Didelphidae is the largest family of marsupials in the Americas and the only family in order Didelphimorphia; it represents 19 genera and 95 species. These animals have fairly diverse modes of locomotion including terrestrial, arboreal, scansorial and semi-aquatic. Although members of family Didelphidae may range from 10 g to over 2 kg, their size and physical diversity may be limited by their early developmental requirements. These animals are characterized by their long rostrum, low body, well-developed sagittal crest and polyprotodont incisors; their very generalized body plan is considered ancestral for metatherians. Didelphids can have a chromosomal diploid number of 14, 18 or 22, although 22 is most common. (Astua, 2009; Carvalho, et al., 2002; Feldhamer, et al., 2004; Gardner, 2008)
Family Didelphidae is an ancestral group, not surprisingly; these animals maintain many morphological similarities to early mammals including a long rostrum, a well-developed sagittal crest and short limbs. Didelphids range in size from 10 g in genus Monodelphis to over 2 kg in genus Didelphis. Including their tails, the smallest members may be 170 mm long, whereas the largest members may be over 1,000 mm long. Sexual dimorphism is present in many didelphid species, with males larger than females. Their pelage varies between woolly or fine and often includes guard hairs or underfur. Their fur is typically darkly colored, but may vary to a pale gray or yellow-brown. Many species of didelphids also have a dark facial mask that encircles their eyes. All didelphids have the same dental formula: 5/4, 1/1, 3/3, 4/4, with a total of 50 teeth. These animals have conical upper incisors, polyprotodont lower incisors and large canines. In general, the larger members of the family have a pouch; smaller members may have simple lateral folds on their abdomen instead. In either case, these animals may have anywhere from 4 to 27 functional mammae. Their tails are often very long and prehensile with very little hair; however, some members have short, non-prehensile, furry tails. Some species are specialized with crassated tails that store fat for periods of torpor. These animals have 5 digits on each foot, the hallux and pollex is opposable in many species. The semi-aquatic water opossum is specialized with webbed hind feet. Many of these animals also have long sensitive vibrissae. (Astua, 2009; Feldhamer, et al., 2004; Gardner, 2008; Jones, et al., 2003; Nowak, 1999; O'Connell, 2006; Volchan, et al., 2004)
Family Didelphidae has a wide geographic range throughout the Nearctic and Neotropical zones. These animals can be found throughout North, Central and South America and may also be found on the surrounding continental shelf and Caribbean islands. The northernmost species, Virginia opossums, are the only extant didelphid found north of Mexico. Since the arrival of Europeans, Virginia opossums have expanded their northern range into southwestern Ontario, Canada. The southernmost species, Patagonian opossums, are found into Santa Cruz, Argentina. (Cuaron, et al., 2012; Feldhamer, et al., 2004; Gardner, 2008; Pearson, 2008)
Didelphids can be found in a wide range of habitats. These animals may survive in dry environments, tropical forests, grasslands, mountains, temperate forests and near human settlements. In some cases, human settlement may even facilitate range expansion. Virginia opossums have expanded into southern Canada since European settlement, partially due to the increased shelter and food provided by human habitation. Likewise, other species such gray four-eyed opossums and common opossums may supplement at least part of their diet with garbage produced by humans. Although these animals may survive in a wide variety of habitats, their northward expansion is limited by cold temperatures and deep snows. These animals may be found from sea level up to 3,400 m in elevation. (Castro-Arellano, et al., 2000; Cuaron, et al., 2012; Feldhamer, et al., 2004; Harmon, et al., 2005; Kanda, 2005; O'Connell, 2006; Reid, 2009)
Members of family Didelphidae are considered polygynous. Males from studied species compete for reproductive females. In laboratory tests, male didelphids often follow, chase and circle females, while producing clicking noises. Generally, didelphids show neither courtship displays nor pair bonds. (Fernandes, et al., 2010; O'Connell, 2006; Saunders and Hinds, 1997)
Members of family Didelphidae become sexually mature between 6 and 10 months of age and produce 1 to 4 polytocous litters per year. Didelphids are primarily seasonal breeders, although some species, such as gray short-tailed opossums, breed nearly year round. Their breeding season is largely dictated by food availability. In the seasonal tropics, breeding typically coincides with the dry season, whereas in less seasonal environments the breeding season may be longer. These animals have a very short gestation period that often lasts no more than 2 weeks, after which, several altricial young are born. Their tiny newborns are about 1 cm long and weigh about 0.13 g. At birth, many of their organs and systems have not fully formed; however, these animals have very precocial forelimbs with sharp deciduous claws that allow them to climb from the cloaca to their mother’s mammae. Although these animals have 4 to 27 mammae, many species produce more offspring than they can nurse and a large number of newborns perish. Larger species, such as members of the genera Didelphis, Chironectes, Philander and Lutreolina, typically have a pouch; smaller species have lateral abdominal folds located near their mammae. Once the newborns are attached, they nurse constantly for several more weeks, during this time they complete their development. Males of some species show partial semelparity, after breeding, these males have high mortality rates and a low body condition. (Astua, 2009; Feldhamer, et al., 2004; Gardner, 2008; Lee and Cockburn, 1987; Martins, et al., 2006; Nowak, 1999; O'Connell, 2006; Wilson, 2000)
