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Home , Didelphis, Opossum, Virginia opossum

OF MISSISSIPPI 1:1-8 ( virginiana)

BRITTANY L. WILEMON

Department of Wildlife and Fisheries, Mississippi State University, Mississippi State, Mississippi, 39762, USA

Abstract.—Didelphis virginiana is a small more commonly known as the opossum. Found primarily in the eastern , it is a very hardy that is usually gray with a lighter shade in the north and a darker shade in the south. Known for its opposable and its ability to feign death, this primarily nocturnal mammal prefers wooded and moist areas. Didelphis virginiana is a of little concern, with populations expanding to the north and west.

Published 5 December 2008 by the Department of Wildlife and Fisheries, Mississippi State University

Virginia opossum spectrum. Weight ranges from 1.9 to 2.8 kg Didelphis virginiana (Kerr, 1792) (McManus 1974). Average life expectancy is approximately 1.5 years. The length of the CONTEXT AND CONTENT tail is relatively large compared to the body Didelphimorphia, Family Didelphidae, length. The tail is usually around 90 percent of Subfamily Didelphinae, Didelphis. Four the body length (McManus 1974). The tail is subspecies are recognized. hairless and scale like. The ears are hairless • Subspecies virginiana and are dark gray or black in coloration. The • Subspecies californica adult dental formula (Fig. 2) of the Virginia • Subspecies pigra opossum is i 5/4, c 1/1, p 3/3, m 4/4, 50 total • Subspecies yucatanensis (McManus 1974). • GENERAL CHARACTERS DISTRIBUTION The ranges in color from The Virginia opossum has been noted as one a light gray in the north to a dark gray in of the most successful mammal species in the southern part of the range. Males are history. This is due to the recent expansion relatively larger than females (McManus of the species northern and western range 1974). Body length ranges from 38 to 51 cm (Weber and O’Conner 2000). The Virginia with males occupying the larger end of the opossum is found in the south and northeastern

