically important to our understanding of observed population fluctuations, and for monitoring population status over time. It also provides data on a suite of parameters that are important in population viability analyses. When we began the research, very little was known about reproduction in these cryptic lizards, but we are now able to fill in some important gaps in our knowledge, and attempt to answer questions from genomic analyses that would not be possible with traditional demographic data alone.
Abstract
We investigated reproduction in female Flat-tailed Horned Lizards (Phrynosoma mcallii) on the Barry M. Goldwater Bombing Range near Yuma, Arizona from 2017-2019. We palpated and weighed females to determine gravidity. We affixed radio transmitters to the dorsal surface of lizards using silicone adhesive. Individuals were radio tracked two to three times per day until they laid their eggs. We placed nest enclosures around nest burrows using aluminum flashing and we checked the enclosures daily until hatchlings appeared or ten weeks had passed. We radio tracked 29 individual females in 2017, 17 in 2018, and 29 in 2019. Sixteen individuals produced one clutch, and two individuals produced two clutches in 2017. No individuals produced clutches in 2018 and 16 individuals produced one clutch in 2019. Females moved relatively long distances prior to oviposition, which may be a dispersal mechanism leading to increased genetic diversity. Upon exiting nest burrows, females backfilled the entrance to their burrows (25 of 35 exhibited this behavior). Hatchlings emerged from nest burrows approximately six weeks after oviposition. Clutch sizes ranged in size from 1 to 6 and there was an average clutch size of 4 ± 0.35. In addition to demographic and life history data, we are also examining population ecology, dispersal, and parentage using genomic analyses.
Allyson Hawkins and Anthony Pawlicki setting up nest enclosures.
Methods
Lizard Surveys
We find the vast majority of Flat-tailed Horned by first finding their characteristic tracks in the sand. We start surveys at sunrise and stop when temperatures exceed 40°C, or when the sun rises too high in the sky, making it difficult to see tracks . When we capture a lizard, we record its location using a GPS unit, take data on temperature and humidity, and take multiple measurements on morphology, such as mass, snout-vent length, and parietal horn length. We sex lizards based on scales at the base of their cloaca, and permanently mark individuals with a tiny microchip inserted under the skin. We take genetic samples for genomic analyses by clipping the very tip of the lizard’s tail so we can obtain a blood blot and a tissue sample at the same time. We do this all in the field, so we can quickly release the lizard at its exact point of capture.
Radio Telemetry
While surveying for Flat-tailed Horned Lizards, we focused on finding gravid females. We identified gravid females based on a higher than average mass and/or palpating for developing ova. We attach a radio transmitter to the females and hold them in a controlled environment for 24 hours until the adhesive is cured. We radio track the lizards twice per day; once in the morning to weigh them and once at midday after they take shelter in burrows that they excavate themselves. We radio track lizards until a week after they lay their eggs.
Nest Enclosures
When we find the burrow containing the clutch, we erect a circular enclosure (5 m radius) using aluminum flashing buried to a depth of 10 cm. We install enclosures four weeks after the female lays her eggs and check them every day until hatchlings emerge, or eight weeks have passed, at which time we are able to determine that the nest was not successful.