Reproductive strategies of female Didelphis

Abstract

A two-year experimental field study was conducted simultaneously in Venezuela and Florida to test the hypothesis that in polygynous mammals offspring sex ratio is affected by maternal capacity for reproductive investment. Female common opossums (Didelphis manupialis) and Virginia opossums (D. virginiana) were used as model animals. At each study site three groups of adult virgin females (10/group/year) were trapped prior to the breeding season, weighed, radiocollared and released. Females were randomly assigned to Control, Provisioned, or Delayed Reproductive groups. Provisioned females were located at daytime rest sites and provisioned with about a 125 gm mixture of sardines and cat food at 2-day intervals from at least three weeks prior to conception until the end of the breeding season. Delayed Reproductives were those whose embryonic young were removed repeatedly from the pouch to assess the potential reproductive cost to females who conceived but lactated for only a brief time. All females were recaptured at monthly intervals for weighing and to record the sex and size of pouch young or, in the case of Delayed Reproductives, to remove any new pouch young. In both Florida and Venezuela, reproduction was highly seasonal and most (> 90%) females in the population produced two litters per year. The mean litter size (mode=7) for both sites did not differ significantly between the first and second litters, nor was there any effect of treatment on litter size. The interval between the birth of the first and second litters was of similar duration for Provisioned and Control females. Length of lactation was also similar for Provisioned and Control females, although the length of lactation for the second litter was 1-2 weeks longer in both Florida and Venezuela. There was no differential investment in male or female offspring during the teat-attachment phase, but Provisioned females exhibited greater parental investment. The weight at weaning of young from Provisioned females was significantly higher than young of Control females. Provisioned females invested disproportionately in males by producing male-biased sex ratios and by producing a higher percentage of litters that were male biased. Females, whether provisioned or not, that were in good condition tended to produce more male-biased litters. A drought in Florida in 1985 disrupted the feeding program, consequently few females received their dietary supplement and a small percentage of litters were male biased. In Venezuela, male offspring of Provisioned females were recaptured post-weaning significantly more often than males of Control females, although the comparison was not significant for the Florida data. Females who were denied the stress of lactation tended to survive longer, the difference in survivorship approached significance in Venezuela. The extended survivorship of Delayed Reproductives was not biologically significant as few lived more than two years, and mean longevity for all groups was less than two years. Opossums of both species appear to be geared to perform a maximum reproductive effort in the year following their birth.