Estimating offspring production using capture-mark-recapture and genetic methods in red squirrels
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Reproductive rate is a key demographic parameter of life history and population ecology. In traditional population-ecology studies of small mammals, this and other vital rates are inferred from capture-mark-recapture (CMR) data. However, CMR assumes that immigrants at first capture can be distinguished from unmarked locally born offspring, an assumption not always met. We verified CMR estimates of locally born red squirrel (Sciurus vulgaris) offspring as a measure of reproductive rate, with candidate offspring (CO)–candidate parent (mothers, CPs) assignment by CERVUS, using ten DNA microsatellite loci. Seventy-two of 122 candidate offspring (59%) were assigned to 52 of 125 CPs in six populations. Estimates of mean litter size were 1.5 young (range 1–3). The 50 CO (41%) not assigned to a reproducing female in the study site were considered immigrants. Parentage assignment also provided evidence of dispersal between two of our sites. Overall, CMR and CERVUS agreed in 77% of cases. Considering only the 55 juveniles determined as locally born by CMR, 50 (91%) were also assigned as local offspring with CERVUS. The main discrepancy between the two methods was that 22 subadult squirrels classified immigrants by CMR, were assigned by CERVUS to females which had reproduced in our sites. It is concluded that although in our study system agreement between CMR and CERVUS in determining local offspring was high, using genetic parentage assignment helped to correctly classify some subadults, considered immigrants by CMR, as locally born. Hence, in large-scale demographic studies, combining CMR with parentage assignment will allow more precise estimates of reproduction and dispersal.
KeywordsBody mass DNA microsatellite analysis Immigrants Parental assignment Sciurus vulgaris
We are grateful to Ambrogio Molinari, Massimiliano Zaninetti, and Giuseppe Airoldi for their help with the fieldwork. This study is part of the ASPER (Alpine Squirrel Population Ecology Research) project, funded by the province of Sondrio (Servizio Agricoltura, Caccia e Pesca, Settore Risorse Ambientali), the Gran Paradiso National Park, the Stelvio National Park and the Parco Regionale delle Orobie Valtellinesi to Istituto Oikos NGO, Milan. Additional financial support was given by MIUR (Ministero dell’Istruzione, dell’Università della Ricerca, project COFIN 2003, number 2003053710-006) to Insubria University of Varese. Trapping and handling squirrels complied with the current laws on animal research in Italy and were carried out under permission of the Region of Lombardy. This is paper #17 of the ASPER project.
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