Chironectes minimus
Creators: name: Astúa, D.
Publisher: name: Lynx Edicions
Year: 2015
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Mammalia
Didelphimorphia
Didelphidae
Chironectes
Chironectes minimus

description: 49.
Water Opossum
Chironectes minimus
French: Yapock / German: Schwimmbeutler / Spanish: Zariglieya acuética
Other common names: Yapok
Taxonomy. Lutra minima Zimmermann, 1780,
“Gujana.” Restricted by A. Cabrera in 1958 to “ Cayenne, French Guiana.”
A taxonomic revision using modern techniques may change the status of subspecies. Four subspecies recognized.
Subspecies and Distribution.
C.m.minimusZimmermann,1780—E&SEColombia,Venezuela(SoftheOrinocoRiver),andtheGuianastoNBrazil(EtoNWMaranhao),alsoWAmazonianBrazil,EEcuador,EPeru,andW&CBolivia.
C.m.argyrodytesDickey,1928—SMexico(SofOaxacaandTabasco)andNCentralAmerica.
C.m.panamensisGoldman,1914—SNicaraguathroughCostaRicaandPanama,toN&WColombia,NVenezuela,TrinidadI,andSintoWEcuadorandNWPeru.
C. m. paraguensis Kerr, 1792 — S & SE Brazil, E Paraguay, N Uruguay (Cerro Largo), and NE Argentina (Misiones).
Descriptive notes. Head-body 25-40 cm, tail 27-43 cm; weight 510-790 g. The Water Opossum is possibly the most distinctive species of Didelphidae, both ecologically and morphologically. Its fur pattern is unique. Dorsal fur is pale silvery gray, with four blackish transverse bars (located over shoulders and on center of back, hips, and lower rump) connected by blackish mid-dorsal stripe that runs from crown to base of tail. Shoulder and rump patches extend laterally over legs. Head fur is blackish, with dark eye-rings continuous with blackish crown fur, resulting in pale bar over each eye. Tail length is c.110% of head-body length, and tail is round and has fur on proximal onesixth or less. Naked rest of tail is blackish with a whitish tip. Ventral fur is pure bright white. Fur is short, dense, smooth, and water-repellent. Feet are reddish-brown or dark gray, with fleshy carpal tubercles in both sexes, and hindfeet are fully webbed from base to terminal phalanges. Ears are moderately large, naked, and rounded. Females have a distinct pouch that opens backward and can be tightly closed. Five mammae are present, two on each side, and a medial mamma. Water Opossums are the only species of opossums with distinct urogenital and rectal openings rather than a single cloacal opening. Male Water Opossums also have a rudimentary “pouch” anterior to the scrotum. Cremaster muscle in tunica vaginalis allows scrotum to be pulled up and kept in contact with abdominal body wall, at the rudimentary pouch area, so pouch protects scrotum while a male is swimming and diving or even moving on land. The Water Opossum has a 2n = 22, FN = 20 karyotype, with all acrocentric autosomes, and with an acrocentric X-chromosome similar in size to autosomes and a small acrocentric Y-chromosome. Skull shape is sexually dimorphic.
Habitat. Tropical and subtropical habitats, with permanent water (rivers, streams, and ponds), including moist lowland and lower montane forests, from sea level up to elevations of 1860 m. Water Opossums inhabit slow and fast-flowing waters but are normally absent from waters with high levels of sediment. In south-eastern Brazil, they occur only in fast-flowing streams, preferring wide waterways with dense forest on banks and stones on bottom. In cerrado, Water Opossumsare found near ponds or in gallery forests.
Food and Feeding. Due to its semi-aquatic habitat, the Water Opossum is also unique in its foraging habits. It swims by paddling with hindfeet only, while keeping forefeet extended forward, fingers wide open, feeling substrate for prey. Prey is often captured with hands and then brought to the bank to be eaten. They have larger digital pads and reduced claws compared with other big-bodied species of opossums. Fingertips of Water Opossums have epidermal scales surrounded by finger-like conical structures that are radially oriented, enabling perception of tactical stimuli from all directions. These structures, along with less-clawed fingertips, likely improve tactile abilities underwater. Diet of the Water Opossum includes mostly aquatic or semi-aquatic vertebrates (fish, particularly slow-moving and bottom-dwelling species such as silurids and cichlids, and frogs) and aquatic invertebrates (crustaceans and mollusks) but also crickets; occasionally, fruits and aquatic plants have been found. Water Opossums were also seen opportunistically preying on bats (Seba’s Short-tailed Bat, Carollia perspicillata, and the Little Yellow-shouldered Bat, Sturnira lilium) that were entangled in a mist-net set over a stream. Captive individuals readily consumed freshly killed mice, chicks, young rats, and crayfish, and they also fed on slices of fish or meat mixed with cod-liver oil and chopped meat. Consumption offish eggs has also been reported. Nutritional contents of preferred diets, determined with cafeteria experiments in captivity where individuals were free to choose food items according to their needs, resulted in 8 g of proteins, 7-1 g of carbohydrates, 0-8 g oflipids, and 2-:2% offibers per 100 g of dry matter.