Young didelphids are cared for solely by their mother, there are no reports of male parental investment. While attached to the mammae, young are kept warm by their mother’s body heat. After the attachment period, young remain associated with their mother for several more weeks and continue nursing. During this time, it is common for offspring to ride on their mother’s fur during nighttime excursions. Typically, young leave their mother’s care during the wet season. Litters born late in the season tend to be smaller and have higher mortality rates. Likewise, older females often have fewer offspring per litter. Regardless of age, females with only one offspring may not be able to produce milk due to the low level of stimulation. (Feldhamer, et al., 2004; Nowak, 1999; O'Connell, 2006; Wilson, 2000)
In general, didelphids have a fairly short lifespan. In the wild, these animals typically live 1 to 3 years; however, their infant mortality rate is also very high. In captivity, many didelphids survive up to 8 years. (O'Connell, 2006)
Members of family Didelphidae are nocturnal and are often crepuscular as well. Generally, these animals are solitary but they may congregate while feeding, although they do not usually interact. In laboratory tests, male didelphids were extremely aggressive with other male conspecifics and fought often. Males did not show a similar level of aggression toward female conspecifics. These animals have several modes of locomotion including terrestrial, scansorial, arboreal and semi-aquatic (water opossums). Many species utilize nests by either creating them, using the nests of other species or using tree hallows. Nesting substrates include dry leaves, roots and grasses. Some species enter torpor due to low food availability and extreme temperatures. (Astua, 2009; Gardner and Dagosto, 2008; Lemelin, 1999; Nowak, 1999; O'Connell, 2006; Pires, et al., 2010; Saunders and Hinds, 1997; Wilson, 2000)
Members of family Didelphidae are not often noisy. When they do produce sound, it typically varies between four distinct types, tonal chirps, growls, clicks/hisses or screams. These animals are sensitive to high frequency sounds, but their sensitivity to general noises may be low. Their hearing is somewhat limited compared to placental mammals and has to do with their primitive inner ear morphology. The hearing range of studied species typically was most acute at 8 to 64 kHz. Regardless, didelphids communicate vocally during breeding season and to maintain contact with their young. Young didelphids may produce bird-like screams and adult females may produce clicking and lip smacking sounds. Didelphids have fairly well-developed eyesight, as is evident from their ability to capture prey and walk on narrow vines. As compared to other groups, such as primates, their visual acuity is fairly poor, although some species are able to discriminate between prey items at a distance of about 60 cm. Similar to other nocturnal species, the light gathering ability of their eyes is extremely well developed. Although it is still somewhat debated, current research suggests that these animals have dichromatic vision. When they perceive danger, these animals may produce visual displays such as opening their mouth and curling their lips. Olfaction is an extremely important mode of perception among didelphids. These animals have scent glands, which are used during breeding season to attract mates. In these situations, males scent mark more frequently than females using their head, flank and chest. Sexually immature males do not show scent marking behaviors. Likewise, olfaction is an important means of communication between mothers and their offspring. After parturition, females often produce a musky secretion that stains their fur and the fur of their offspring. This may help maintain contact if they are separated. Likewise, didelphids use their vomeronasal organ in selecting their food. Didelphids with impaired vomeronasal organs show no preference when selecting food as opposed to non-altered animals, which can be rather selective. (Aitkin, 1998; Fadem and Cole, 1985; Frost and Masterton, 1994; Gutierrez, et al., 2011; Halpern, et al., 2005; Holmes, 1992; O'Connell, 2006; Oswaldo-Cruz, et al., 1979; Volchan, et al., 2004)
