Encyclopedia of Animal Cognition and Behavior

Living Edition
| Editors: Jennifer Vonk, Todd Shackelford

Squamate Morphology

  • Angele MartinsEmail author
  • Roberta A. Murta-Fonseca
Living reference work entry
DOI: https://doi.org/10.1007/978-3-319-47829-6_150-1


Squamates are grouped within the paraphyletic “Reptilia,” with Testudines (turtles), Crocodylia (crocodiles, alligators, and gavials), and Rhynchocephalia (tuataras). Rhychocephalia is the closest related to the Squamata, being its sister-group (clade Lepidosaura) and sharing many characters, as the skin covered by scales/plates that are partially impermeable and changed periodically, and a transversal cloacal slit. Squamata is the largest and most diversified clade of extant reptiles, comprising about 95% of its current diversity, with around 6500 lizard species, 3700 snake species, and 190 amphisbaenian species (Uetz and Hosek 2018).

It is not hard to recognize a squamate and morphologically distinguish it from other reptiles. The basic corporeal plan of a lizard, with an elevated head, short neck, four limbs, and long tail, distinguish them from most other reptiles (McDiarmid 2012). The differences between Squamata and Rhynchocephalia are subtler – the main difference regards the presence of an ancestral skull in tuataras, with complete temporal bars defining the upper and lower temporal openings. Lizards lost the lower temporal bar, while snakes lost both (Kardong 2014). Many lizards and amphisbaenians, as well as all snakes, evolved the apodal elongated body, which is also distinct from other reptiles. Some important morphological features among snakes are the absence of eyelids and external ear, numerous vertebrae with modification on its connections, many paired internal organs elongated and some modified or lost (McDiarmid 2012). Moreover, all squamates have paired copulatory organs defined as hemipenis.

External Morphology

Squamates are extremely diverse and can be quite specialized, not only with respect to their habitat or locomotion type, but also their morphology (Klein and Gorb 2014). Both snakes and amphisbaenians are legless elongated reptiles, even though a few snake species (blindsnakes and a few boas) exhibit rudimentary pelvic girdles that project externally as spurs, and a few amphisbaenians (Bipes spp.) exhibit reduced forelimbs. Additionally, there are different degrees of reduction in the number of limb elements to complete limblessness. The tail tends to be long and slender, but some snakes and lizards might exhibit bulbous or laterally expanded, or dorsoventrally or laterally compressed tails (Zug et al. 2001; Lillywhite 2014).

Several external morphological features associated to their sensory system include a parietal eye on the top of the head in a few lizards, distinct by the presence of a modified head scale, which is important for their photosensitivity. Additionally, a few snakes exhibit sense organs that respond and detect infrared radiation, very useful at night when visible light is usually unavailable. Such receptors are present as modified lip scales (= labial pits) in boids and as a facial pit (=loreal pits), located between the eye and the nostril, in pitvipers. Additionally, all snakes, amphisbaenians, and several lizards present biphid tongues, which are extended to sweep air in front of them. The tongue collects airborne particles and is posteriorly retractes to perceive chemical signals, transferred to the vomeronasal organ located on the roof of the mouth. Tongues may also serve for prey capture, as in some chamaeleonids lizards (Kardong 2014).

The eyes of most squamates are large and conspicuous, especially in terrestrial and arboreal species. However, several fossorial species might present extremely reduced/degenerate eyes that can lie under the scales. Additionally, eyes of lizards exhibit bony plates (scleral ossicles) embedded in the sclera and surrounding the cornea (Zug et al. 2001). Pupils may vary from round to elliptical and are usually oriented vertically, even though it can be horizontally oriented in a few species. Extreme specializations in the squamate eye include the protrude lateral eyes of chameleons, with distinct anatomy of nodal and center points, in which the amplitude of movement is very large and the eyes move and focus independently (Zug et al. 2001; Lillywhite 2014).

