Encyclopedia of Animal Cognition and Behavior

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Marsupial Morphology

  • Gabby Neves GuilhonEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-319-47829-6_1195-1

Definition

Marsupial

Group of mammals which may have a marsupium pouch to carry their young

Morphology

The biological category concerning size, shape, and general structure of organisms

Introduction

Marsupials are a group of mammals characterized by their distinct reproduction. They have a short gestation period, followed by an extensive lactation sometimes inside a pouch, also called marsupium. Despite what the group name implies, several marsupials do not have a pouch. Another interesting characteristic is that all marsupial neonates are altricial, which means that they have an incipient state of development at birth. Young are born excessively small, blind, and without fur but with the ability to climb the mother’s inguinal region until they find a teat to attach to and complete its development.

The earliest fossil known of metatherians (mammals closely related to marsupials than placentals, besides modern marsupials) is from the early Cretaceous of Asia (Kielan-Jaworoska et al. 2004). However, there are few well-preserved records of late Cretaceous metatherians with complete skull and/or skeletons; most are composed only by jaw fragments and teeth, which already show similarities with modern marsupials. After the mass extinction at the Cretaceous-Paleogene boundary, several metatherian families were extinct. Still in late Cretaceous, North American opossums from Didelphimorphia order have a rich fossil record until middle Miocene. Along the Cenozoic, marsupials migrated to southern continents and spread throughout South America, probably including Antarctica and Australasia regions. The Oceania isolation in late Eocene allowed a great diversification of marsupials, with animals of very different sizes, shapes, habits, and habitats. At first glance, it looks like marsupials are phylogenetically split due to its geographical distribution: the cohort Ameridelphia (for those that occur in the Americas) and cohort Australidelphia (for those that occur in the Australasia region). In fact, this division was first proposed by the tarsal morphology of calcaneum and astragalus bone (Szalay 1982) lumping one order found in South America with the Australian marsupials. This classification is used until today and corroborated by several morphological and molecular phylogenetic analysis.

The cohort Ameridelphia is represented by two orders, Didelphimorphia and Paucituberculata. The first one occurs in diverse habitats ranging from south Canada to Argentinian Patagonia, and the latter occurs in high altitudes of Andes Mountains, northern and western South America (Astúa 2015; Patterson 2015). The cohort Australidelphia is represented by five orders: Dasyuromorphia, Diprotodontia, Notoryctemorphia, Peramelemorphia, and Microbiotheria. Most of them occur in the Australasia region (Wilson and Mittermeier 2015), except for Microbiotheria order, that although it belongs to the Australidelphia cohort, it is present only in South America, in southern Andean forests. This latter is also considered as an Australian or Antarctic radiation survivor.

This entry will be categorized in three important topics of (I) morphological design; (II) dentition; and (III) reproductive system and biology. In each topic, the group identification could be by the cohorts or by the orders of marsupials, to facilitate the understanding of their general morphology. The information provided in these sections were mainly based on Vaughan et al. (2000), Szalay (2006), Gardner (2007), Flores (2009), Pough et al. (2012), Feldhamer et al. (2015), and Wilson and Mittermeier (2015). Additional or specific data will be presented with its respective reference.

Morphological Design/General Morphology

Skull

Marsupial skull is generally characterized by its tight and small braincase and a flared narrow long nose. When present, they have ossified auditory bullae that are formed predominantly by the alisphenoid bone instead of the tympanic, basisphenoid bones and/or petrosal, similar as most eutherian mammals. Marsupials do not present the postorbital bar (bone bar behind the orbit), while the jugal bone composes portion of jaw glenoid. The palate is very distinct, with a characteristic large vacuity in the posterior part. The angular process of the dentary bone is normally inflected medially and is not present in eutherian mammals, being a synapomorphy of marsupials.

Skeleton

Most marsupial skeletons barely resemble eutherian skeletons, and they are often compared. Notwithstanding, there are a few important differences between them.

