Polygyny threshold model

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The polygyny threshold model is an evolutionary explanation of polygyny, which is the mating of one male of a species with multiple females. The model illustrates how females can achieve a higher level of biological fitness by mating with a male who already has a partner. The female makes this decision despite the presence of other males because the chosen male's genetics, territory, food supply, or other vital characteristics are superior to those of his competitors, even with two females in the territory.

Graphical depiction

The graphical depiction of the model presented in Gordon Orians' 1969 paper is often used to explain the concept. The graph illustrates two curves on a plot of biological fitness versus environmental quality. Environmental quality refers to the quality of a male's territory. The left curve, labeled " monogamous, " represents the perceived biological fitness for a female entering into a monogamous relationship with a specific male. The right curve, labeled " bigamous, " indicates the fitness of the same female entering into a relationship with another male who already has one female mate but has defended more resources. The second curve is essentially the first curve shifted to the right. The shapes of these curves will change with other intrinsic factors, such as genetic quality and male paternal investment. It is important to note that the terms "female" and "male" are often accurate; however, in some mating systems, the operational sex ratio favors females, who may then be motivated to engage in resource defense polyandry (provided the requirements of economic defendability are met).

The intersection of the vertical dotted line on the left with the monogamous curve indicates the biological fitness of a female who opts for a monogamous male despite lower environmental quality. Conversely, the intersection of the vertical dotted line on the right with the bigamous curve reflects the biological fitness of a female who enters a bigamous relationship with a male of higher environmental quality. The difference between these two intersection points, labeled PT, represents the polygyny threshold. This measure indicates the environmental quality gain for the female when she chooses the bigamous relationship, highlighting the minimum environmental quality difference required to make bigamy advantageous for her. Additionally, the vertical line drawn from the intersection of the line with the bigamy curve to the monogamy curve above illustrates the fitness advantage for a female who prefers monogamy over bigamy, attributed here to resource holding differences.

Orians predicted that animals exhibiting resource defense polygyny, such as the fish Neolamprologus pulcher, would fit this model when living in successive habitats where territory quality is highly variable. This model uses territory quality to determine whether to pursue a monogamous or polygynous mating relationship. An example is illustrated by Pribil and Searcy (2001) in the red-winged blackbird. Female red-winged blackbirds prefer to mate with males whose territories are over water and with unmated males. In their trials, females were given the choice between unmated males and previously mated males with superior territories over water. In 12 out of 14 trials (86%), females chose the already mated male with the superior territory.

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 summing all component costs and benefits. Costs include reduced parental care and increased competition among females for the male's provisions and food, along with other resources. A benefit could be the group defense of territory and resources. Searcy and Yasukawa graphically defined the distance between curve 1 (the monogamous line) and curve 2 (the bigamous line) in the polygyny threshold model graph (see above) as the cost of polygyny. N. B. Davies further clarified it as the cost of sharing to emphasize that the term refers to the fitness cost to females breeding in the same territory.

Searcy and Yasukawa conducted studies on Pennsylvania red-winged blackbirds that showed females would mate in territories already settled by another female, indicating that there was no cost to polygyny. However, a later study by Pribil and Picman on Ontario populations of red-winged blackbirds indicated that there was indeed a cost to polygyny. In this study, the females were given a choice between adjacent territories: one with an established female (defined by many researchers as the primary female) and the other without a female present. In all 16 situations, the females chose the unsettled territory where they could be monogamous. Davies, Krebs, and West, in their textbook An Introduction to Behavioural Ecology, cited another Pribil study that noted the costs of polygyny to the red-winged blackbird females. The effects of polygyny made them less effective mothers when removed and relocated to a more isolated population, evidenced by the fact that mothers from monogamous relationships adapted better to the new environment. In an earlier text, Davies explores examples of costs, showing that the burden is not always on the second and subsequently joining females. He asserts that there are situations in which the cost is shared between the primary and secondary females and also mentions scenarios where the primary female experiences a decrease in her fitness following the addition of the secondary female to the harem.

There are many studies concerning the polygyny threshold model and its costs to polygyny across various species. Staffan Bensch conducted a study on the great reed warbler, revealing that the primary cost of polygyny for these females is the higher mortality rate of nestlings belonging to the primary female. Johnson, Kermott, and Lien examined the house wren (Troglodytes aedon) and found inherent costs of polygyny for these female populations as well. Secondary females lost more of their broods largely due to starvation, and they also experienced lower reproductive success in other areas. A key factor in their decreased fitness was reduced male assistance. Kyle Summers and David Earn studied female poison frogs of the genus Dendrobates to determine whether the costs of polygyny influenced the evolution of their parental care system from female care to biparental or paternal care. They concluded that these costs could not be identified as the sole driver behind this parental transition. The numerous studies on polygyny costs highlight the various factors that not only contribute to these costs but are also influenced by them.

Polygyny threshold model in humans

Studies in human populations have shown a decline in polygyny in highly stratified societies where wealth is concentrated among a small elite class (economic inequality).

See also [ Hypergamy ]

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