Amphisbaenians exhibit the most conserved external morphology amongst Squamates, being characterized by their elongated bodies that are usually less than 150 mm long, with an extreme reduction (Bipes spp.) or complete loss of the limbs, and rudimentary/reduced eyes. Most of their distinctive features regard the dramatic modification of the head shapes, which is functionally correlated with specific tunneling behaviors. Heads may vary from (1) “shovel-headed,” with snouts dorsoventrally flattened with a strong craniofacial angle; (2) “keel-headed” forms; (3) “spade-headed” forms; or (4) “round-headed” forms – representing the most common form amongst amphisbaenians (Kearney 2003).

Within snakes, most small species belong to Scolecophidia, a group of fossorial snakes with reduced/absent eyes, rudimentary pelvic girdles that might be present as spurs, and a row of ventral scales that does not differ from those located dorsally in the trunk. Such snakes exhibit a relatively conserved external morphology, with variation occurring mostly in their head shape and scales pattern, dorsum colors and patterns, and presence/absence of a tail spine. The Alethinophidians comprise all the other snakes, with an extreme diversity in sizes, colors, scale patterns, and shape (Fig. 1).
Fig. 1

Examples of diversity in Alethinophidia – (a) Bothrops neuwiedii, (b) Corallus caninus, (c) Phimophis guerini, (d) Erythrolamprus aesculapii, (e) Philodryas mattogrossensis, (f) Philodryas argentea. (Photos: Roberto Murta)

Among squamates, lizards exhibit the highest level of morphological diversity (Fig. 2), varying from the ancestral type to forms that are completely legless. Many arboreal species are laterally compressed and may exhibit head and dorsum projections that provide an effective method for obscuring their surroundings. Several lizards are long with long tails, while others are short and robust with short tails. Some have strongly prehensile tails, used as fifth leg in climbing, others use their tails as whips to defend themselves, and many have laterally compressed tails used in swimming. Among lizards with limbs, there are species with four long/short limbs, as well as forms with only two (posterior) limbs. A few lizards have webbed feet and/or skin flaps that can be expanded and used as partial parachutes when jumping from arboreal perches. Some have adhesive pads on their toes, allowing them to climb vertical smooth surfaces; a few even have adhesive pads on the typo of the tail, providing a fifth point of secure contact. Heads of lizards vary from flat and wide to long and thin. A wide variety of ornamentation exists as well, some for crypsis (camouflage) and some having to do with social interactions. All lizards (except for a few gekkonids) have claws, which aid on locomotion on vertical surfaces (Pianka and Vitt 2003).
Fig. 2

Examples of diversity in lizards – (a) Amphisbaena alba, (b) Polychrus acutirostris, (c) Notomabuya frenata, (d) Salvator merianae, (e) Tropidurus sp., (f) Ophiodes sp., (g) Hemidactylus mabouia, (h) Ameiva ameiva, (i) Plica umbra. (Photos: Roberto Murta)


Squamates in general exhibit a water-conserving integument (Withers and O’Shea 1993) with the skin modified into scales, which might be named as plates, tubercles, lamellae, etc., depending on the taxonomic group and location of the scales (Zug et al. 2001). Scales in various squamates have evolved in different sizes, geometries and gross structure, also varying regionally in the body, what is extremely relevant for systematics (Lillywhite 2014). The scales of a few lizards might be underlain by bony plates, called osteoderms. The integument of Amphisbaenians is characteristic in representing a disconexion to the trunk, enhancing its underground locomotion (Pough et al. 2018).

The epidermis of Squamata is composed by several keratin layers formed from cells of a basal layer, stratum germinativum, that is renewed together during shedding process (ecdysis). The most external cell layer, Oberhäutchen, presents three-dimensional microstructures on its surface (=microornamentation; Fig. 3), which are studied with Scanning Electron Microscopy. Since the first works about lizards and snakes microdermatoglyphics, these morphological patterns have been constantly suggested as an important taxonomic and systematic tools, and also to provide novel and interesting data on their biology.
Fig. 3

Microornamentation of the dorsomedial scale of snakes on a 10 k zoom exhibiting the lamellate (a) and dotted (b) patterns. (Photos: Luciana O. Ramos)

