EXTRA-PAIR PATERNITY IN WILLOW PTARMIGAN BROODS - MEASURING COSTS OF POLYGYNY TO MALES

Willow ptarmigan are one of only three monogamous grouse species in No rth America. However, in some populations between 5 and 20% of individ uals pair polygynously. It has been suggested that monogamy may be mai ntained by the high cost of polygyny to males. We have used DNA finger printing to assess the actual reproductive success of both monogamous and polygynous adults. We determined whether or not the putative paren ts were the biological parents of the chicks from 38 broods. Of these clutches 30 were from monogamous matings, and 8 were from bigamous mat ings. Of the 207 chicks from monogamous matings 96% were within-pair o ffspring, compared to 67% of the 49 chicks from bigamous matings. All extra-pair offspring chicks resulted from extra-pair fertilizations (E PFs), and there were no instances of intraspecific nest parasitism. Ma te guarding by monogamous males seems to be a highly effective method for maintaining genetic monogamy, as the only cases in which EPFs occu rred were when the resident female left the territory for a few days o r when a second female visited the territory. Our results support the notion that certainty of parentage may be one factor constraining will ow ptarmigan males to be monogamous.