In Australian marsupial foot, a strong unification of digits II and III only by the skin, also known as syndactyly, is present in all species from the Diprotodontia and Peramelemorphia orders. The true ecomorphological function of this phenotype is still on discussion.

Another feature frequently considered as unique within marsupials are the epipubic or marsupial bones, paired bones articulated in the pubic symphysis, in the pelvic girdle. These bones are ventrally projected to the front of the belly and were considered, for a long time, to function only as a marsupium pouch support. However, there are registers of epipubic bones in cynodonts, the nonmammalian group that originated modern mammals. They were oviparous species that did not carry their young during development. Besides that, it is known that these bones, in addition to two extra muscles, perform a cross-couplet linkage during marsupial locomotion (Reilly and White 2003). Nowadays, these two non-excluding hypotheses for epipubic bone presence are accepted.

Ameridelphia Morphology

The Ameridelphia group is composed of two orders, Didelphimorphia and Paucituberculata. They are small to medium-size opossum-like marsupials with very conserved characteristics. Considering the Didelphimorphia order (Fig. 1), they have a long snout with many vibrissae, round eyes, and big flattened ears. The coloration is variable, with individuals presenting dark gray or brown tones, and could have cream-colored belly. Some species with lighter shades may be brownish-orange and white, with details in black or gray, especially in eye masks. Body length ranges from 10 to 95 cm and can be as light as the 30 g Tlacuatzin or as heavy as the 5.9 kg Didelphis. Body shape may be associated with its locomotion habit. Terrestrial and scansorial species tend to be elongated while walking and form a lumbar curve when sitting, standing or jumping. Arboreal species are likely to walk with members flexed because of their gravity point, which needs to be closer to the substrate (Argot 2001). The contrasting animal in this group is the unique and truly semiaquatic marsupial species, Chironectes minimus. It has a hydrodynamic body with dense and water-repellent fur and webbed foot adapted to swim. There are no fossorial or gliding marsupials in this group. All Didelphimorphia has a prehensile tail, but it may be short, as in the short-tailed opossums Monodelphis, long or fat as in the fat-tailed opossum Thylamys, and especially thick, as in the thick-tailed opossum Lutreolina. In Didelphimorphia, the skull is homogeneous, with a few differences related to size and shape, preserving their general morphology. The rostrum is long and the sagittal crest is prominent. Caluromys shows a rounded palate, where a shift in orientation occurs between the upper post-canine rows and the development of a strongly curved post-palatine torus, which are not known for any marsupial so far (Flores et al. 2010). Their feet are plantigrade, and all species have an opposable and clawless thumb, being probably inherited from arboreal ancestors.
Fig. 1

White-eared opossum, Didelphis albiventris, representing the Didelphimorphia order. Photo: Fernando Perini.

The Paucituberculata order (Fig. 2) is known as small and terrestrial shrewlike opossums due to its long snout and small eyes. It has distinct lip flaps right after the vibrissae and flattened ears smaller than Didelphimorphia. Their feet are unspecialized (not fused) and do not have opposable thumbs. Tail is not prehensile and can store fat seasonally, expanding from 4–5 mm (in diameter) during summer to 9–11 mm in winter. Their size generally ranges 15–25 cm, and they could weight 25 g as in the Dusky shrew opossum (Caenolestes fuliginosus) and up to 53 g as in the Sangay shrew opossum (Caenolestes sangay). The dorsal coloration does not diverge from black, gray, and dark brownish and they could or could not present whitish belly.
Fig. 2

Shrew-opossum Lestoros inca, representing the Paucituberculata order. Note the unique lip flaps. Photo: Bruce Patterson.

Australidelphia Morphology

As mentioned earlier, the Oceania isolation during Cenozoic Era supports the significant diversification of Australian marsupials, when compared to American marsupials. In Australian marsupials, their entire body can be remarkably different, even in the same order or family. They also have huge differences in size. The smallest ones are the pygmy possums, ranging from 5 to 12 cm, and they could weigh between 10 and 50 g. The largest ones are the red kangaroos, with up to 2 m height and a body mass up to 90 kg. To comprise all these distinct designs, this section will be divided according to their orders and main differences.