Internal Morphology


“Reptiles” exhibit diapsid skulls (Fig. 4), with two pairs of temporal fenestrae that are disconnected by the superior temporal bar (formed by the squamosal and postorbital bones) and the inferior temporal bar (formed by jugal and quadradojugal bones). Among Lepidosauria, only the tuataras retained both fenestrae – squamates present exclusively the superior fenestra (lizards) or none (snakes) (Zug et al. 2001). The loss of the arch and fenestrae is associated to the increase of the flexibility (skull kinesis). The kinesis is derived from the presence of joints between many sections of the skull. A joint may occur in the posterior part of the skull (meta-kinetic joint), and between the dermic skull and braincase, at the parietal/supraoccipital suture (being the oldest kinetic articulation and, nowadays, occurring only in tuataras). Two other joints evolved in braincase dermic bones – the dorsal meso-kinetic joint, present between frontals and parietal in many lizards and, in many snakes, a pro-kinetic joint, in the contact between nasals and prefrontals or frontals. The most impressive skull kinesis of Squamata is exclusive of snakes – streptostyly or quadrates rotation. Each quadrate is loosely connected to the dermatocranium and has the ventral extremity free. These lost ligaments allow the quadrate to rotate and swing back and forth and to the inside and outside (Zug et al. 2001).
Fig. 4

Scheme of a Dipasid skull based on tuatara

The basic form of the squamate skull has the quadrate suspension with reduced dorsal and ventral processes of squamosal; loss of quadratojugal; fusion of the parietals; reduction of palatal dentition and anterior plate of the pterygoid; enclosure of the Vidian canal; shortening of the maxilla/dentary; among others (Evans 2008). Additionally, the snake skull seems to be paedomorphic in relation to lizards as claimed by several ontogenetic and anatomical studies. Considering the limbs, 15 synapomorphies are known for Squamata, and some of it are: elongate, gracile limbs; specialized joints (ulna-ulnare and radius-radiale; first metacarpal-wrist; locked tibio-atragalar; ankle); intermedium reduced or absent; and second distal tarsal absent (Russel and Bauer 2008).

The “lizards” are divided into two major clades: Iguania and Scleroglossa which according to several studies includes the snake lineages. Iguanians show some primitive skull features (Fig. 5), as the retention of complete postorbital and upper temporal bars, retention of large upper temporal fenestrae and the absence of a hypokinetic joint. Still, their long period of independent evolution provided them with some specializations, such as frontals fused in the embryo, postfrontal reduced/absent, splenial absent/reduced, and teeth frequently tricuspid. Some Scleroglossa characters include prominent anterior descending processes of the frontals, loss of dorsal process of squamosal, vomers reduced in length, and septomaxillae dorsally expanded and convex (Evans 2008).
Fig. 5

Scheme of the dorsal/ventral views of a lizard skull (based on Plica plica). Pm = premaxilla, Na = nasal, Mx = maxilla, Prf = prefrontal, L = lacrimal, J = jugal, Fr = frontal, Pa = parietal, Po = postorbital, Sq = squamosal, Vo = vomer, Pt = pterygoid, De = dentary, Ar = articular, Bo = basioccipital

The morphological diversity of the lizards’ axial skeleton reflects the wide extent of their morphological specializations, as previously shown. The number of presacral vertebrae varies from 16 in some Chamaleonidae to as much as 116 in some Dibamidae. The Sauria (lizards) shows two main morphological types of vertebrae, those with an amphicoelous centrum and those with a procoelous centrum. Sauria (=lizards) ribs are usually called holocephalous (one of the two articulations between the rib and the vertebra has disappeared), but the first cervical ribs of Varanus may still be dichocephalous. On the following vertebrae, and generally in the cervical and anterior trunk of lizards, the two articulations fuse to form a single synapophysis, but the latter remains oblong, keeping a vestige of dichocephaly. In the posterior part of the trunk, the single articular facet becomes hemispherical and the ribs clearly holocephalous. The vertebral column is divided into cervical, trunk, sacral/cloacal, and caudal regions (Hoffstetter and Gasc 1969).