Fig. 2. Dorsal and lateral view of adult Didelphis Fig. 1. Virginia opossum. Photograph by Karen. virginiana. Average length of is 11 cm. Courtesy of www.fl ickr.com Photograph courtesy of Unlimited. overall appearance (McManus 1974). Sweat glands have proved to be nonfunctional and skin glands are located on the ventral side of males. Female teat numbers can range from ten to seventeen, but thirteen is most common. The Virginia opossum has a relatively small brain case, and the corpus callosum is absent in this species. Vertebral numbers remain fi xed throughout growth and maturation – 7 cervical, 13 thoracic, 6 lumbar, 2 sacral, and 26 to 29 caudal (McManus 1974). The species possesses an opposable hallux on the hind feet. This hallux does not a , unlike the other digits. Each of the opossum’s feet is covered by ridges used for friction in climbing (McManus 1974). The reproductive system of the male is composed of a scrotum and a hemipenis. The female reproductive system is composed of three , two vagina, which receive the sperm and are laterally placed, and Fig. 3. Geographic distribution of Didelphis virginiana. Photo courtesy of the Smithsonian Book a centrally placed median vagina, which serves of North American Mammals. the purpose of a birth canal (McManus 1974). regions of the United States. The species is Function.—The Virginia opossum has a widely distributed through all of Mississippi. much lower amount of expanded energy due This species is found as far south as Central to thermoregulation than any other placental America and has recently expanded as far mammal (Weber and O’Conner 2000). It north as southern (Kanda 2005). is a homeotherm that can maintain its body The only limitation is the climatic limitation of temperature at ambient temperatures that are temperature. The northern boundary of the lower than zero degrees Celsius (McManus region is limited by the low winter 1969). Many thermoregulatory measures are temperatures (Kanda et al. 2005). It is very taken by the opossum in low temperatures; common for species to experience frostbite or this species has been known to use shivering, mortality due to starvation at the northern edge vasoconstriction, piloerection, and even of their boundaries (Kanda 2005). avoidance of the low temperatures (McManus 1974). Signs of temperature regulation were FORM AND FUNCTION fi rst noted in young at the age of 55 to 60 Form.—The Virginia opossum is a relatively days old, at which the young are still living hardy and stout individual. Males, at 2.8 kg, in the of the female (McManus 1969). are slightly larger than females, which usually At 95 days of age young Virginia opossums reach a weight of about 1.9kg. Species in the were found to be able to hold a deep body north tend to have a denser underfur, which temperature constant at ambient temperatures serves a thermoregulatory function. The as low as fi ve degrees Celsius for a time period underfur tends to be white in the northern of two hours. The average heart rate of a regions and may have dark coloring on the normal Virginia opossum is approximately 200 tips. The species also has guard . beats per minute. After ten days of age, young Opossum populations in the south tend to have develop the means to fi ght infection (McManus a thinner underfur and the guard hairs tend 1974). to be darker. This gives the a darker ONTOGENY AND REPRODUCTION particular study to be approximately four miles The mating season of the Virginia opossum per year (Long and Copes 1968). The sex ratio begins in January or February and lasts is slightly preferential to males at 52 percent through June or July in southern states that compared to females at 48 percent. Of the 106 have been studied such as , , adults examined, 56 were males and 50 were and Texas (McManus 1974). The reproduction females (Llewellyn and Dale 1964). rate of the Virginia opossum was found to be approximately 5.5 (Llewellyn and Dale 1964). Space use.—The Virginia opossum uses a Reproduction yields the highest percentages wide range of habitats. These habitats can be during late January through late March and arid, dry environments, but the species usually May through early June. The prefers wet areas, near a stream or other body averages about 30 days (McManus 1974). of water. It is very diffi cult to estimate home The female is receptive for no longer than range due to the excessive movement of a 36-hour period. Receptiveness ends with opossums. One study provided a home range copulation (McManus 1967). Each of the of approximately 11.5 acres but that is only an two ovaries releases on average 11 ova. estimation. The opossum’s home ranges are Fertilization actually occurs in the fallopian more often a longer course rather than a circle tubes, and the fertilized ovary reaches the shaped course (McManus 1974). One study approximately 24 hours after fertilization. showed that one in every four dens in the range Parturition or birth usually takes place 13 days was occupied by the opossum. The fact that after copulation. Even though the female more dens were always available than were in may posses as many as 13 teats the average use proves that den factors place no limitation litter size is approximately 7 to 9 due to death on the opossum. The preferred dens were of some of the neonates. A female usually usually covered with woody cover or another produces two litters per year with young usually type of protection (Lay 1942). weighing approximately 0.15 grams at the time of birth. The young are considered to be Diet.