Opossum (Didelphis virginianus)
Survival and Mortality
ResultsWe used 89 opossums (49 males, 40 females) to estimate survival and mortality patterns. Causes of death were automobile collision (n = 1), predation (n = 1), hunting (n = 2), unknown factors (n = 50), and mortalities resulting directly from collar related injuries (n = 10). Annual survival did not differ between sexes (P = 0.16). Mean annual survival was 0.09 (range: 0.0 - 0.33) for males and 0.03 (range: 0.0 - 0.09) for females. Based on limited sample sizes after the breeding season, we were unable to compare survival among seasons by sex, except during the breeding season. Male survival during the breeding season across years (0.23) was lower (P <0.01) than for females (0.63).
Unknown factors accounted for most (78%) opossum deaths; however, harvest was low. Most (66%) unknown deaths occurred within 60 days of capture and radio-marking. Deaths resulting from effects of radio-marking were only confirmed during 1996, accounting for 38% of mortalities.
SummaryWhile opossums are short-lived mammals with a high reproductive capacity, the additive effects of radio-marking on survival in wild populations is unknown. Previous studies have reported low annual survival rates of opossums. Further, our telemetry data indicate that male survival is noticeably lower than that of females during the breeding period. Male breeding activities may result in reduced self-maintenance and subsequently lower survival. Although similarities in survival patterns exist between our data and others, we feel that several factors concerning radio-marking of opossums warrants discussion.
Our findings suggest that radio-marking may affect opossum survival, potentially violating assumptions inherent in survival analyses. For comparative purposes, we estimated survival of non-radio-marked opossums (n = 208) during 1996 using mark-recapture data. Probability of survival from 1 February 1996 to 1 February 1997 pooled across sexes was 0.26. Survival estimation using mark-recapture differs from estimates using telemetry data; however, this comparison does provide evidence that radio-marking may affect survival.
Most causes of death in 1993-1995 resulted from unknown factors. During this period, inadequate collar design and limited signal range hindered timely determination of cause-specific mortality. Further, the fact that most opossums died within 60 days of capture during all years of the study make it likely that capture or radio-marking techniques were causal in some instances where mortality agents were classified as unknown.
The potential exists that low annual survival reported in previous radio-telemetry studies may be due, in part, to collar-induced mortalities. Our data suggests that collar material and attachment technique are determining factors in effects of radio-collars on opossums. After no annual survival in 1994 and 1995, we attempted to improve collar design to examine if radio-marking was lowering survival. Plastic and leather collar materials were replaced with pliable nylon and collar weight was reduced > 10 g. In 1995, opossums captured, radio-marked, and subsequently recaptured with infections at collar attachment indicated that collars were possibly being placed too tightly around the neck. Thus, in 1996 we attempted to place collars just tightly enough to prevent opossums from slipping collars over their heads. This modification may have facilitated opossums pushing front limbs beneath collars to remove collars, resulting in increased mortalities.