Dasyuromorphia

This order comprehends two families, Dasyuridae and Myrmecobiidae. Dasyuridae (Fig. 3) are known as carnivorous marsupials, and some of them have similar skull to Didelphimorphia order. Their forefoot has five digits, and the hindfoot can have four or five digits, with a clawless thumb that could be vestigial or absent in some cursorial genera, such as the tasmanian devil, Sarcophilus. Except for the long-limbed jumping kultarr (Sminthopsis laniger) and the cursorial quoll (Dasyurus viverrinus), they all have plantigrade posture. The tail is never prehensile. Their sizes range from 10cm and 3.6g in very small possums as Planigale, with 10 cm and 3,6 g to Tasmanian devils (Sarcophilus) that can reach 131 cm and weights up to 14 kg. Myrmecobiidae is represented for only one species, the numbat. It is slender and small long-nosed anteater, with a long narrow and sticky tongue to capture termites. It has a colorful dorsum with orangish, black, and gray gradient, with well-marked lighter strips and a non-prehensile tufted tail.
Fig. 3

Tasmanian-devil, Sarcophilus harrisii, representing the carnivorous marsupials from Dasyuromorphia order. Photo: Gabby Guilhon.

Diprotodontia

The most diverse order of marsupials have several and extensive radiations. It is composed by very distinct 11 families (Wilson and Mittermeier 2015), organized in 3 suborders: Vombatiformes, Phalangeriformes, and Macropodiformes (Fig. 4).
Fig. 4

Vombatiformes as the koala (Phascolarctos cinereus) and Macropodiformes as the red-kangaroo (Osphrancter rufus), representing the Diprotodontia order. Photos: Gabby Guilhon.

Vombatiformes comprehend koalas and wombats. Koalas can range up to 82 cm length, and males can be two times heavier than females. The species significantly changes between South Australia population, which are heavier and darker than North and East Australia population. It has rounded tufted ears, large and flat nose, and eyes with vertical iris. They have opposable clawless thumbs and a rudimentary tail. The fur is mostly gray on the dorsum and white belly, with dark brownish tons on the rump in South populations. Wombats resemble a terrestrial and burrowing koala, with smaller ears and short and large nose. It could be up to 115 cm in length and 30 kg, and pelage can be gray or brown, some with yellowish tons. In contrast with the koala, they have a poorly developed cecum and cubic feces.

Phalangeriformes have five families which include arboreal primate-like forms as cuscuses and large opossums, great, lesser, and feather-tailed gliders, pygmy possums, and the exclusive nectarivorous marsupial, the honey-possum. They have medium-small size, ranging from 10 cm to 1,2 m length, and can weight up to 4.5 kg. The fur color greatly varies between the families, and they can be dorsum-striped as gliders and honey-possum, or with exceptionally different spotted patterns as several cuscuses, white with gray, black, and also brown yellowish stains on the dorsum. Some of them can present prehensile tail as in ring-tailed opossum. In the two glider forms, the gliding membrane is formed by strong connective tissue from the elbow to the ankle, comprising all the lateral side of the body.

Macropodiformes are the hopping marsupials as bettongs, potoroos, musky-rat kangaroos, wallabies, and kangaroos. They all have short forelimbs and large hindlimbs and hindfeet. The first two represents the Potoroidae family; they are small, ranging from 55 to 85 cm and up to 3 kg. It can present greyish, brownish, or yellowish dorsum fur, with lighter belly. They have medium furred ears, small eyes, and naked nose. The tail can be prehensile. Musky-rats are the smallest macropodiformes, ranging up to 45 cm and 0.5 kg. The body fur is mostly orangish brown, with a prehensile naked tail, short naked ears, and small rounded eyes. Rostrum is short. Macropodidae is the largest family in the order Diprotodontia, they can change up to 340 cm length and 90 kg in the red kangaroos. They could be terrestrial or arboreal, with a huge variation on pelage color, varying from homogeneous gray to flash yellow and brownish in tree kangaroos. Sexual dimorphism of size is notable, with males significantly heavier than adult females. Besides, it is not only size, but males can be very muscularized and boxing with other males, for sexual competition. The rostrum is long, usually, and movable big rabbit ears, and eyes size can vary. Tail is robust and provides equilibrium during rapid bipedal hopping locomotion. Their big rump is useful as low gravity center to help them maintain balance while jumping.

