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. 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. 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. Juveniles of a species may appear similar in size to mature adults, but possess little developed hemipenes that might not be easily recognizable. However, they will be smaller than male hemipenes and have no visible blood vessels, though a red tip may be visible. This will cause the hemipenis to evert out of the body.
Juveniles of a species may appear similar in size to mature adults, but possess little developed hemipenes that might not be easily recognizable. Hemipenes also have an outer groove called the sulcus spermaticus, which transports sperm through the outside, rather than the inside, of the organ. In snakes and lizards, morphological differences in the reproductive organs are believed to exist to help the male copulate with the female. Cryptic female choice[ edit ] Oftentimes, females 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. This is one hypothesized reason for males having two penises instead of one: Background[ edit ] The hemipenis is the intromittent organ of squamata, which is the second largest order of vertebrates with over species distributed around the world. It is important to distinguish between cryptic female choice and facultative parthenogenesis , a form of asexual reproduction. Lock-and-key mechanism[ edit ] The lock-and-key mechanism or hypothesis is the idea that genital morphology has evolved to maintain reproductive isolation between species, by ensuring that mating can only occur between a male and female of the same species. 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 go longer. However, embryonic and molecular research is beginning to shed light on the origin of the hemipenis. 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. 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. Therefore, it is in their interest to mate with as many females as possible to increase their chance of passing on genetic material. Consequently, females can store sperm internally  for as long as five years, or possibly longer. 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. Unlike squamata, however, their singular penis is four-headed. This will cause the hemipenis to evert out of the body. The idea was first postulated in by French entomologist, Leon Dufour, who observed the diversity of genital morphology among Dipteran flies. 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. 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. However, they will be smaller than male hemipenes and have no visible blood vessels, though a red tip may be visible. A product of this system is that males are not guaranteed to successfully reproduce and bear offspring from a single mating. 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. 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. Unpaired penis as ancestral form[ edit ] This theory proposes that the single unpaired penis is the ancestral state for amniotes, and that this trait was retained by most amniotes today.
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