Hemipenis
A hemipenis (pl.: hemipenes) is one of a pair of intromittent organs of male squamates (snakes, lizards and worm lizards).[1][2][3] Hemipenes are usually held inverted within the body, and are everted for reproduction via erectile tissue, much like that in the human penis. They come in a variety of shapes, depending on species, with ornamentation such as spikes.
Background
The hemipenis is the intromittent organ of Squamata, which is the second largest order of vertebrates with over 9000 species distributed around the world. They differ from the intromittent organs of most other
Evolution
Much debate continues regarding the evolutionary origin of hemipenes and their relationship to the intromittent organs of other species. However, embryonic and molecular research is beginning to shed light on the origin of the hemipenis.
Unpaired penis as ancestral form
This theory proposes that the single unpaired penis is the ancestral state for amniotes, and that this trait was retained by most amniotes today. A look at the embryonic underpinning of hemipenes and penises of other animals suggests that there are fundamental differences in their developmental stages, particularly their origin of development relative to the embryonic cloaca. Specifically, the hemipenes of Squamata are found to develop on the posterior side, while the paired genitals of non-squamate amniota develop on the anterior side.[6] This developmentally significant difference suggests that the two types of penises could have distinct homologies, and it is thought that this could be attributed to variance of signaling genes during embryological development.[6]
Speciation
Hemipenes are also being used to study speciation among Squamata, especially in identifying cryptic diversity and understanding taxonomy at a species level.[7] One study conducted in 2015 investigated anole speciation through hemipenis variation, and found that anole hemipenial morphology evolved six times faster than other non-genital morphological features.[8] Such studies can help researchers understand adaptive radiation and recover phylogenetic relationships, especially between species that are morphologically very similar.
Morphology
Identification
Hemipenes can be examined on a living snake or lizard by gently squeezing on the tail and massaging towards the vent, which is the cloacal opening on the underside of the tail. This will cause the hemipenis to evert out of the body. However, this method can also make it difficult to identify the sex of a dead specimen if it is damaged or dry.
While the presence of hemipenes is an indicator of a specimen being male, its absence should not be immediately inferred as the specimen being female. Juveniles of a species may appear similar in size to mature adults, but possess little-developed hemipenes that might not be easily recognizable. It is also possible to misinterpret the scent gland papillae by the cloaca of the female as a hemipenis, as it can also protrude and be quite large. However, they will be smaller than male hemipenes and have no visible blood vessels, though a red tip may be visible.[9]
Structure and shape variation
Hemipenes can be found in a variety of shapes and sizes, but foundationally, have the same general structure. They are made up of two hemipenises tucked under the tail side by side or stacking; bottom and top, with each of the lobes exhibiting a range of ornamentation, including spicules and hooks. Hemipenes also have an outer groove called the sulcus spermaticus, which transports sperm through the outside, rather than the inside, of the organ. This is structurally different from the human penis, which has sperm travel inside the organ through the vas deferens and the urethra.
The surface of hemipenes is one of the most interesting and unique features, and is often covered in sharp spines and spicules that are organized in formations called rosettes. However, species with relatively smooth hemipene surfaces also exist. For example, the hemipenis of the Siamese spitting cobra (
Function
Lock-and-key mechanism
The lock-and-key mechanism or hypothesis is the idea that genital morphology has evolved to maintain
In snakes and lizards, morphological differences in the reproductive organs are believed to exist to help the male copulate with the female. Spikes and hooks are thought to assist the male in fixing the hemipenis in place during mating, and are made specifically compatible to the female of the species. For example, species with branched hemipenes have females with branched cloacas, and species with many spikes have females with thicker cloacal walls, compared to those of species with males having little or no spines. This huge variety among the reproductive organs of Squamata is of interest to taxonomists, as it may be able to shed light on evolutionary relationships among reptiles.[9]
Sexual conflict theory
The function of the ornamentation of the hemipenes is still being explored by researchers, but one study supports the theory that the spines that can be found on many hemipenes is a trait to assist in longer, and hence more successful, reproduction for males. Through surgically removing the large basal spine of the red-sided garter snake, researchers found that while males were still able to mate, the duration of mating and the depth of copulation were much shorter than the unaffected control males, indicating that the spines play a crucial role in the male's ability to mate successfully with the female.[13] Females responded to the male hemipenis with strong vaginal contractions that prevented longer mating. To test that the purpose of the contractions was to shorten mating time, researchers anesthetized the female to prevent the contractions and found that mating did indeed last longer.