—The Virginia opossum is omnivorous, altricial at the time of birth (McManus 1974). but the opossums’ diet is mainly composed In the southern distribution range two litter per of insects when they are available. Insects year are most common even though a very low consumed ranged from different species of percentage of females may produce three (Lay crickets to caterpillars. Opossums will also 1942). The further north the opossum lives, consume a wide range of larvae. The diet of the less likely they are to produce two litters the opossum is also comprised of vegetation, per year, but on average the same amount grasses, and fruit. Most of the fruit that is of young are produced in the southern and consumed has fallen from the tree or bush and northern regions (Kanda and Fuller 2004). is readily eaten off of the ground (Hamilton The young remain in the pouch, attached to 1953). The species has also been known the nipple up to 65 days of age. Mortality to consume amphibians, snakes, and birds may occur in the females pouch. The young on occasion. The contents of an are then weaned from the mother at 95 days. opossum population were examined and the Estrus can be resumed after the young or contents proved that the species will consume weaned (McManus 1974). small mammals such as cottontail , squirrels, and mice. It was also found that the ECOLOGY species will consume other members of the Population characteristics.—Young begin species (Taube 1947). In one particular study, to disperse and forage alone around 90 days the Virginia opossum was found to heavily of age, but they still occasionally return to the rely on earthworms during the winter season mothers den during the day (Gillete 1980). The (Dexter 1951). dispersion rate for this species was noted in a Diseases and Parasites.—Opossums can BEHAVIOR become infected with protozoa’s such as a Grouping behavior.—The Virginia opossum trypanosome. The Virginia opossum was uses three main types of locomotion, which fi rst found to be infected with this disease includes arboreal, terrestrial, and aquatic. The in Texas. Virginia opossums can also be terrestrial locomotion of the Virginia opossum infected with Leptospirosis, which is caused is characterized as -quadrupedal. by L. Pomona. The fi rst opossums to ever be Running speeds of the species have been noted as carriers of L. Pomona were found in recorded up to approximately 7.4 kilometers per Virginia and Louisiana (Barr 1963). Opossums hour. is facilitated mainly are also susceptible to yellow fever and they by the and friction ridges that are do not produce a good antibody response to found on the plantar surfaces on the bottom of the disease. Opossums have been know to the feet. The opossum also has an opposable be carriers of . Rabid opossums have hallux that aids somewhat in arboreal a very widespread distribution and have been locomotion. Arboreal locomotion is typically noted to be present in many different areas slow (McManus 1974). It has been noted of the United States. Opossums have been that the opossum will employ swimming but known to contract rabies mainly through mainly as an escape mechanism. The Virginia contact with other infected species. In one opossum has been described as a strong particular study, 34 opossums were examined swimmer and has been noted to have no fear and only 4 of them showed symptoms of the of water. All accounts state that immediately virus. The symptoms recorded were signs after entering the water the opossum would of an affected . The engage in underwater swimming. The species 4 opossums that were infected were capable engages in two different types of swimming of transmission (Barr 1963). The main (Doutt 1954). The swimming technique used ectoparasite that affects opossums is . A most often resembles terrestrial locomotion study examined 56 individuals and only 5 of the used by the animal and the other has been individuals were completely free of ticks. The described as “similar to that of a pacing horse” infected individuals were somewhat less fi t, (Doutt 1954). The toes are usually spread but there was no major impact on the infected apart during swimming, and the tail has been individuals. Fleas have also been observed in noted to move from side to side. The species is many opossums (Lay 1942). known to defecate in the water while engaging in swimming activities. The opossum fl oats Interspecifi c interactions.—The great horned with relative ease. The animal controls the owl is the major predator for Virginia opossums. closing of its nostrils with ease and has been Other predators known to the opossum are observed resting while completely submerged carnivores such as dogs and (Fitch under water. The eyes may also remain open and Shirer 1970). Many avian predators can during under water swimming. Shivering has be detrimental to young populations of young been recorded in response to swimming in opossums (McManus 1974). Opossums low temperature water. It has been stated have often been found living in dens that are that large bodies of water, especially cold characteristic of armadillos, and on certain water, might serve as a distribution barrier occasions a male opossum was found to be due to the opossum’s rapid exhaustion and sharing a den with an armadillo (Lay 1942). shivering response (Doutt 1954). It has also It has been noted in some specifi c research been noted that the opossum will dive and experiments that opossums and will travel under water up to a distance of 15 feet often compete for resources that are necessary without surfacing to breath. The opossum for survival when their habitats are being was examined upon exiting the water and reduced (Ginger et al. 2003). the pouch carrying the young was sealed so tightly that it could not be opened. This may prove previously stated knowledge to be true that a mother opossum can close her pouch noted to be nest building. On many occasions so tightly that water may not enter (Moore the opossum has been observed gathering 1955). Extensive grooming is carried out by leaves and twigs with the mouth and passing the hind foot. Many defensive techniques and them under the arched body with the help of the behaviors have been noted and studied in the forefeet. The material was then placed in the opossum. In response to threatening stimuli curled tail and immediately taken back to the the animal will hiss and growl, bare teeth, and nest box (Layne 1951). Another observation discharge a secretion from two anal glands of use of the tail for nest building described the (McManus 1974). Feigning death is one of opossum using its tail to hold its hindquarters the opossums most noted and described off the ground while placing the gathered behavioral traits. This trait allows the opossum leaves in the curled tail. This was repeated to greatly decrease mortality due to predation. many times and the opossum took the gathered Feigning death has been described as the materials into the hole being observed. The animal becoming very still or “freezing” and opossum defended the hole to a degree before somewhat curling its body and falling onto running away (Smith 1941). its side. The tail is tucked between the legs and the mouth is drawn back and the tongue Communication.