ABSTRACT

In a single blind study the gender of nine adult Mexican beaded lizards, Heloderma horridum and 35 adult Gila monsters, Heloderma suspectum of known sex was determined by two dimensional ultrasound imaging of the ventral tail just distal to the cloaca. In each case subjects could be sorted into two distinct morphologic groups. Group 1 animals had large, variably hypoechoic, paired soft tissue structures in the ventral aspect of the tail spanning 2 cm distal to the vent, while the distal tail lacked these structures. Group 2 animals lacked this characteristic altogether, giving the ventral tail a much more consistent ultrasonic complexion along the entire length. The most logical conclusion for this difference was the presence of paired hemipenes in group 1 animals. Using this premise, subjects in group 1 were declared males, while group 2 animals were declared females. The predictions matched the known sexes of each individual in every case for adult lizards. One juvenile Gila monster could not be accurately sexed based on this technique. From these data we conclude that ultrasound imaging of structures in the proximal 2 cm of the tail provides a non-invasive and accurate means of determining gender in adult Gila monsters and Mexican beaded lizards.