Cryptic female choice
Often times, female snakes and lizards also have the ability to control whether they get pregnant immediately after mating or not, to account for the fact that the ideal time for mating may not translate to the most optimal time for ovulation and gestation. Consequently, females can store sperm internally[14] for as long as five years, or possibly longer.[15] This phenomenon is known as cryptic female choice, as the physiological mechanisms making this possible are hidden within the body, and the female has the ability to influence when egg fertilization occurs.
While the exact mechanism of how the female controls the stored sperm to fertilize her eggs remains unclear, it is believed that the specialized pockets found in the reproductive tract play a key role.[16] As a result, a female can mate with multiple males and choose when to fertilize her eggs. Furthermore, females can also produce offspring that can feature genetic material from multiple males in a single clutch if she mated with more than one male.
A product of this system is that males are not guaranteed to successfully reproduce and bear offspring from a single mating. Therefore, it is in their interest to mate with as many females as possible to increase their chance of passing on genetic material. This is one hypothesized reason for males having two penises instead of one: as each hemipenis is associated with one testis and only one side can be used during mating, having a second hemipenis functions as a "backup" and ensures that mating can continue even if one side were to run out of sperm.
It is important to distinguish between cryptic female choice and facultative parthenogenesis, a form of asexual reproduction. Due to the female's ability to get pregnant long after she has been in contact with a male, it is difficult to distinguish between the two in ambiguous cases. In such circumstances, molecular testing techniques can be used to identify whether her offspring share all or some of their genetic make-up with their mother.
Hemiclitoris
Most research in the area of squamate reproductive organs has focused on the male hemipenis, but recent studies have investigated the homologous paired erectile structure in females, dubbed "hemiclitoris" (pl.: hemiclitorises or hemiclitorides).[17] Researchers first investigated females of Phymaturus and Liolaemus lizards and discovered a structure that was previously undescribed. The hemiclitoris was observed to be smaller than a hemipenis,[18] and the organs were consistently observed in the lizards in the study. While the specific functions are yet to be identified, this study brings attention to a Squamata apomorphy that will likely continue to be investigated in detail. A 2022 study showed that nine species of snakes also have hemiclitorises of widely differing structure.[19][20]
Other vertebrates
Among vertebrates, penises can be found in a variety of shapes, sizes and structures, such as the lymphatic erection mechanism of ratites and the single vascular erectile body of turtles.
Many
See also
Notes
- ^ "Hemipenes".
- ^ "Male reproductive behaviour of Naja oxiana (Eichwald, 1831) in captivity, with a case of unilateral hemipenile prolapse". Herpetology Notes. 2018. (Full text via https://www.biotaxa.org/hn/article/view/34541/37750.)
- S2CID 18482667.
- S2CID 8964417.
- ^ Laslo, M. (2015). "When two become one: the evolution and development of external genitalia on land". Evo Bites.
- ^ S2CID 14361363.
- S2CID 86734273.
- S2CID 8228959.
- ^ a b Mattison, Chris (1998). The Encyclopedia of Snakes. London: Blanford.
- ^ Dufour, L. (1844). "Anatomie générale des diptères". Annales des Sciences Naturelles (in French). 1: 244–264.
- ^ De Wilde, J. (1964). "Reproduction". Physiology of Insecta: 9–58.
- PMID 16701234. Archived from the original(PDF) on 2017-10-12. Retrieved 2018-08-26.
- PMID 24225467.
- ^ Arnold, C. "Sperm Hoarders: Why Animals Store Semen". National Geographic. Archived from the original on July 28, 2017.
- .
- PMID 11754034.
- ISBN 978-3-31941-674-8. Retrieved November 10, 2023.
- S2CID 86218746.
- ^ Fox, Alex (14 December 2022). "Scientists Overlooked the Snake Clitoris, Until Now". The New York Times. Retrieved 14 December 2022.
- PMID 36515117.
- .
- PMID 21708713.
- S2CID 40632746.
- S2CID 31095809.
External links
- Media related to Hemipenis at Wikimedia Commons
- Why do snakes have two penises? at Life is Short but Snakes are Long