Persistent fenestrae occur between frontal or parietal bones in skulls of various rodents, especially species of Andinomys and Ctenomys. The frequency of occurrence and the shape and position of the fenestrae may vary with age or among individuals, populations, and species. Different families and species from both high and low elevations exhibit fenestrae. They have arisen in several different evolutionary lineages. Genetic factors contribute, but no clear functional significance of the fenestrae has been established.