Trapping was conducted from mid March through early June and in 60 forest patches distributed throughout the study area. A detailed outline of the trapping methods are described elsewhere Beasley and Rhodes Traps were placed in a grid m spacing and maintained for 10 consecutive nights. Maternity was thus assumed unambiguous because offspring tissue samples were collected prior to pouch emersion.
Only offspring sired during the early first breeding season were sampled in this study, as offspring sired during the late second breeding season are not present within the marsupium until mid June in northern Indiana.
DNA was extracted from tissue samples using an ammonium acetate protocol modified from the purgene kit; Gentra Systems. Specific amplification conditions and annealing temperatures for each locus are provided in Fike et al.
Genotypes were manually assigned with GeneMapper v3. Based on the constraints of our quality control measures and tests for Hardy—Weinberg equilibrium HWE , the loci OP30 and OP41 were removed from all paternity analyses. Because GERUD uses exclusion to estimate the number of male genotypes contributing to a given progeny array, estimates using this program are considered conservative and should never overestimate the number of sires for a litter Jones Using known maternal genotypes, GERUD calculates the minimum number of fathers contributing to a given progeny array by subtracting the known maternal alleles from offspring genotypes, simulating all possible paternal genotypes, and determining the combinations of the remaining alleles that yield the fewest possible sires Jones For example, if a given mother—offspring array exceeded the computational capacity of GERUD using 10 loci, we systematically excluded the least polymorphic loci until a result was achieved.
Within these constraints, we performed an exhaustive search of the number of possible combinations of fathers that could explain each progeny array. To evaluate the power of our microsatellite loci to detect multiple paternity, we used the program GERUDsim 1. Using the populations observed allele frequencies, GERUDsim simulates sets of offspring genotypes based on user specified litter sizes in our case 10 , draws a sample of offspring, and then estimates the number of sires present in each litter.
We ran iterations of the simulation, each using a single multilocus maternal genotype and up to 3 randomly generated paternal multilocus genotypes based on observed levels of multiple paternity to evaluate the probability of correctly determining the number of fathers within litters. Parameter estimates for use in the simulation were based on the average number of males captured per forest patch in a concomitant study occurring in the same landscape i.
For each offspring, paternity was assigned based on the candidate father with the highest LOD score. However, as a conservative estimate of the number of fathers identifiable within our forest patches, only fathers assigned to offspring with no mismatches at all 11 loci were considered true fathers. In all instances where candidate fathers were assigned offspring mismatching at 1 locus, we regenotyped assigned fathers, mothers, and offspring to confirm genotypes.
We captured and successfully genotyped adult male opossums during and from within our 60 study patches. During , we captured and sampled 67 mother—offspring litters consisting of offspring.
Of these, we successfully genotyped 64 mothers and offspring from 34 forest patches. Mean litter size of females was 8. The 11 loci used in our analyses exhibited high levels of polymorphism Frequency of multiple paternity in litters of Virginia opossums sampled in northern Indiana, USA, The mean number of sires within litters was 1. Of the offspring evaluated for evidence of paternity, CERVUS identified 13 unique fathers for 14 litters 1 male sired 2 separate litters with no mismatches at any loci from within our sampled population of males.
For those fathers not captured in the same patch as their putative offspring, the average Euclidean distance between the original capture sites for known father—offspring pairs was However, of the 6 sires which were not captured in the same patch as their assigned offspring, 4 were initially tagged during the year prior to sampling of mother—offspring pairs but were not captured in Distribution of all father—offspring pairs identified using Cervus 3. Matching letters represent father—offspring pairs, with capital letters representing the location of sires and lowercase letters representing mother—offspring locations, for individuals sampled in northern Indiana, USA in — This figure appears in color in the online version of Journal of Heredity.
Due to the fact that we only were able to identify 14 fathers of sampled litters, we felt it was inappropriate to conduct further statistical analyses on our paternity data to explore the relationship between paternity success and male body size. The results of this study indicate that multiple paternity is common in litters of wild Virginia opossums and that both sexes employ a promiscuous mating strategy. Over the last decade multiple paternity has been reported in several species of marsupials, suggesting that female promiscuity is an important aspect of the reproductive ecology for this infraclass of mammals Taylor et al.
For many species of mammals promiscuity in females likely evolved as a means of confusing paternity in order to minimize infanticide by males Wolff and Macdonald However, in marsupials male infanticide likely is uncommon as young emerge from the marsupium as precocial and relatively mobile infants Wolff and Macdonald Nonetheless, many species of marsupials clearly exhibit female promiscuity and thus, alternative evolutionary benefits e.
For Virginia opossums it is unknown if alternative benefits explain the development and maintenance of female promiscuity; however, this mating strategy can facilitate increased litter sizes, Hoogland ; Kraaijeveld-Smit, Ward, and Temple-Smith , increased genetic diversity within litters Madsen et al.
Alternatively, female promiscuity may simply be a function of male sexual coercion, with no direct benefit to females and potential fitness or evolutionary costs to those individuals who mate with multiple males Clutton-Brock and Parker ; Head and Brooks Nonetheless, our finding that females commonly mate with multiple males suggests that sexual selection could be occurring within opossum populations at multiple biological scales: 1 direct competition among males for access to females and 2 competition among sperm from disparate males within the reproductive tract of females.
Previous research indicates that competition among male opossums is extensive with male mating success presumably tied to body mass Ryser Given the brief period of fertility 12 h within the estrous cycle of female opossums Reynolds , competition among males for access to receptive females undoubtedly is extensive and thus, sexual selection favoring large body mass would not be surprising for this species. In highly fragmented landscapes such as the UWB, competition among males likely is magnified as a result of the brief female receptivity period coupled with the patchy distribution of females.
Consequently, our estimates of the rate of multiple paternity for opossums occupying a fragmented agricultural landscape may be an underestimate when compared with populations in more contiguous habitats Banks et al. A rather interesting finding of this study was that despite our intensive sampling regime within local patches, we only were able to capture 14 fathers out of the possible 92 fathers estimated to have contributed to our 64 litters.
Although the lack of identifiable fathers could be due to an inability to capture males present within sampled patches, we believe this is unlikely due to our intensive trapping effort i.
Due to the large size of our study area and logistical constraints of trapping, a number of habitat patches were not sampled which, combined with opossum movements extending beyond the effective area of our trapping grids, likely contributed significantly to the limited number of fathers detected. High levels of male mortality prior to sampling also could have contributed to our inability to identify fathers of litters.
At the time of mating, according to one story, the forked organ of the male is inserted into the nostrils of the female, and the male cells or sperm are deposited within the nasal passages.
The female then supposedly thrusts her nose into her pouch and blows vigorously. In this way, the male cells are deposited in the pouch where it is believed, fertilization of the eggs takes place and the young pass through their complete development.
The male opossum does have a forked reproductive organ, but reproduction does not take place as outlined above. However, what actually happens afterward is almost as amazing as the story just related. You are commenting using your Twitter account. You are commenting using your Facebook account. Notify me of new comments via email. Notify me of new posts via email. Opossum in tree click to enlarge. Opossums mating click to enlarge. The right side roll click to enlarge.
Virginia Opossum click to enlarge. Share this: Twitter Facebook Email. Like this: Like Loading Leave a Reply Cancel reply Enter your comment here Fill in your details below or click an icon to log in:. Email required Address never made public.
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