Microbiotheria

In this order, there is only one extant species, the monito del monte Dromiciops gliroides (Fig. 5). It is a small species with 15–25 cm in length and could weight up to 32 g. Its snout is short when compared with other opossums, and they have medium-sized ears and eyes, with a rounded well-marked mask. It has dense body fur with a blended pattern color with white venter and mixed cream, yellow, brown, and grayish dorsum. The tail is semi-prehensile, and it stores fat, in case of food scarcity and/or cold temperatures to enter in a period of hibernation (Rageot 1978; Palma and Valladares-Gómez 2015). It has a greatly enlarged tympanic bullae which includes an ectotympanic bone, an unknown feature among other marsupials (Hershkowitz 1992)
Fig. 5

Dromiciops gliroides, the only extant member of the Microbiotheria order. Photo: P. L. Meserve, ASM Image Library.

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Notoryctemorphia

This noteworthy order is represented by the unique marsupial mole (Fig. 6), presenting many congruent characteristics with eutherian moles. It is small, completely blind with vestigial eyes covered by skin, the ear does not have a pinnae, and the nose is similar to a hard shield with a slim incision. Members are robust, spread positioned and closer to the ground, like a “sand-swimmer.” The fore claws of digits III and IV are greatly expanded and spade-shaped, while in the hindfeet the first digit has a nail; the II, III, and IV are enlarged; and the fifth is vestigial. Five cervical vertebrae are fused to stiffen the neck while digging and epipubic bones are reduced. The fur is long and bright white or yellowish red due to sand stains (Aplin 2015)
Fig. 6

Marsupial mole, Notoryctes sp. representing the Notoryctemorphia order. Photo: P. Aitken, ASM Image Library.

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Peramelemorphia

This order comprises bandicoots and bilbies (Fig. 7). Both present non-prehensile tails and lack clavicles. Bandicoots have a slender rostrum with very lengthy nose. The smallest bandicoot is up to 100 g and the largest up to 5 kg. In general, they have a dorsum fur darker than the belly, and the dorsum can vary from gray to orangish brown. They present short forelimbs with the II, III, and IV digits well developed with claws and elongated hindlimbs with the fourth digit very prominent. Bilbies can range from 50 to 85 cm in length, and sexual dimorphism in size is evident; males can range twice the size of females, up to 2.5 kg. It is recognized as the Australian marsupial “rabbit” celebrated on “Easter Bilby”, due to its long nose and ears and hopping habit. It has silky and mostly gray fur on the dorsum and white belly, with a tufted tail abrupt divided anteriorly in black fur and posteriorly in white fur
Fig. 7

Long-nosed bandicoot Perameles nasuta, representing the Peramelemorphia order. Photo: J. Hall, ASM Image Library.

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Dentition

The marsupial molar teeth are tribosphenic or tritubercular, due to its three sharp and large cusps disposed in a triangular pattern. Upper molars are formed by the paracone, protocone, and metacone, which in between the first and second cusps are the paraconule and between the second and third cusps are the metaconule. Lower molars, otherwise, have a paraconid, protoconid, and metaconid as sharp cusps and also a talonid, which is the contact space with the protocone from the correspondent upper molar. Nevertheless, particularities of marsupial dentition different from eutherians are a well-developed stylar shelf present only on the upper molars in the labial side. In the lower molars, they have a twinned hypoconulid and entoconid.

