Many mammalian species have developed keratinized penile spines along the glans 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.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. " Evolutionary Ecology 16.2 (2002): 123-137.
Non-human mammals
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.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.
Penile spines in chimpanzees and mice are small surface projections made by the piling up of keratinized cell layers in the outermost skin surface.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) They occur in wombats,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. koalas, spotted hyenas,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. fossas, echidnas, primates,Harcourt, A. H., and J. Gardiner. "Sexual selection and genital anatomy of male primates." Proc. R. Soc. Lond. B 255.1342 (1994): 47-53. bats, and several rodent species.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.
Penile spines are often found in primate species that form copulatory plugs after mating.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. In galagos, penile spines may form a "genital lock" during copulation.Dixson, A. F. "Sexual selection, genital morphology, and copulatory behavior in male galagos." International Journal of Primatology 10.1 (1989): 47-55.
Humans 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.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) These are sometimes described as vestigial remnants of penile spines. However, the relationship between the structures is still uncertain.Penile spines versus pearly penile papules in humans
When the hominin lineage split into the genera Homo and Pan, a regulatory DNA sequence associated with the formation of small keratinized penile spines was lost in the Homo lineage. This simplification of penis anatomy may be associated with the sexual habits of humans. In some species which retain full expression, penile spines contribute to increased sexual sensation and quicker orgasms.Paleoanthropology, Genetics, and Evolution 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.
Birds The penises of some bird species feature spines and brush-like filaments.
See also Fordyce spots
Lion#Reproduction and life cycle
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