A new study has revealed that lizard's penis evolves six times faster than any of its other parts.
Study researcher Julia Klaczko, a biologist at the University of Campinas in Brazil, said that the study is the first to directly measure the evolution rate of the penis of any species, though researchers have long suspected that the male genitalia evolve faster than other body parts.
Because penises are often so different even in species that otherwise look almost identical, researchers frequently use genitals to discriminate between different species and Klaczko and her team chose to measure the genitals of 25 species of Anolis, a group of lizards that live in the Caribbean.
The lizards' hemipenes are tubular structures with a groove through which semen can flow. The researchers measured the length and width of the hemipenes in several specimens of each species. For comparison, they also measured the length of the lizards' limbs, which evolve in response to the vegetation in the animals' habitats, and the size of their dewlaps, which are the flaps of tissue near the throat that the lizards use for communication.
Next, using mathematical modeling, the researchers estimated the rates of evolution necessary to arrive at the differences in genitals, limbs and dewlaps and found that male genitalia change six times faster than either legs or throat flaps, making them more diverse in shape and size from one another than the other body parts.
The study was published in the Journal of Zoology.
