Many mammalian species 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 (strepsirrhines). Penile spine morphology may be related to mating system.
Felines, especially domestic cats, are well known for having penile spines. Upon withdrawal of a cat's penis, the spines rake the walls of the female's vagina, which may serve as a trigger for ovulation. Many other felid species have penile spines, but they are relatively small in jaguars and pumas, and do not occur in margays.
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 wombats, koalas, spotted hyenas, fossas, echidnas, primates, bats, and several rodent species.
In contrast to chimpanzees, a common morphological variant found in humans called Hirsuties coronae glandis, or pearly penile papules, 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. These are sometimes described as vestigial remnants of penile spines. However, the relationship between the structures is still uncertain.
In the primate line, 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 sexual habits of humans. In some species which retain the full expression of penile spines, penile spines contribute to increased sexual sensation and quicker orgasms. An hCONDEL (highly conserved region of DNA that contains deletions in humans) located near the locus of the androgen receptor gene may be responsible for the loss of penile spines in humans.
- Alan F. Dixson (26 January 2012). Primate Sexuality: Comparative Studies of the Prosimians, Monkeys, Apes, and Humans. Oxford University Press. ISBN 978-0-19-954464-6. Retrieved 27 September 2013.
- Orr, Teri J., and Patricia LR Brennan. "All Features Great and Small—the Potential Roles of the Baculum and Penile Spines in Mammals." Integrative and Comparative Biology (2016): icw057.
- Stockley, P. "Sperm competition risk and male genital anatomy: comparative evidence for reduced duration of female sexual receptivity in primates with penile spines. Archived 2017-02-06 at the Wayback Machine" Evolutionary Ecology 16.2 (2002): 123-137.
- Virginia Douglass Hayssen; Ari Van Tienhoven (1993). Asdell's Patterns of Mammalian Reproduction: A Compendium of Species-specific Data. Cornell University Press. ISBN 978-0-8014-1753-5. Retrieved 27 September 2013.
- Aronson, L. R.; Cooper, M. L. (1967). "Penile Spines of the Domestic Cat: Their Endocrine-behavior Relations" (PDF). Anat. Rec. 157 (1): 71–78. doi:10.1002/ar.1091570111. PMID 6030760. Archived from the original (PDF) on 2017-06-20.
- Swanson, W. F., et al. "Reproductive status of endemic felid species in Latin American zoos and implications for ex situ conservation." Zoo Biology 22.5 (2003): 421-441.
- Hill, W.C.O. Note on the male external genitalia of the chimpanzee. Proc.Zool.Soc. Lond. 116, 129–132 (1946)
- Murakami, R. A histological study of the development of the penis of wild-type and androgen-insensitive mice. J. Anat. 153, 223–231 (1987)
- Hogan, Lindsay A., Tina Janssen, and Stephen D. Johnston. "Wombat reproduction (Marsupialia; Vombatidae): an update and future directions for the development of artificial breeding technology." Reproduction 145.6 (2013): R157-R173.
- Larry Vogelnest; Timothy Portas (1 May 2019). Current Therapy in Medicine of Australian Mammals. Csiro Publishing. ISBN 978-1-4863-0752-4.
- Drea, C. M., et al. "Exposure to naturally circulating androgens during foetal life incurs direct reproductive costs in female spotted hyenas, but is prerequisite for male mating." Proceedings of the Royal Society of London B: Biological Sciences 269.1504 (2002): 1981-1987.
- R. F. Ewer (1998). The Carnivores. Cornell University Press. p. 116. ISBN 978-0-8014-8493-3. Retrieved 23 July 2013.
- Köhncke, M.; Leonhardt, K. (1986). "Cryptoprocta ferox" (PDF). Mammalian Species (254): 1–5. doi:10.2307/3503919. JSTOR 3503919. Retrieved 19 May 2010.
- Larry Vogelnest; Rupert Woods (18 August 2008). Medicine of Australian Mammals. Csiro Publishing. ISBN 978-0-643-09928-9. Retrieved 23 July 2013.
- Harcourt, A. H., and J. Gardiner. "Sexual selection and genital anatomy of male primates." Proc. R. Soc. Lond. B 255.1342 (1994): 47-53.
- "Observations on the evolution of the genitalia and copulatory behaviour in male primates". 1987. doi:10.1111/j.1469-7998.1987.tb03718.x. Cite journal requires
- L. Alterman; Gerald A. Doyle; M.K. Izard (9 March 2013). Creatures of the Dark. Springer Science & Business Media. ISBN 978-1-4757-2405-9.
- Elizabeth G. Crichton; Philip H. Krutzsch (12 June 2000). Reproductive Biology of Bats. Academic Press. ISBN 978-0-08-054053-5.
- Dewsbury, Donald A. "On the function of the multiple-intromission, multiple-ejaculation copulatory patterns of rodents." Bulletin of the Psychonomic Society 18.4 (1981): 221-223.
- Dunham, A. E., and V. H. W. Rudolf. "Evolution of sexual size monomorphism: the influence of passive mate guarding." Journal of evolutionary biology 22.7 (2009): 1376-1386.
- Dixson, A. F. "Sexual selection, genital morphology, and copulatory behavior in male galagos." International Journal of Primatology 10.1 (1989): 47-55.
- Glicksman, JM and Freeman, RG. Pearly penile papules. A statistical study of incidence. Arch. Dermatol. 93:56-59 (1966)
- Agrawal, SK et al. Pearly penile papules: a review. Int. J. Dermatology 43:199-201 (2004)
- Penile spines versus pearly penile papules in humans
- McLean, CY, PL Reno, AA Pollen, AI Bassan, TD Capellini, C Guenther, VB Indjeian, X Lim, DB Menke, BT Schaar, AM Wenger, G Bejerano, and DM Kingsley. Human-specific loss of regulatory DNA and the evolution of human-specific trait. Nature 471: 216-219 (2011). 
- Paleoanthropology, Genetics, and Evolution
- Frank B. Gill (6 October 2006). Ornithology. Macmillan. pp. 414–. ISBN 978-0-7167-4983-7. Retrieved 5 December 2012.
- Media related to Penile spines at Wikimedia Commons