Abstract
Using recent developments in capture–mark–recapture modelling, I analysed survival rates of adults of two species of water frogs, the parent species Rana lessonae and its sexual parasite, the hybrid Rana esculenta. Frogs were caught in four different breeding ponds between 1995 and 1998 and the effects of genotype (= species), sex, pond, and time on survival rates and recapture probabilities were tested. Survival rates were consistently higher in R. lessonae than in R. esculenta. Recapture probability was higher in males than in females. In both species, survival rates were constant during spring and summer and similar in all years of the investigation, average monthly survival rates being lower than those during autumn and winter. The variation in annual survival rates (72–84% for R. lessonae and 53–70% for R. esculenta) is probably caused by differences in winter survival rates. Capture–mark–recapture models cannot separate mortality and emigration and hence usually underestimate survival rates. To eliminate this source of error, I quantified emigration, which ranged from 0 to 29% at the four ponds. After correcting for these emigration rates, I found no differences in survival rates among the four ponds. The overall high survival rates of adult R. lessonae compared with R. esculenta partially compensate for the hybrid's initial reproductive advantage in terms of mating, fertility, and larval development and, hence, contribute to stabilising mixed populations.
Holenweg Peter, A K