The amphisbaenians are highly specialized lizards that have a completely different morphology and effect penetration of the substratum by movements of their head. Compared to other lizards, the amphisbaenians skull (Fig. 6) has the following peculiarities: absence of palatal and suborbital fenestrae; lateral parietal downgrowths enclosing the braincase; fusion of bones in the occipital segment and mandible; presence of secondary palate; absence of major mobile articulations; presence of a tabulosphenoid and of a paired elements-X (without clear homology to any other lizard bone), among others. Most of the bones overlap to avoid slippage when the large pressures associated with excavation are applied to the head. Their skull can be divided into five morpho-functional units: an occipital segment, intermediate segment, snout, palatal series, and mandibulae (Gans and Montero 2008).
Fig. 6

Scheme of the dorsal/ventral views of an amphisbaenian skull (based on Amphisbaena fuliginosa). Pm = premaxilla, Na = nasal, Mx = maxilla, Fr = frontal, Pa = parietal, Pf = prefrontal, Pl = palatine, Qd = quadrate, Oc = occipital complex, De = dentary, An = angular, Cb = compound bone, E-x = element-X, Pbs = parabasisphenoid

In amphisbaenians, the number or precloacal vertebrae varies from 64 to 145. Each vertebra is generally depressed with transversely widened condyle, and flat ventral face of the centrum with parallel lateral borders. Neural spines are lacking in the trunk region, and prezygapophysial processes are more or less clearly present. The division of the column into regions is difficult. There is generally a vestigial pectoral girdle with no connections with the ribs, and a pelvic girdle, also without connection with vertebral column – thus, there is no differentiated sacrum. A series of vertebrae that bear forked ribs may be identified as cloacals. The caudal region is always short, comprising fewer than 30 vertebrae (Hoffstetter and Gasc 1969). Among amphisbaenians, only one family/genus presents limbs (Bipedidae, Bipes), and it is restricted to the forelimbs.

In snakes’ skull, the frontals and parietal have expanded ventrally around the sides of the skull, forming the major part of the braincase wall. Its enlargement resulted in the loss of many other dermic bones. Skull characters are so important to snakes that the taxon is diagnosticated based in some features, as the absence of the lacrimal, squamosal, and epipterygoid bones (Estes et al. 1988) and the absence of bony mandibular symphysis and contact between frontal and maxilla bones (Conrad 2008) (Fig. 7).
Fig. 7

Scheme of the dorsal/ventral views of a snake skull (based on Farancia abacura). Na = nasal, Fr = frontal, Pa = parietal, Pf = prefrontal, Po = postorbital, Ep = ectopterygoid, So = supraoccipital, Ex = exoccipital, St = supratemporal, Qd = quadrate, Pm = premaxilla, Vo = vomer, Pl = palatine, Mx = maxilla, Pt = pterygoid

“Scolecophidians” snakes present several exclusive skull features, especially in braincase and snout. The snout forms a bulbous terminus to the end of the skull and is attached to the braincase by peripheral sutures rather than by a central strut (Cundall and Irish 2008). The skull of such species has been interpreted as an adaptation to its burrowing and feeding habits (scolecophidians feed primarily on small arthropods). Such snakes have a shortened lower jaw and suspensorial elements that are angled anteroventrally, rather than vertically or posteroventrally, palatines and pterygoid without teeth, supratemporal lost, mandible shorter than combine length of braincase and snout, among other features (Fig. 8) (Cundall and Irish 2008). Alethinophidians, unlike scolecophidians, present medial pillars on the frontal bones, a laterosphenoid bone in the middle of the trigeminal foramen, and a toothed anterior process on the palatine. None of these features is uniformly present among the group though. All the species present a lower jaw as long as or longer than the skull and a snout both prokinetic and rhinokinetic. The most conspicuous consequence of such differences among Scolecophidians and Alethinophidians is the enlargement of the gape size in the latter (Cundall and Irish 2008). According to the dentition, such snakes are divided into four categories: aglyphous – does not possess specialized teeth to venom injection; opisthoglyphous – a pair of grooved teeth (fangs) located in the bottom of the maxilla; proteroglyphous – a pair of grooved teeth (fangs), located in the anterior portion of the maxilla; and solenoglyphous – a pair of large and movable teeth (fangs), with a canal by which the venom is inoculated, located in the anterior portion of maxilla.
Fig. 8

Skull and lower jaw of Trilepida salgueiroi in lateral view (Scolecophidia). Ver Vertebrae, Pro Prootics, Par Parietal, Fro Frontal, Pfr prefrontal, Pma Premaxillae, Ma Maxillae, Lj Lower Jaw, Tr Trachea

