Polygyny Threshold Model - Costs of Polygyny

Costs of Polygyny

According to William A. Searcy and Ken Yasukawa, the term cost of polygyny is defined as the net costs of polygyny after the summation of all of the component costs and benefits. Costs include less parental care and increased competition between females for the male’s provision and food among other resources. A benefit could be group defense of the territory and resources. Searcy and Yasukawa graphically defined the distance between curve 1 (monogamous line) and curve 2 (bigamous line) of the polygyny threshold model graph (see above) to be the cost of polygyny. N. B. Davies further defined it as the cost of sharing in order to be clear that the term refers to the fitness cost to females that are breeding on the same territory.

Searcy and Yasukawa conducted studies on Pennsylvania red-winged blackbirds that showed that females would mate on territory already settled by another female, which indicated that there was no cost of polygyny. However, in a later study by Pribil and Picman conducted on Ontario populations of red-winged blackbirds the results indicated that there was indeed a cost of polygyny. The females in this study were given a choice between adjacent territories, one in which there was already a settled female (defined by many researchers as the primary female) and the other in which there was no female present. In all 16 situations, the females chose the unsettled land in which she could be monogamous. Davies, Krebs, and West, in their textbook An Introduction to Behavioural Ecology, cited another Pribil study noting the polygyny cost to the red-winged blackbird females. The effect of polygyny made them less effective mothers when they were removed and taken to a more isolated population, proven by the fact that the mothers from monogamous relationships had better adaptation to the new environment. In an earlier text, Davies explores the examples of costs, showing that the cost is not always to the second and subsequently joining females. He asserts that there are situations in which the cost is shared between the primary and secondary female. He also mentions scenarios in which the primary female receives a decrease in her fitness upon addition of the secondary female to the harem.

There are many other studies concerning the polygyny threshold model and costs to polygyny using other species. Staffan Bensch conducted a study on the great weed warbler that showed the only cost of polygyny to these females to be higher mortality of nestlings that were belonging to the primary female. Johnson, Kermott, and Lien conducted a study on the house wren (Trglodytes aedon) showing that there were inherent polygyny costs to these female populations, also. The secondary females lost more of the broods largely because of starvation, and they also experienced lesser reproductive success in other areas. One of the main factors in their decreased fitness was less male aid. Kyle Summers and David Earn studied female poison frogs, genus Dendrobates, to see if the polygyny costs drove the evolution of the parental care system from a female care to biparental or paternal care. They concluded that the costs could not be concluded to be the sole cause of this parental transition. The numerous studies concerning polygyny costs show the different factors that not only cause these costs, but are also affected by these costs.