Overall, didelphids are opportunistic omnivores; these animals eat a broad range of invertebrates, vertebrates and plant material. There is variation in the group regarding the degree of insectivory, carnivory and frugivory, but almost all members consume at least a small amount of each of these food items, although their diet generally changes based on food availability. Species found in urban environments may feed on garbage produced by humans and food intended for domestic pets. Many members of family Didelphidae are immune to the snake venom of family Vipiridae and may actively hunt rattlesnakes. Likewise, although didelphids are impacted by the chemical release produced by harvestmen spiders, many opossums still consume them. (Almeida-Santos, et al., 2000; Caceres, et al., 2002; Castro-Arellano, et al., 2000; Feldhamer, et al., 2004; Machado, et al., 2005; McRuer and Jones, 2009; Nowak, 1999; Perales, et al., 1994; Pires, et al., 2010; Reid, 2009)
Didelphid opossums are prey items for many predators within their various habitats including felids, canids, birds of prey, rattlesnakes and coatis. Not surprisingly, young opossums are the most vulnerable to predation. These animals may run or climb a tree to avoid a predator, when cornered, didelphids may attempt to bite their attacker. Some species may even enter a catatonic state when they are exposed to extreme stress; colloquially known as ‘playing opossum’, this response is relatively rare. (Bianchi, et al., 2011; Campos, et al., 2007; Ferreira, et al., 2013; Gustavo, et al., 1990; Motta-Junior, et al., 2004; Parera, 2002; Pires, et al., 2010; Rotenberg, et al., 2012; Sant'anna and Abe, 2007; Shripat, 2011)
Many didelphids act as important seed dispersers due to their at least partially frugivorous diet. These animals are also used as hosts for a variety of internal and external parasites including fleas, ticks, chiggers and mites, as well as cestodes, nematodes and acanthocephalan worms. (Durden and Nixon, 1990; Jimenez, et al., 2011; Medellin, 1994)
Didelphid opossums may transmit parasites and human ailments such as Chagas disease and Leishmaniases. Opossums living near human settlements also have a tendency to knock over trash cans in an effort to feed on human refuse and may consume food left for domestic pets. Large didelphids often raid poultry farms, killing chickens. Several of the more frugivorous species may steal crops from orchards and pineapple plantations, however, the crop loss and damage from didelphids is generally considered minimal. These animals may also disrupt scientific research due to their propensity to kill animals captured in mist nets. (Adler, et al., 2012; Brito, et al., 2008; Cabello, 2006; Castro-Arellano, et al., 2000; Cuaron, et al., 2012; Deane, et al., 1984; McRuer and Jones, 2009; Monge-Meza and Linares-Orozco, 2010; Reid, 2009)
Members of family Didelphidae may be used for human benefit in a variety of ways. Several species are hunted for sport or food and their meat has been used in the wild game trade. Likewise, the pelts of some species have been used for relatively low quality fur. In addition, several opossum species are used as study organisms in scientific laboratories. A few species, such as Virginia opossums and gray short-tailed opossums have been used in the pet trade. These pets are illegal in many areas of the United States but are reportedly trainable. Historically, opossum fat, meat and bones have been used to treat and prevent a variety of ailments such as stomach discomfort, inflammation, labor pains, asthma, headaches, toothaches, ear aches, sore throat, allergies, epilepsy, dermatitis, coughing and heart attacks. (Alonso-Castro, et al., 2011; Alves and Rosa, 2006; Alves and Rosa, 2007; Cuaron, et al., 2012; Feldhamer, et al., 2004; Jacobo-Salcedo, et al., 2011; Junior, et al., 2010; Keyte and Smith, 2008; McManus, 1974; McRuer and Jones, 2009; Nowak, 1999)
Members of family Didelphidae are under a variety of conservation risk levels according to the IUCN Red List of Threatened Species. At least 2 species are currently considered critically endangered, both Handley's slender mouse opossums and one-striped opossums, are endangered due to their small amount of viable habitat. At least 7 species are currently considered vulnerable including Karimis fat-tailed opossums, red three-striped opossums, Reigs opossums, junin slender opossums, Chacoan pygmy opossums, little woolly mouse opossums and dryland mouse opossums, the immediate threats to these species include habitat conversion to agriculture, logging and the encroachment of human settlements. At least 3 additional species are currently near threatened including wood sprite gracile mouse opossums, Handleys short-tailed opossums and Paraguayan fat-tailed mouse opossums. Likewise, there is one species that is recorded as newly extinct, red-bellied gracile mouse opossums have not been seen since 1962, their entire recorded habitat has been converted to agricultural land and used for human industry. The conservation status of several other species has yet to be evaluated. (Diaz and Barquez, 2008; Flores and Solari, 2011; Flores and Teta, 2011; Lew, et al., 2011a; Lew, et al., 2011b; Pacheco, et al., 2008; Perez-Hernandez, et al., 2011; Pires Costa and Patterson, 2008; Solari and Patterson, 2011; Teta and de la Sancha, 2008; Weksler and Bonvicino, 2008; Weksler, et al., 2008; de la Sancha and Teta, 2011)
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