The genus Phrynosoma, commonly known as horned lizards, comprises unique reptiles found across North America. These lizards are characterized by a wide, flattened body and elaborate cranial and body spines. Determining the sex of a horned lizard is important for breeding programs and conservation efforts, but the sexes often appear superficially similar. While some visual differences exist, definitive identification relies on examining specific anatomical structures.
General Visual Clues
A primary difference between the sexes is overall body size, known as sexual size dimorphism. Females are typically larger and heavier than males in many species. For example, a female Texas horned lizard (Phrynosoma cornutum) can reach a snout-vent length of about 5 inches, while the male measures closer to 3.7 inches.
The female’s body also tends to be bulkier and more rounded, providing space for egg development. This size difference relates to the female’s need to produce a large number of offspring. Males may have evolved to be smaller, allowing for greater mobility when searching for mates. Coloration can also offer a hint, as patterns on both sexes may become more vibrant during the breeding season.
Key Anatomical Features
The most reliable method for sexing a horned lizard involves inspecting the area immediately behind the cloaca, or vent. Male lizards possess paired reproductive organs called hemipenes, which are stored internally at the base of the tail. The presence of these organs creates a distinct swelling or bulge on the underside of the tail, directly behind the vent.
This hemipenal bulge is absent in females, resulting in a smoother, more immediate taper of the tail from the body. When viewed from the side, a male’s tail base appears thicker and more elevated due to the stored hemipenes.
Femoral Pores
Another anatomical feature involves the femoral pores, which are small openings found along the underside of the lizard’s thighs. Both sexes have these pores, which secrete waxy lipids used for chemical communication. However, the pores in males are noticeably larger and more prominent, often appearing as a row of raised dots. These pores become noticeable in mature males and may be filled with waxy plugs that release pheromones for scent-marking territory and attracting mates.
Distinguishing Behaviors
Behavioral displays provide additional context, though they are seasonal and less conclusive than anatomical inspection. Male horned lizards exhibit more pronounced territoriality, often displayed through aggressive posturing toward other males. The male’s home range can be significantly larger than the female’s, especially during the mating period, as they actively search for receptive partners.
A common communication method is the head-bob display. Male horned lizards perform more frequent and vigorous head-bobs during courtship and territorial challenges. This display signals dominance to rival males and communicates fitness to potential mates. While females may also perform a head-bob, it is less frequent and subtle than the male’s performance.