Marsupials have a particular pattern of dentition, with distinct numbers of upper and lower incisors. Primary (deciduous, “milk-teeth”) incisors and canines are vestigial and never erupt, being reabsorbed when the secondary teeth (“permanent-teeth”) erupt. The first and second premolars are deciduous and caniniform, the permanent form does not develop. The third premolars have two forms; one is a deciduous (“milk teeth”) molariform premolar or also represented as dP3. This form helps the young to shift their feed pattern from liquid to solid and starts mastication activity. As the animal grows up and other molars develop, the third permanent caniniform premolar (or P3) erupts and takes place in the occlusal plane. Sometimes the dP3 is not lost immediately after the P3 eruption, so it is common to find both teeth coexisting for a while in the jaw. The four molars are unreplaced primary teeth (Cifelli and Muizon 1998). The general dental formula of the cheek teeth in marsupials is three premolars and four molars, usually represented by P3/3 and M4/4 or \( {P}_3^3 \) and \( {M}_4^4 \).

Ameridelphia

The dental formula of Didelphimorphia is I 5/4, C 1/1, P 3/3, M 4/4 = 50. They have small, delicate and unspecialized incisors, canines are robust and large, and molars do not have critical changes. Although their feed habits are highly diverse, it is not strong enough to override the pre-existing morphological differences in molar shape that occur among didelphids (Chemisquy et al. 2015).

In Paucituberculata, the dental formula is I 4/3-4, C 1/1, P 3/3, M 4/4 = 46 or 48. The distinguish feature about them is their first lower incisors that are strongly procumbent and large, while the others incisors, the canine, and the first premolar are narrow and unicuspid. Their lower incisors are used like rapiers to stab prey (Kirsch 1977), and it appears to be homologous with the second lower incisors of didelphids.

Australidelphia

Dasyuromorphia

The dental formula is I 4/3, C 1/1, P 2-3/2-3, M 4/4 = 42 or 46. They have small, pointed or blade incisors with large sharp-edge canines. Their upper molars have three sharp cusps that are adapted to an insectivorous and carnivorous diet.

Diprotodontia

The name of Diprotodontia order is due to their modified lower incisors, which are procumbent and project forwardly, similar to rodents (Greek di = two, proto = first and dont = tooth). This order comprehends herbivorous and omnivorous extant forms. Their dental formula is variable within the families.

In Vombatiformes, the dental formula of koalas (Phascolarctidae) is I 3/1, C 1/0, P 1/1, M 4/4 = 30 and in wombats (Vombatidae) is I 1/1, C 0/0, P 1/1, M 4/4 = 24; in the latter, teeth are open-rooted and grow continuously, unique in marsupials and similar to rodents.

In Phalangeriformes, the pygmy possums (Burramyidae) dental formula is I 3/1-3, C 1/0, P 2-3/2-3, M 3-4/3-4 = 30-42, with the first incisor much larger than the other two. In cuscuses and possums (Phalangeridae), dental formula is I 3/2, C 1/0, P 2-3/2-3, M 4/4 = 36-40, with a second deciduous premolar. Ring-tailed and greater gliders (Pseudocheiridae) have I 1-3/1-2, C 1/0, P 1-3/1-3, M 4/4 = 26-40, while lesser gliders (Petauridae) have I 3/1-2, C 1/0, P 3/1-3, M 4/4 = 34-40, with the three premolars single-cusped and crest-shaped crowns in molars. The tiny honey possums (Tarsipedidae) have fragile jaws that are not able to chew, and the dental formula is reduced due to its nectarivorous habit, I 2/1, C 1/0, P 1/0, M 3/3 = 22. Feather-tailed gliders (Acrobatidae) have uncertain dental formula, usually I 3/1, C 1/1?, P 3/3?, M 3/3 = 36, and can differ from feather-tailed possums.

In Macropodiformes, the dental formula in the Potoroidae is I 3/1, C 1/0, P 1/1, M 4/4 = 30, with first incisors larger than the other two and short diastema, with well-developed upper canine and a large blade-like premolar. In Macropodidae the dental formula is variable, I 3/1, C 0-1/0, P 1/1, M 4/4 = 28-30, with upper incisors and procumbent lower incisors usually well-developed. They have a long diastema and are lophodont, with high-crowned molars.

