ZEITSCHRIFT FÜR SÁUGETIERKUNDE Z. Sâugetierkundc 63 (1998) 169-172 © 1998 GustavFischer INTERNATIONAL JOURNAL OFMAMMALlAN BIOLOGY WISSENSCHAFTLICHEKURZMITIEILUNGEN Population dynamics of the Common opossum, Didelphismarsupialis (Mammalia, Marsupialia), in southern Brazil ByN. C. CÁCERES and E. L. A. MONTEIRO-FILHO Departamentode Zoologia, Universidade Federal do Paraná, Curitiba, Brasil Receipt of Ms. 01.04.1997 Acceptance of Ms. 10. 12.1997 Keywords: Didelphismarsupialis, population density, sexual rate, age structure Nowadays, the majority of the American marsupial species is found distributed in the Neotropical Region andinc1udedin three families (KIRSCH 1977). The common opossum, Didelphismarsupialis Linnaeus, 1758, one of these marsupials, shows a c1earneotropical distribution occurring from Mexico to northern Argentina (HERSHKOVITZ 1969). Despite itsabundancyin urban environments (HUSSON 1978), the main studies concerning the po- pulationdynamics of the common opossum in Brazil were carried outby DAVIs (1945), CERQUElRAetaI. (1993), and Cr-IEREM et al. (1996). Thus, as there is little demographic data available to understand the general ecology of this opossum, this study aims to veri- fyitssexualrate, age structure, population density, recruitment, and mortality. This study was carried out in an area of Curitiba City, State of Paraná, southern Bra- zil (25°25'Sand 49°18'W), at 940 mabovesealevel, The area has five hectares covered with mixed ombrophyllous forest, and it has many roads, houses, and buildings in the vicinity, The mean annual temperature is 16.5°C (MAACK 1981). It is possible to define two distinct seasons in Curitiba City, the dry season (April to August) and the wet season (September to March), in accordance withc1irnatic data obtained at the Meterorological Station of the Universidade Federal do Paraná, 8 kmdistantfromthe study area. Opossums were captured in 30 tivetraps (40x20x20 em) that were uniformly placed in the study area between February 1995 and January 1996. These livetraps were baited during the afternoon and observed for opossum captures the following morning once per week, The bait was ripe banana with peanut butter and codfish liver oil. After capture, animals were sexed, aged (TYNDALE-BISCOE and MACKENZIE 1976), marked by combina- tionsofholesin each ear and released. Sexes of pouch young were determined taking care not to detach them from teats, A binomial distribution test (ZAR 1984) was utilized to verify if there was a significant difference between expected (1: 1) and observed sexual rates. For abetter understanding of the age structure, the opossum permanence in the area was verified taking into account the time between the first and the latter capture day of each individual. The immigration rate was obtained by taking the proportion of the number of new individuals captured during each period of two months divided per the total of individuals in the area during the last two months, Inversely, theemmigration rate was obtained by taking theindívidualsthatwerenotrecapturedduringeachperiod of two months. The density and migration rates were based on the minimum number known to be alive in the study area. The mortality was verified through direct observa- tioninthestudy area and its surroundings during the field phases,
170 . C. CÁCERES andE. L. A. MONTEIRO-FrLHO After 12monthsof studies, 1761 baited traps resulted in 192 opossum captures. The estimated population size was 18 opossums, with 11 females and seven males (1.6 females: 1.0 male). Utilizing the binomial distribution test (ZAR 1984), the sexual rate observed for adults and subadults did not differ significantly from the expected rate (P > 0.12). The same occurred for pouch young (P > 0.10) with 25 females and 21 males (1.2 females: 1.0 male). Juveniles and subadults (all less than one year old) were captured during the end of the wet season and the beginning of the dry season, respectively. Some of them (threefe- males) remained in the area until the adult stage (seven to 12 months). Two older adult females were recaptured fora time longer than eight months. Adult males (about one year old) and females with pouch young were captured mainly during the wet season (September to January and