Throughout many species, mammalian penises have developed keratinized penile spines along the glans and/or shaft, which may be involved in sexual selection. These spines have been described as being simple, single-pointed structures (macaques) or complex with two or three points per spine (prosimians). Penile spine morphology may be related to mating system[how?].
Penile spines in chimpanzees and mice are small surface projections made by the piling up of keratinized cell layers in the outermost skin surface. They also occur in spotted hyenas, fossas, and echidnas.
In contrast to chimpanzees and mice, the common structures found in humans are substantially larger, appear to be an outpocketing of both surface and underlying connective tissue layers, and lack the rich innervation seen in other animals. Thus, the relationship between the structures is still uncertain.
In the primate line from which humans have evolved, a regulatory DNA sequence associated with the formation of small keratinized penile spines was lost. This simplification of penis anatomy may be associated with the monogamous mating habits of humans. In some species which retain the full expression of penile spines, penile spines contribute to increased sexual sensation and quicker orgasms.
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