The absence of pectoral girdle hampers the definition of the boundary between the cervical and trunk regions. As in amphisbaenians, the site of the cloaca and associated organs is reflected in the vertebral morphology. The vertebral number is always high, varying from 160 to more than 400 (precloacal ranging from 120 to over 320). There are no fundamental differences between the vertebral morphology of snakes and the other squamates, but there are a certain number of typically ophidian characters. The centrum bears a large anterior cotyle (glenoid cavity) faced ventrally, and a dorsally turned posterior condyle, which widely overlaps the transverse section of the centrum. The ventral surface of the centrum is limited on both sides by a more or less clear crest (which are completely lacking in some families). The hypapophyses arise from the posterior part of the centrum in the midventral line. In the middle and posterior parts of the trunk, the hypapophyses may be reduced to a simple haemal keel, disappearing completely in some burrowing families. In the caudal region, they are replaced, when present, by paired haemapophyses which fuse to the centrum. The neural arch consists of a roof and walls. The neural crest is often trilobate in section. The ribs are completely ossified and generally robust with a double articular facet – each part articulated with one of the two areas of the synapophyses (Hoffstetter and Gasc 1969).


Squamates, as well as other amniotes, exhibit two major sets of muscles: the cranial (jaw and pharyngeal musculature and extrinsic eye muscles) and postcranial muscles (appendicular and axial) (Kardong 2014). The loss of the temporal arches in the skull, as well as the great differences in the degree of cranial kineticism, is directly reflected in the wide variation of the squamate cranial muscles. Differences in the position and configuration of the upper temporal arch and the relative width of the dorsal temporal fossa might have caused radical changes in the arrangement of the external and internal adductor muscles. Several fossorial squamates exhibit vestigial or absent extrinsic eye muscles that are possibly directly associated to the reduced eyes (Haas 1973). The muscles of the head of snakes differ strikingly and in many ways from those of lizards and are somehow closely and similar to amphisbaenians. However, the latter has developed an extremely strong biting apparatus, while in the microphagous snakes, cranial kinesis and horizontal movements of the jaws are more important. The process to swallow preys in ophidians is aided by the complex constrictor internus dorsalis group of muscles, which effect the protraction and retraction of the bony palate, and the complicated and diversified concomitant movement of the maxillae and bones of the snout (Haas 1973).

In Squamate (as well as in other amniotes), the horizontal septum that splits the epaxial and hipaxial muscles is lost or indistinct, although the supply by the dorsal and ventral rami of the spinal nerve still aids on the identification of such muscles. In lizards (except legless ones), even though lateral undulations of the vertebral column contribute to locomotion, limbs become more important in providing propulsive forces. Therefore, the epaxial muscles associated to vertebral column are reduced, and the musculature associated to the appendices are much more conspicuous. In snakes, amphisbaenians, and limbless lizards, the axial muscles are very important on providing propulsive forces and, therefore, are prominently developed. The hypaxial muscles form much of the body wall and are associated to breathing as it is attached to the rib cage. Most epaxial and hipaxial muscles in squamates split into several layers forming many differentiate muscles that span several segments (Kardong 2014).

Visceral Morphology

Lungs represent the main respiratory surface of squamates, although some degree of cutaneous respiration might be found in several species (Zug et al. 2001; Kardong 2014). The lungs of snakes and most lizards typically include a single central air chamber into which faveoli open (Kardong 2014). Thoracic aspiration is used to ventilate the lungs, and, in lizards, intercostal muscles between the ribs contract and force the ribs forward and outward (Zug et al. 2001). In several snakes, the faveoli may be reduced in the posterior part of the lung, leaving it as a nonexchange region, traditionally named as air sacs or saccular lungs. In monitor lizards, the single central air chamber is subdivided into numerous internal chambers that receive air from the trachea.

The elongated body plan leads to several rearrangements in the organ topography in snakes, amphisbaenids, and elongated-legless lizards. Among the snake organs, the respiratory system exhibits the highest level of specialization of squamates. In primitive snakes, the lungs are paired, but in many advanced snakes the left lung is reduced and often entirely lost (Kardong 2014). A few snakes (scolecophidians and some advanced snakes) exhibit a tracheal lung, which is characterized by the presence of a vascular portion of the lung located anterior to the heart (Wallach 1998).