Microbiotheria

Dental formula is the same as the American order Didelphimorphia, but their lower canines are smaller than the didelphids. The four lower incisors are uncrowded and evenly spaced in line (Hershkowitz 1992).

Notoryctemorphia

Their teeth are unusually separated from each other. Dental formula is variable, I 3-4/3, C 1/1, P 2/2-3, M 4/4 = 40-44. Incisors, canines, and premolars are unspecialized, unicuspid, and little sharp, while the last upper premolars have two cusps (bicuspidate). Finally, the upper molars are tritubercular and the lower molars lack a talonid.

Peramelemorphia

They are characterized in general by their insectivorous sharp dentition and also are polyprotodont, with four or more incisors on each side of the jaw. The general dental formula is I 5/3, C 1/1, P 3/3, M 4/4 = 46-48, but Echymipera bandicoots have only four upper incisors. Peramelemorphians have small incisors and well-developed canines and molars which can be tritubercular or quadritubercular, with fewer cusps.

Reproductive System and Biology

Marsupial general anatomy is commonly compared with some eutherians, due to its size and shape similarities. The reproductive apparatus of these two groups, however, is remarkably different.

In females, the oviducts are separated by the ureters, in other words, the ureters pass in between the oviducts. This impede the oviducts to meet in the middle to form a single median uterus and vagina, one of the limiting features that possibly explains the small size of marsupial neonates (Renfree 1993). Their reproductive tract consists of two lateral vaginae only for sperm passage, and it is connected anteriorly, following the end of the uteri. During the first birth, a median vagina or pseudovagina is formed, a canal only for the neonate passage. Concerning the placenta, it is commonly stated that marsupials do not have one. In fact, all marsupials and some eutherians have a rudimentary choriovitelline placenta, which is developed from the yolk sac and considered as a primitive placenta. Notwithstanding, bandicoots, koalas, and wombats develop an intermediary chorioallantoic placenta in late gestation, formed from the union of chorionic and allantoic amniote membranes, similar with eutherians placenta.

In males, the penile glans is bifid, to fit in the two lateral female’s vagina. Unlike eutherians, peduncular testicles from marsupial males are in front of the penis. An interesting exception occurs in the marsupial mole (Notoryctemorphia) that due to its digging habit, the testes are located inside the body, between the abdominal wall and the skin, so there is no visible scrotum. In American marsupials, males do not have seminal vesicles, ampullary or preputial glands, as seem in eutherians. In Didelphis, the gonadal differentiation occurs early, with 9 days in the marsupium as altricial neonates. At this same time, the scrotum and the marsupium (in females) are visible in the young (Nogueira 2006). In Microbiotheria, the scrotum has a very short peduncle, being almost sessile.

Both sexes present an external cloaca instead of a separated reproductive and excretory apparatus, except the male of the American water-opossum (Chironectes), which lacks a cloaca and has separated anal and urogenital openings (Nogueira et al. 2004). In Microbiotheria, the cloaca is significantly different, which opens on the ventral side of the base of the tail, as occurs in monotremes (platypus, equidnas) and also in reptiles.

Pouches

The name marsupial comes from the marsupium/pouch, a structure in the abdomen to keep and protect the litter during their external development. Although it is expected that all marsupials have pouches, due to iconic kangaroos with their well-developed pouch, many of them do not have a marsupium, especially the American marsupials. This structure has several morphological changes during reproduction, such as change in color, size, and shape, and could secret fluids (Hesterman et al. 2008). This variation also occurs in pouchless species but in the female inguinal region in both Australian and American marsupials (Calaby and Taylor 1981; Oliveira et al. 1992; Guilhon et al. 2019).