—Vocal noises are comprised extends out of the mouth. The eyes have of hisses and growls, and they are usually been noted to remain open but slightly less used during aggressive encounters. Another than when not feigning death (Franco1969). sound, the “metallic click” is used in a variety Reactions to tactile stimulation are reduced of situations but mainly during mating. All during this display. This can last from a few communications between adult opossums are minutes to up to six hours (McManus 1974). aggressive except during mating (McManus Virginia opossums have been observed to be 1974). most active soon after dusk until dawn. The species is almost exclusively nocturnal. Social Behavioral responses.—Many behavioral behavior is not well developed, and with the responses to increased temperatures have exception of mating, the species is solitary. been noted in the Virginia opossum. The Most encounters between adult opossums response of saliva spreading was the fi rst to be are usually hostile and aggressive (McManus observed. The feet and the snout were usually 1974). the fi rst to be licked. The opossum has also been noted to urinate and defecate in response Reproductive behavior.—The “metallic click” to high temperatures before deep breathing is the only vocalization used by the opossum began. Many of the subjects were observed during mating. Mating occurs when the male lying on their sides while breathing rapidly straddles the female and holds the female’s (McManus 1969). hind legs with his hind feet causing the pair to fall over to the side. Insertion of the penis then GENETICS occurs and the couple can remain joined for The Virginia opossum possesses 22 up to twenty minutes. After insemination has chromosomes and 4 pair of sex chromosomes occurred the female resumes her aggressive (McManus 1974). behavior toward the male (McManus 1967). The female opossum participates in very little CONSERVATION maternal behavior. The mother will groom Conservation of the opossum is not a major the young, known as joeys, as long as they concern due to the widespread distribution remain attached to the nipple. Once they are of the species. Opossums are used as a detached, the young must groom themselves. resource in many southern areas. The fur or Females observed in captivity have never been pelts are not worth much individually, so a large seen defending their young (McManus 1974). number of opossum skins must be harvested The main use of the opossums’ tail has been (Lay 1942). LITERATURE CITED Barr, T. R. B. 1963. Infectious diseases in Lay, D.W. 1942. Ecology of the opossum in the opossum: a review. Journal of eastern Texas. Journal of Mammalogy Wildlife Management 27:53-71. 23:147-159. Dexter, R. W. 1951. Earthworms in the winter Layne, J.N. 1951. The use of the tail by an diet of the opossum and . opossum. Journal of Mammalogy Journal of Mammalogy 32:464. 32:464-465. Doutt, J. K. 1954. The swimming of the Llewellyn, L.M. and F.H. Dale. 1964. Notes opossum. Journal of Mammalogy on the ecology of the opossum in 35:581-583 Maryland. Journal of Mammalogy Fitch, H. S., and H. W. Shirer. 1970. A 45:113-122. radiotelemetric study of spatial Long, C. A., and F. A. Copes. 1968. Notes relationships in the opossum. American on the rate of dispersion of the opossum Midland Naturalist 84:170-186. in Wisconsin. American Midland Franco, E. N. 1969. Behavioral aspects of Naturalist 80:283-284. feigned death in the opossum, Didelphis McManus, J.J. 1967. Observations on sexual marsupialis. American Midland \ behavior in the opossum, Didelphis Naturalist 81:556-568. marsupialis. Journal of Mammalogy Gillete, L. N. 1980. Movement patterns of 48:486-487. radio-tagged opossums in Wisconsin. McManus, J.J. 1969. Temperature regulation in American Midland Naturalist 104 (1) 1. the opossum, Didelphis marsupialis Ginger, S.M., E.C. Hellgren, M.A. Kasparian, virginiana. Journal of Mammalogy L.P. Levesque, D.M. Engle, and D.M. 50:550-558. Leslie, Jr. 2003. Niche shift by Virginia McManus, J.J. 1974. Didelphis virginiana. opossum following reduction of a Mammalian Species 40:1-6. putative competitor, the raccoon. Journal Moore, J. C., 1955. Opossum taking refuge of Mammalogy 84(4):1279-1291. under water. Journal of Mammalogy Hamilton, W.J., Jr. 1953. The food of the 36:559-561. opossum in New York State. Journal of Smith, L. 1941. An observation on the nest- Wildlife Management 15:258-264. building behavior of the opossum. Kanda, L. L., and T. K. Fuller. 2004. Journal of Mammalogy 22:201-202 Demographic responses of Virginia Taube, C.M. 1947. Food habits of Michigan opossums to limitation at their northern opossums. Journal of Wildlife boundary. Canadian Journal of Zoology Management 11:97-103. 82: 1126-1134. Weber, J.M. and T. O’Connor. 2000. Kanda, L.L. 2005. Winter energetics of Virginia Energy metabolism of the Virginia opossums Didelphis virginiana opossum during fasting and exercise. and implications for the species’ northern The Journal of Experimental Biology distributional limit. Ecography 28:731- 203:1365-1371. 744. Kanda, L.L., T.K. Fuller, and K.D. Friedland. Contributing editor of this account was 2005. Temperature sensor evaluation of Clinton Smith. opossum winter activity. Wildlife Society Bulletin 33(4):1425-1431.