Studying the reproductive biology of the common wall lizard using ultrasonography

Ultrasound imaging is a promising technique for studying the reproductive biology of reptiles, but it has yet to be validated for small lizards in field research. This study aimed both at assessing the reliability of ultrasound imaging in field research and the measurement of the breeding effort and timing of reproduction in the northern Italian female population of the common wall lizard, Podarcis muralis. To this end, we kept 22 gravid females in captivity in April and June 2010 and used ultrasonography to predict the number of eggs they laid. The following year, we applied the same technique to monitor the breeding performance of females in their natural habitat. We collected 119 females between March and June 2011 and used ultrasonography to evaluate clutch size and egg developmental stage. Our results showed that 20 out of 22 females (91%) laid the exact number of eggs predicted. The field research showed that the percentage of gravid females varied over the season, showing a clear bimodal pattern with two peaks in late April and late May. These peaks corresponded to the first and second clutch depositions, respectively. Furthermore, female common wall lizards reach sexual maturity at a body size of 50–51 mm snout–vent length, at around 2 years of age. Mean clutch size in our population ranged from 2 to 5.5 eggs, with an average of 3.6 eggs. There was a strong positive relationship between clutch and female size, which was only statistically significant in the first deposition. The female lizards in our study were smaller than those in French and central European populations, they reached maturity at 50.9 mm and they laid few eggs. In this paper, we discuss some potential explanations for such differences.