Why Monogamy? Comparing House Wren Social Mating Systems In Two Hemispheres
The evolutionary basis for strict monogamy remains unclear. It has been suggested that monogamy might be maintained in cases where: 1) female fertility is synchronous within the population, 2) resources are evenly distributed so that male territories are of similar quality, 3) male-male competition prevents males from acquiring more than one female or 4) female-female aggression prevents additional females from settling in monogamous territories. During 2003-2007, I carried out research on the social mating system and parental behavior of a migratory population of northern house wren (Troglodytes aedon aedon) and a resident population of southern house wren (Troglodytes aedon bonariae) breeding in nest-boxes in the U.S. and Argentina, respectively. Northern house wren females bred more synchronously than southern house wrens, so synchrony does not explain the higher prevalence of polygyny in the north. The addition of nest-boxes to increase territory quality of occupied territories did not stimulate polygyny in southern house wrens. Only removal of territorial males and floaters increased the polygyny rate in the southern house wren, suggesting intense competition for breeding territories. Strong competition for breeding sites could be a consequence of higher adult survival in the south leading to lack of enough territories to breed in relation to population density. Indeed, southern house wrens bred in smaller territories than northern house wrens. Northern and southern house wrens visited a caged female close to the primary and secondary boxes in similar proportions. Both were aggressive to the caged female close to the primary nest-box, but a higher proportion of southern house wren females were aggressive when the cage was installed close to the secondary box. Female aggression in the southern house wren might be related to the smaller size of territories. Indeed, southern house wren polygynous males' territories were of similar size to northern house wren territories, but monogamous southern house wren territories were smaller than northern house wren territories. I suggest that southern house wren males are monogamous because they are unable to monopolize more than one female. This pattern seems to be dominated by two processes: male habitat saturation and female territoriality.
dissertation or thesis