August to January, respectively). Males remained in the area for three months at maximum. The mean population density was 1.4 opossums/ha (range: 0.8 to 2.2 individuals/ha). The immigration rate showed a positive correlation with the density (r = 0.91, P<0.05). Theemmigration rate did not contribute to density fluctuations (r = 0.00, P<0.05). Based on births, the density was high during August and November (8.2 and 6.8 pouch young/ ha, respectively), and was zero from February to July. There were two common opossum deaths in the area. One death was as aresultofhu- man contact. On the other hand, several D. albiventris Lund, 1841 were found dead on roads near the study area. Sexual rates observed here corroborate some other studies on Didelphis (STOUT and SOl El SHINE 1974; ATRAMETOWICZ 1986; CREREM et al. 1996). However, there was an untested trend fora greater number of female opossums in the population (see HOLMES and SANDERSON 1965; ATRAMETOWICZ 1986). The higher rate of females in the study area is most probably related to this trend. Studies of DAVIS (1945), ATRAMENTOWICZ (1986), SUNQUIST etal. (1987) and O'CON ELL (1989) also revealed greater female permanence inotberregions. The absence of males in the study area during the dry season is prabably related to non-oestrous females that possibly avoided them during this time (see MOITA etalo 1983). The study of RVSER (1992) on Didelphisvirginiana Kerr, 1792 gives some evi- dencetosupport this, with females showing stable home ranges and males increasing theirs during the breeding season while seeking formates. The presence of pouch young was only observed during a certain period of the year. This is in agreement with studies of FLEMING (1973), TvNDALE-BISCOE and MACKENZIE (1976), CERQUEIRA et al. (1993), and CHEREM et al. (1996) concerning the same species. Juveniles were also seasonally captured, which is in agreement with the study of FLEMING (1972) inPanamá. The common opossum probably showed this population pattem in re- sponseto advantages that their young gain if they are weaned during a season with widely available resources (see FLEMING 1973; JULIAN-LAFERRIERE and ATRAMENTOWICZ 1990). Some cited evidences of seasonality in the population succession were already reported by O'CONNELL (1989) and CERQUETRA et al. (1993) from other sites in South America. Our observations in southern Brazil were in accordance with other authors that reported a rapid tumover, every two or three years, for the common opossum in northern South America (ATRAMETOWICZ 1986; SUNQUIST et al, 1987; SUNQUIST and EISE BERG 1993). Hence, we noticed that the great majority of captured opossums originated from the preceeding breeding season as reported by O'CoNELL (1989) from Venezuela. Except for the study of AUGUST (1984) in the llanos of Venezuela, other estimated densities for the common opossum (FLEMING 1972; CRARLES-DoMINIQUE et al. 1981; ATRAMENTOWICZ 1986; SUNQUIST et al. 1987; O'CONNELL 1989; CERQUElRAeta!. 1993) were similar to the density verified here. Considering the greater difference between the high densities of pouch young and the
Population dynamics of the common opossum Didelphismarsupialis in southem Brazil 171 low abundance of independent opossums in our field area, it is probable that relatively fewindividuaIs survive until maturity, which is in agreement with data of ATRAMENTOWICZ (1986). The main factor of opossum mortality in the study area was human action. Based on our data, thecornmon opossum must be less often runover by cars than the white-eared opossum in Curitiba City. GARDNER (1983) reported that thecornmon opossum is often runover by cars in Costa Rica. On the other hand, ATRAMETOWICZ (1986) and Su QUTST andEISENBERG (1993) reported that the main mortality causes in natural environrnents outside urban areas are wild predators. Therefore, the common opossum showed a popuJationdynarnic subject to seasonal variations, exhibitinga population density strongly related to births and immigrations. Evidences of sexual demographic differences were observed, with females tending to remain in forested sites, when the species occurs in urban environrnent. This trend may re- flectanattemptto better guarantee reproductive success in these sites
