Horned lizards (Phrynosoma) are reptiles recognized by their flattened bodies and crown of cranial spines. Determining the sex is often necessary for successful breeding programs or understanding social dynamics. Sexual dimorphism in horned lizards is subtle, making accurate identification challenging. Effective sexing relies on careful examination of specific physical traits, primarily those related to reproductive anatomy and overall body morphology.
Primary Sexual Dimorphism: Tail Base and Vent Shape
The most reliable way to distinguish an adult male from a female horned lizard is by inspecting the vent, the region immediately following the cloacal opening. This method is definitive because it relates directly to the presence of the male reproductive organs.
Male lizards possess paired copulatory organs called hemipenes, which are stored internally in the tail base. The presence of these organs creates two distinct bulges, or swellings, located just behind the vent. When viewed from the side, this post-anal swelling gives the tail base a noticeably thicker, more cylindrical appearance before it tapers off.
In contrast, the female tail base lacks these internal structures, resulting in a smoother, more slender profile that tapers quickly from the body. The vent itself, which is a transverse slit, may also appear slightly different between the sexes. The male’s vent tends to be narrower and more slit-like, positioned slightly more forward due to the underlying hemipenes.
The female’s vent often appears wider and more rounded to accommodate egg-laying. While vent shape difference is useful, the presence or absence of the distinct hemipenal bulges remains the most accurate marker for sexing mature horned lizards. Inspection requires gently elevating the tail to fully expose this ventral area.
Secondary Physical Indicators: Size and Horn Structure
Beyond the vent area, secondary physical traits provide supporting evidence for sex, though they are less reliable alone. In most Phrynosoma species, females exhibit a larger body size and greater mass than males. This female-biased size dimorphism is tied to the need to produce and carry clutches of eggs.
For example, adult female Texas Horned Lizards (P. cornutum) can reach a snout-vent length of up to five inches, while males typically remain smaller, averaging around 3.7 inches. Males, however, sometimes display a slightly broader or bulkier head relative to their overall body size, a characteristic often linked to male-to-male competition.
Another secondary sign involves the femoral pores, small openings visible along the underside of the hind legs. These pores are more prominent, waxy, or pronounced in males, as they secrete pheromones for communication during breeding. Note that the length or structure of the cranial horns is not a reliable indicator of sex, as research suggests no consistent sexual dimorphism in horn development.
Important Considerations for Accurate Sexing
Accurately determining the sex of a horned lizard is only possible once the animal has reached sexual maturity. For most species, this occurs when the lizard is between one and a half to two years of age. Juvenile lizards do not exhibit the development of secondary sex characteristics, such as hemipenal bulges, making visual sexing unreliable.
The degree of difference in size and other features can vary significantly across the twenty-one recognized species of horned lizards. What is pronounced in a Desert Horned Lizard (P. platyrhinos) might be much subtler in a Short-horned Lizard (P. hernandesi), requiring careful species-specific knowledge.
When handling the lizard for inspection, approach calmly and avoid grabbing the animal from above, which triggers a stress response. Support the lizard’s entire body gently, especially the limbs and torso, while briefly elevating the tail to view the vent area. Minimizing handling duration reduces stress, ensuring the process is safe.