Polygyny threshold model

The polygyny threshold model is an explanation of polygyny, the mating of one male species with multiple females. The model shows how females may gain a higher level of biological fitness by mating with a male who already has a mate. The female makes this choice despite other surrounding males because the choice male's territory, food supply, or other important characteristics are better than those of his competitors, even with two females on the territory.

Graphical Depiction

The graphical depiction of the model presented in Gordon H. Orians' 1960 paper is often used to explain the concept. The graph shows two curves on a graph of biological fitness versus environmental quality. Environmental quality refers to the quality of the male's territory. The left curve, labeled monogamous, is the perceived biological fitness for a female entering into a monogamous relationship with a given male. The right curve, labeled bigamous, shows the fitness of the same female entering into a relationship with a different male who already has a mate but who has a better environmental quality.

The intersection of the vertical dotted line on the left with the monogamous curve indicates the biological fitness of a female who chooses a monogamous male with a lower environmental quality. The intersection of the vertical dotted line on the right with the bigamous curve indicates the biological fitness of a female who enters into a bigamous relationship with the male of a higher environmental quality. The difference between these two intersection points, labeled PT, is the "polygyny threshold". It is the gain of environmental quality for the female when she chooses the bigamous relationship.

References

*Orians, G. H. (1969). "On the Evolution of Mating Systems in Birds and Mammals". The American Naturalist, 103(934), 589-603.