Squamates show a typical double circulation, which includes a three chambered heart, with the retention of the aortic arches III, IV, and VI. The ventral aorta is subdivided forming the left and right aortic arches and the pulmonary trunk. Such configuration in the aortic arches provides one pulmonary circuit and two systemic circuits, each of which arising independently from the heart. Although the ventricle is given as a single and undivided cavity, three interconnected compartments are present and separated from each other by a muscular ridge. Thus, a few authors might assign the squamate heart as five-chambered, composed of two atria and three compartments of the ventricle, or six chambers if the sinus venosus is counted (Kardong 2014).

Two types of tooth implantation are present in squamates: acrodonty (a few lizards) and pleurodonty (lizards, snakes and amphisbaenids). Additional features of the squamates digestive system include a buccal cavity lacking a secondary palate and several oral glands that support food digestion. In both snakes and lizards, such head glands have evolved independently several times to venom glands that support not only food ingestion, but prey capture. The alimentary canal is very diverse in size and regionalization, mostly depending on type of food ingested. However, its general pattern consists on a short esophagus, with gradual transition to stomach, which is followed by the small and large intestines that finally open in the cloaca. The cloaca is partially differentiated into the coprodeum, a chamber into which the large intestine empties, and the urodeum, into which the urogenital system empties. The alimentary tract of herbivores, particularly the large intestine, is consistently longer and more voluminous than that of similar sized carnivores (O’Grady et al. 2005). In some herbivorous lizards, a cecum is present between the small and large intestines (Kardong 2014). The body elongation on snakes resulted on an elongate digestive system, with inconspicuous folds in the small intestine.

Squamates exhibit a series of oral glands: supralabial and infralabial glands are present along the upper and lower lips; lingual and sublingual glands; premaxillary and nasal glands, in association with the snout; and palatine gland, along the roof of the mouth (Fig. 9). These glands release mucus to lubrificate the prey during intraoral and esophageal transport. The lacrimal and Harderian glands release secretions that bathe the eye and vomeronasal organ. Duvernoy’s gland, situated along the posterior upper lip, is found in many nonvenomous snakes and releases its serous secretion via a duct adjacent to the posterior maxillary teeth. In venomous snakes, the venom gland, homologue of Duvernoy’s gland, secretes a cocktail of different chemicals with various functions – some toxic, some digestive (Kardong 2014). While venom and Duvernoy’s glands are always located in the upper jaw, transferring the venom through maxillary teeth, lizards venom glands are usually located in the lower jaw and transferred through both maxillary and mandibular teeth (Fry et al. 2006).
Fig. 9

Illustration of the head glands of Atractus potschi in lateral view. Na = Nasal glands, Ha = Harderian gland, Su = Supralabial gland, In = Infralabial gland, lao = Musculus levator anguli oris, aes = Musculus adductor mandibulae externus superficialis, aem = Musculus adductor mandibulae externus medialis

All squamates bear metanefric kidneys, with primary uricotelism as their nitrogen waste. However, nephron structure can be quite different from one taxonomic group to the next and may appear at first to have no obvious correlation with the phylogenetic position, but most likely to their environmental demand. The kidney typically lacks the distinct color and functional distinction of the mammalian cortex and medulla. The kidneys are paired, lobular (weakly so in some lizards), elliptical, pink or red structures that are located retroperitoneally (extracoelomically). Kidneys are posterior to the level of the ilial crest in most lizards and usually lack distinct lobes. Snake kidneys are distinctly lobed and elongated, found in the posterior third of the bodies, with the right kidney occurring anterior (approximately 69–77% of snout-vent length [SVL]) to the left kidney (~74–82% of SVL). Snakes lack urinary bladders; nitrogenous wastes are refluxed from the cloaca into the rectum, where uric acid is stored and further ion reclamation may occur. Several species of lizard lack urinary bladders or develop just a vestigial bladder, including some varanids, agamids and a few gekkonids (Wyneken 2013).