Breeding. Nests of the Water Opossum are located in chambers at the end of tunnels that are ¢.60 cm long. Entrances to tunnels are just above water level. Like other species of opossums, the Water Opossum usesits tail to drag material for nest building. Nest entrances have a diameter of c.10 cm and can found not only on banks of waterways but also on the ground, among rocks, or among roots. The nest is lined with leaves and grass. Pre-copulatory behavior of the Water Opossum is similar to that of other species of opossums, with males circling or chasing females and engaging in oral-genital contact. Males also grab females during copulation. Reported litter sizes vary from one to five young, but only 2-4 young/litter were observed in south-eastern Brazil. Pouch young are apparently tolerant of low oxygen levels and resistant to asphyxia, enabling the mother to dive while carrying young in her pouch. This may not be unique to Water Opossums because Virginia Opossums also dive and swim underwater with pouch young. In Brazil, young are born in December—January, and females are found with pouch young in January-February. In Argentina, there are records of young born in August, and in Venezuela, females with pouch young were captured in January, July, and November. Throughout the distribution of the Water Opossum, females with pouch young are captured all year long, suggesting that they do not breed seasonally. Young detach from mammae at c.48 days but continue to nurse. As in other big-bodied species of opossums, the female sometimes carries large young on her back. Juveniles have been seen swimming with their mothers in the wild.
Activity patterns. Water Opossums are nocturnal. They become active right after sunset and concentrate their activity during the first one-half of the night; during this period, males and females have alternate preferred foraging times, with males more active during the first one-half of the activity period and females during the second one-half. Males have longer activity periods during the dry season, and females have longer activity periods during the wet season.
Movements, Home range and Social organization. Water Opossums swim in a unique way. All other opossums that have been seen swimming use all four legs to paddle, but the Water Opossum paddles with hindfeet only, in an alternate stroke. Water-repellent fur provides additional buoyancy, and they are able to swim keeping their backs almost parallel to water surface, with entire head and back above water level. Speed ranges from 0-7 km/h to 2:6 km/h. Water Opossums usually flee from danger by swimming, but some individuals have been seen leaving water and escaping into dense streamside vegetation or entering a burrow. They also reportedly splash noisily while wading or swimming. Captive individuals have been seen jumping and climbing with agility on vertical supports, sometimes to reach hollow logs above ground level that were used as dens. Home ranges are linear, and lengths in south-eastern Brazil are 844-3742 m along rivers. Densities are hard to compare with other opossums because they relate to river length and not area, but current estimates can be up to 1-3 ind/km. Home ranges of male Water Opossums are up to four times longer than those of females, and there is considerable overlap of home ranges within and between sexes. Water Opossums are solitary, although in a single situation two individuals (one male and one female) were found sharing a den. Although some captive individuals did not tolerate presence of another individual, mutual grooming has been observed in captivity after swimming, and in some cases, up to three adults shared the same enclosure without any agonistic activity.
Status and Conservation. Classified as Least Concern on The [UCN Red List. The Water Opossum has a wide distribution and presumably a large population, and it occurs in several protected areas. They are sometimes considered rare, but this is probably due to difficulty of locating and capturing them. Populations of Water Opossums are not only threatened by deforestation, as are most forest-dwelling species, but additionally by contamination and deterioration of freshwater ecosystems. Artisanal gold mining in French Guiana and other parts of the distribution of the Water Opossum might degrade watercourses, posing a serious threat.
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description: Published as part of Astúa, D., 2015, Didelphidae, pp. 129-186 in Handbook of the Mammals of the World – Volume 5 Monotremes and Marsupials, Barcelona :Lynx Edicions on pages 157-158, DOI: 10.5281/zenodo.6685333