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Chironectes minimus
Physical Description
Chironectes minimus, or the water opossum, is a grayish-white color with some brown
mixed in. The underside of the chin back to the beginning of the tail on the ventral side is
a prominent white color. This is in marked contrast to the grayish-black dorsal side.
Four gray stripes streak the blackish sides, with a solid band of black following the spinal
column. From the tip of the nose back through the eyes to the crown of the head, is a
blackish-brown band. A grayish colored band extends dorsally from the eyes back to the
ears in a crescent shape. Arms and legs are white inside with grayish outside. Hands and
feet are nearly naked, brown proximally and silver-gray on digits. The tail is blackish
with a light yellow cast to the tip.
Chironectes minimus is the only marsupial that is well-adapted for semi-aquatic habitats,
having short, dense, water-repellant fur, a streamlined body, and webbed hind feet
(McLean 1993). The forefeet are unwebbed, with long, hairless, tactile fingers and
reduced claws. The head and body range from about 27cm to 40cm and when the tail is
included they vary from about 31cm to 43cm.
Both sexes have a well developed pouch. The female pouch can close tightly enough to
not allow any water to enter. This is beneficial for carrying young underwater. In
addition to the previous feature, the young can endure several minutes without oxygen
which improves survival capabilities in the mother’s pouch. In the males the pouch seal
isn’t as tight as in females. In males the pouch serves to hold the scrotum which is drawn
up when the animal swims or moves quickly on land.
Distribution
Chironectes minimus extends from southern Mexico through Central America, Columbia,
Venzuela, Guianas, Ecuador, Peru, Paraguay, eastern Brazil and northeastern Argentina.
In Paraguay it is limited to areas east of the Paraguayan River (Eisenberg 1989). It is
confined mainly to freshwater streams and lakes in tropical and subtropical habitats.
Ontogeny and Reproduction
A typical litter consists of two to three offspring but up to five have been reported. The
young are typically born in December and January. Hair first appears on the offspring on
day 22 with body pigmentation beginning around day 28. On day 34, distinct color bands
are visible and four days later the ears and eyes begin to open. By day 40-43 the eyes are
fully open. The young are too big to fit completely in the pouch at this point. By day 48,
they had begun to detach themselves from the female. They still continue to go to the
pouch to suckle and sleep beside her head or body, generally remaining very close
(Rosenthal 1975).
Precopulatory behavior includes close association between the male and the female. The
male circles or follows the female and then pulls the female towards him as he mounts
her.
Ecology and Behavior:
The water opossum dens in a cavity that is usually accessed through an entrance directly
above the water line of a stream. Excavation of a stream den showed an entrance about
10cm in diameter, just above water level, followed by a tunnel extending about 0.6m at a
45 degree angle. The end of the tunnel was a somewhat enlarged nest chamber (Marshall
1978). The water opossum was never observed defecating in their dens, although they
made no effort to remove soiled nesting material. During the day the water opossum, if
inactive, nests in dark areas of the forest in nests made of leaves or other readily available
material. They have easily accessible escape routes at these nests in case an unwanted
intruder approaches. The water opossum is known to be active during the day and at
night. It was once thought to be nocturnal but further research has shown water opossum
activity during all times of the day.
Chironectes minimus are freshwater stream predators. They prefer aquatic prey such as
crayfish, shrimp, fish, frogs and insects. In captivity water opossums were given a
horsemeat/mink chow mix and smelt and they preferred the fish (Rosenthal 1974). They
locate food in the water by searching with their forefeet, which are held in front of them
while they are swimming. Water opossums are known to be voracious feeders and adults
show agonistic behavior while feeding.
Water opossums, for the most part, are antisocial and solitary. It is thought to be a rare
animal in the wild but this fact lacks hard scientific evidence. Since the water opossum
lives in remote and hard to reach areas, it is hard to know the true population of this
animal.
Literature Cited
Eisenberg J.F., Kent H. Redford. 1989. Chironectes minimus (Zimmerman, 1780) Yapok,
Water Opossum, Cuica de Agua. Mammals of the Neotropics – The Southern
Cone. 2: 18
Gonzalez. 1998. First record of Chironectes minimus (Mammalia, Didelphidae) in
Uruguay. Communicaciones zoologicas del Museo de Natural de Montevideo.
12(192): 2.
McLean. 1993. A first record of the water opossum (Chironectes minimus) from
Guatemala. The Southwestern Naturalist. 38(4): 402.
Marshall L. 1978. Chironectes minimus. Mammalian Species. 109(109): 1.
Rosenthal. 1975. Observations on the water opossum or yapok (Chironectes minimus) in
captivity. International Zoo Yearbook. 15: 4.
Reference written by Ross Crandall, Biology 378 student. Edited by Christopher Yahnke.
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