Walter H. Piper á David C. Evers á Michael W. Meyer Keren B. Tischler á Joseph D. Kaplan á Robert C. Fleischer Genetic monogamy in the common loon (Gavia immer ) Received: 8 August 1996 / Accepted after revision: 22 March 1997 Abstract We conducted behavioral observations and genetic analysis on breeding pairs of common loons in the upper Great Lakes region from 1993 through 1995 to look for behavioral evidence of extrapair copulations (EPCs) and to determine parentage of young. Pairs re- mained close to each other (usually within 20 m) during the pre-laying period, leaving little opportunity for EPCs to occur. Males and females both maintained physical proximity by approaching each other when they became separated. Copulations were obvious but infrequent, occurring about once every other day during the pre- laying period. Multilocus DNA ®ngerprinting was con- sistent with behavioral ®ndings: 58 young from 47 dif- ferent families were all genetic ospring of parents that raised them. Perfect genetic monogamy (genetic par- entage of young by parents that rear them) in loons might arise as a consequence of the need for vigorous territorial defense to prevent territorial takeover. Key words Loon á Parentage á DNA ®ngerprinting monogamy Introduction The recent burst of molecular analysis of parentage has revealed that many socially monogamous animals, es- pecially perching birds (Order: Passeriformes), engage in extrapair copulations (EPCs) with other, often neigh- boring, individuals (Westneat 1990; Gibbs et al. 1990; Stutchbury et al. 1994; Dixon et al. 1994). A number of hypotheses have been oered to explain EPCs. Females might bene®t from EPCs by mating with males that are superior to their social mates in terms of genetic quality, thus acquiring superior genes for their ospring (Smith 1988; Mùller 1991). Females might seek or accept EPCs as insurance against the possibility of their mate's infer- tility (Wetton and Parkin 1991; Wagner 1992). Finally, EPCs might bene®t females through increasing genetic variability of their ospring (Westneat et al. 1990). For their part, males stand to bene®t from EPCs by pro- ducing additional ospring for which another individual cares (Trivers 1972). Modern models of mating behavior have begun to incorporate costs and bene®ts of EPCs as a fundamental aspect of breeding strategies (Westneat et al. 1990; Birkhead and Mùller 1992). The abundant evidence for extrapair paternity in passerine birds (e.g., Westneat 1990; Gibbs et al. 1990; Stutchbury et al. 1994; but see also Gyllensten et al. 1990) has created the impression that this genetic pattern is the norm for birds generally. This perception results largely from a bias towards studies of passerines, which make tractable study animals. Before it is possible to determine the importance of EPCs to the evolution of avian mating systems, however, it will be necessary to examine parentage patterns in a broader range of avian groups. Indeed, inspection of rates of extrapair fertil- ization (EPF) across avian taxa (e.g., Birkhead and Mùller 1992, pp. 228±229) and consideration of many Behav Ecol Sociobiol (1997) 41: 25±31 Ó Springer-Verlag 1997 W.H. Piper 1 (&) á R.C. Fleischer Molecular Genetics Laboratory, National Zoological Park, Smithsonian Institution, Washington, DC 20008, USA D.C. Evers University of Minnesota, Department of Fisheries and Wildlife, 200 Hodson Hall, St. Paul, MN 55108, USA D.C. Evers á J.D. Kaplan BioDiversity, Inc., 16 Lafayette St., Yarmouth, ME 04096, USA M.W. Meyer Wisconsin Department of Natural Resources, North Central District Oce, 107 Sutli Ave., Rhinelander, WI 54501, USA K.B. Tischler 813A Washington St., Kiel, WI 53042, USA Present address: 1 Department of Biology, George Mason University, Fairfax, VA 22030-4444, USA Tel.: (703) 993-1337; Fax: (703) 993-1046; e-mail:wpiper@gmu.edu