The gonads (ovaries and testes) are located dorsally in the body cavity, posterior to the lungs (except in elongated individuals, where lungs might overcome the right ovarium), and ventral to the kidneys and peritoneal wall. The female reproductive tract is composed of paired ovaries, and oviducts (that might be missing in a few snake species) supported by mesenteries. In lizards, at least the caudal part of each ovary is attached to the peritoneum along the ventromedial surface of each kidney. In some lizards with highly modified lungs, such as chameleons, the ovary may extend cranial between the two lungs. In snakes the ovaries tend to be well posterior to the lung(s) and saccular lung and anterior to the kidneys, attached to the dorsal body wall by the mesovarium, while the testes can be round or fusiform in shape (Mader and Wyneken 2002).


Unlike other reptiles, Squamata have a pair of hemipenis, an intromittent copulatory organ that originates at the junction of the cloacal vent and the base of the tail. Although squamates exhibit two hemipenis, during copulation, only one of them is inserted in the cloaca of the female (Kardong 2014). Each hemipenis is usually grooved (sulcus spermaticus) to allow sperm transport. A retractor muscle returns each hemipenis to the body, a process called invagination, storing the organ at a pocket located at the base of the tail, posterior to the vent. During erection, muscle action and hemotumescence force each hemipenis through the cloaca, expanding it out through the vent, as a process named evagination (Kardong 2014).

The hemipenial morphology is widely used in squamates taxonomy and systematics. Hemipenis may be uni- or bilobated and can bear many different ornaments, such as large spines, spicules, body calyces, lobular crest or ridge, calycular pockets, capitular groove, and capitular calyces (Zaher 1999) (Fig. 10).
Fig. 10

Assulcate view of the hemipenis of (a) Atractus paraguayensis, (b) Hydrodynastes bicinctus, and (c) Elapomorphus quinquelineatus. (Scale = 5 mm)