In Didelphimorphia, the pouch is present only in medium-large opossums (6/18 genra) as Didelphis, the wolly-possum Caluromys, the black-shouldered opossum Caluromysiops, thick-tailed opossum Lutreolina, and the black four-eyed opossum Philander. The brown four-eyed opossum Metachirus has similar size but lacks a pouch. The water-opossum Chironectes is the only American marsupial in which both sexes have pouches and notably specialized: a sphincter is responsible for closing females’ pouches, sealing it entirely and preventing water incoming. On the other hand, among males, pouches protect their scrotum from water and cooling, keeping spermatogenesis at optimum level. No other species in Didelphimorphia and Paucituberculata orders have pouches, maintaining their litter in the inguinal region during teat-attachment phase, avoiding moving long distances with the neonates. After this first development phase, the female leaves the nestling unattended while foraging. In Microbiotheria, females of the only extant species Dromiciops gliroides possess a pouch with four nipples. A further characteristic of this order is that pouch-young males lack mammary anlagen (the first discernible group of cells destinated to form a mammary gland) during development, classified as another morphological feature shared only with Australian marsupials (Frankham and Temple-Smith 2012).

Most of Australian marsupials, however, have pouches, in variable forms. It could be well-developed as in several diprotodonts (e.g., kangaroos, wallabies, koalas, wombats, etc.), poorly developed (e.g., some dasyurids) or absent (e.g., sugar gliders, mouse opossums, honey-opossum). The extinct Tasmanian tiger Thylacinus had pouch in both sexes. In females, the pouch opening was faced posteriorly, while males had a rudimentary pouch, believed to protect the penis and scrotum during cursorial running. Posteriorly oriented pouch opening in females can have several functions, as in wombats, which females can continue to dig with pouch-young, protecting them from the dirt entering. This also occurs in the mole Notoryctemorphia, in which a fold in the abdominal wall divides incompletely the pouch into two compartments, and each one contains a nipple Grassè (1955). In males, the marsupium is sometimes inconspicuous, consisting of a slight transverse skin fold that resembles the internal conformation of the female’s pouch.

Neonates

Marsupial newborn is very altricial (Fig. 8), which is characterized by the pup being blind and hairless, with a very reduced mobility and with a high chance of not surviving without the maternal parental care. The neonate is born with the minimum anatomical development necessary to live outside the uterus, with mouth, arms, and deciduous claws to be able to climb the female’s inguinal region to find a teat and attach to it until its development is completed. This teat-attachment phase is variable among species but always exceeds gestation period. It is possible to occur an incompatibility between the number of nipples and litter size. The number of nipples varies from 2 in Notoryctemorphia and some Dasyuromorphia to more than 12 in Didelphimorphia species. The individual variation in this number also occurs within a species. This expresses a very selective phase for the newborn, in which only the first one able to find a teat will survive.
Fig. 8

Altricial neonates from Didelphis albiventris, blind, hairless and with a very reduced mobility. Photo: Diego Astúa.

Conclusions

Marsupials are a group of mammals characterized by their peculiar reproductive strategy, with a short gestation period and a long period of post-partum development and lactation, which could happen inside a marsupium pouch. They are commonly divided in two groups, American and Australian marsupials. The first represents the generalized opossum-like forms, weighing from 30 g to 5.9 kg, terrestrial, arboreal, scansorial and the only truly water-adapted opossum. Most of them are small and pouchless species with similar skull/skeleton and dental formula, usually appropriate to an omnivorous diet. Australian marsupials, however, are more diverse in size and shape, including kangaroos, koalas, sugar gliders, marsupial rats, and mole, weighing from 10 g in the honey-opossum to 90 kg as the red kangaroo. Their skull and skeleton can be significantly different from each other. Most of them present pouches, which could be well or poorly developed, with posterior or anterior openings. All female marsupials possesses two lateral vaginae for reproduction, and one medial pseudovaginal canal develops only for the neonate birth, while most male marsupials have a bifid penile gland, positioned posteriorly to their scrotum. The marsupial young are born in a very altricial state, blind, hairless and with very low mobility, depending on the maternal parental care to survive.

Cross-References

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Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Departamento de VertebradosMuseu Nacional/Universidade Federal do Rio de JaneiroRio de JaneiroBrazil

Section editors and affiliations

  • Marieke Cassia Gartner
    • 1
  1. 1.Zoo AtlantaAtlantaUSA