  1. Conrad, J. L. (2008). Phylogeny and systematic of Squamata (Reptilia) based on morphology. Bulletin of the American Museum of Natural History, 310, 182.CrossRefGoogle Scholar
  2. Cundall, D., & Irish, F. (2008). The snake skull. In C. Gans, A. S. Gaunt, & K. Adler (Eds.), Biology of the reptilia, Vol. 20. Morphology H. (pp. 349–692). New York: Society for the Study of Amphibians and Reptiles.Google Scholar
  3. Estes, R., De Queiroz, K., & Gauthier, J. (1988). Phylogenetic relationships within Squamata. In R. Estes & G. K. Pregill (Eds.), Phylogenetic relationships of the lizard families (pp. 119–282). Stanford: Essays Commemorating Charles L. Camp. Stanford University Press.Google Scholar
  4. Evans, S. E. (2008). The skull of Lepidosauria. In C. Gans, A. S. Gaunt, & K. Adler (Eds.), Biology of the reptilia, Vol. 20. Morphology H (pp. 1–348). New York: Society for the Study of Amphibians and Reptiles.Google Scholar
  5. Fry, B. G., Vidal, N., Normal, J., Vonk, F., Scheib, H., Ramjan, S. F., Kuruppu, S., Fung, K., Hegdes, S. B., Richardson, M., Hodgson, W., Ignjatovic, V., Summerhayes, R., & Kochva, E. (2006). Early evolution of the venom system in lilzards and snakes. Nature Letters, 439, 584–588.CrossRefGoogle Scholar
  6. Gans, C., & Montero, R. (2008). An atlas of amphisbaenian skull anatomy. In C. Gans, A. S. Gaunt, & K. Adler (Eds.), Biology of the reptilia, Vol. 21. Morphology I. New York: Society for the Study of Amphibians and Reptiles.Google Scholar
  7. Haas, G. (1973). Muscles of the jaws and associated structures in the Rhyncocephalia and Squamata. In C. Gans & T. S. Parsons (Eds.), Biology of the reptilia (Vol. 4, pp. 285–490). New York: Society for the Study of Amphibians and Reptiles.Google Scholar
  8. Hoffstetter, R., & Gasc, J. P. (1969). Vertebrae and ribs of modern reptiles. In C. Gans, A. d’A. Bellairs, & T. S. Parsons (Eds.), Biology of the reptilia, Vol. 1. Morphology A. London/New York: Academic.Google Scholar
  9. Kardong, K. V. (2014). Vertebrates: Comparative anatomy, function, evolution (7th ed.). New York: McGraw-Hill.Google Scholar
  10. Kearney, M. (2003). Systematics of the amphisbaena (Lepidosauria: Squamata) based on morphological evidence from recent and fossil forms. Herpetological Monographs, 17(1), 1–74.CrossRefGoogle Scholar
  11. Klein, M. C., & Gorb, S. (2014). Ultrastructure and wear patterns of the ventral epidermis of four snakes (Squamata, Serpentes). Zoology, 117(5), 295–314.CrossRefGoogle Scholar
  12. Lillywhite, H. B. (2014). How snakes work: Structure, function and behavior of the world’s snakes. Oxford: Oxford University Press.Google Scholar
  13. Mader, D. R., & Wyneken. (2002). The anatomy and clinical application of the renal portal system and the ventral abdominal vein. Proceedings of the Association of Reptilian and Amphibian Veterinarians, 2002, 183–186.Google Scholar
  14. McDiarmid, R. W. (2012). Reptile diversity and natural history: An overview. In R. W. McDiarmid, M. S. Foster, C. Guyer, J. W. Gibbons, & N. Chernoff (Eds.), Reptile bioderversity: Standard methods for inventory and monitoring. London: University of California Press.Google Scholar
  15. O’Grady, S., Morando, M., Avila, L., & Dearing, M. D. (2005). Correlating diet and digestive tract specialization: examples from the lizard family Liolaemidae. Zoology 108, 201–210.Google Scholar
  16. Pianka, R., & Vitt, L. (2003). Lizards: Windows to the evolution of diversity. London: University of California Press.Google Scholar
  17. Pough, F. H., Janis, C., & Heiser, J. (2018). Vertebrate life (10th ed.). London: Oxford University Press.Google Scholar
  18. Russell, A. P., & Bauer, A. M. (2008). The appendicular locomotor apparatus of sphenodon and normal-limbed Squamates. In C. Gans, A. S. Gaunt, & K. Adler (Eds.), Biology of the reptilia, Vol. 21. Morphology I. New York: Society for the Study of Amphibians and Reptiles.Google Scholar
  19. Uetz, P., & Hosek, J. (2018). The TIGR reptile database. Available at: http://www.reptile-database.org. Accessed 10 Jan 2019.
  20. Wallach, V. (1998). The lungs of snakes. In C. Gans & A. S. Grant (Eds.), Biology of Reptilia (Vol. 19, pp. 93–296). New York: Society for the Study of Amphibians and Reptiles.Google Scholar
  21. Withers, P., & O’Shea, J. (1993). Morphology and physiology of the Squamata. In C. J. Glasby, G. J. B. Ross, & P. L. Beesley (Eds.), Fauna of Australia, Vol. 2. Amphibia and reptilia (pp. 172–196). Canberra: Australian Government Publishing Service.Google Scholar
  22. Wyneken, J. (2013). Reptilian renal structure and function. Proceedings Association of Reptilian and Amphibian Veterinarians, 2013, 72–78.Google Scholar
  23. Zaher, H. (1999). Hemipenial morphology of the south American Xenodont ine snakes, with aproposal for a monophyletic Xenodontinae and a reappraisal of Colubroid hemipenes. Bulletin of the American Museum of Natural History, 240, 1–168.Google Scholar
  24. Zug, G., Vitt, L., & Caldwell, J. (2001). Herpetology: An introductory biology of amphibians and reptiles (3rd ed.). New York: Academic.Google Scholar

Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Laboratório de Anatomia Comparativa de Vertebrados, Departamento de Ciências Fisiológicas, Instituto de Ciências BiológicasUniversidade de BrasíliaBrasiliaBrazil
  2. 2.Departamento de Vertebrados, Museu NacionalUniversidade Federal do Rio de JaneiroRio de JaneiroBrazil

Section editors and affiliations

  • Alexis Garland
    • 1
  1. 1.Ruhr UniversityBochumGermany