- Email: [email protected]
1969 The polygyny threshold model
CHAPTER THIRTY ONE
1969 The polygyny threshold model The concept Many birds are monogamous but some species exhibit polygynous mating systems. Gordon Orians (1969) developed early ideas about bird mating systems into a formal algorithm to predict when a female might accept a polygynous pairing rather than a monogamous pairing. This algorithm incorporated aspects of the quality of male territory and is called the “polygyny threshold model”.
The explanation Orians (1969) suggests that mate choice may evolve if (i) acceptance of one mate precludes mating with another and (ii) rejecting a mate will likely be followed quickly with another choice of mate. Males that control access to nesting sites may vary in the quality of these sites. Thus, one aspect of female choice among birds would be the relative quality of the sites controlled by different males. However, if a male has already paired with a female then a second possible mate must take into account (i) the reduced time the male may have for care of the young and (ii) the increased demand for resources on a site with two nests. Orians argues that if two males have identical quality sites but one male is paired and one is not, then a female should always prefer the unpaired male. However, it is possible that if the nest site quality is sufficiently different, then a paired male on a high quality site may be preferred to an unpaired male on an inferior site. The difference in environmental quality required to make such a decision possible is called the polygyny threshold. The term was first used by Verner and Willson (1966). Orians (1969) expands upon the ideas of Verner and Willson and makes seven different predictions from his model, “(1) polyandry should be rare, (2) polygyny should be more common among mammals than among birds, (3) polygyny should be more prevalent among precocial than among altricial birds, (4) conditions for polygyny should be met in marshes more regularly than among terrestrial Conceptual Breakthroughs in Evolutionary Ecology ISBN: 978-0-12-816013-8 https://doi.org/10.1016/B978-0-12-816013-8.00031-4
© 2020 Elsevier Inc. All rights reserved.
73
j
74
Conceptual Breakthroughs in Evolutionary Ecology
environments, (5) polygyny should be more prevalent among species of early successional habitats, (6) polygyny should be more prevalent among species in which feeding areas are widespread but nesting sites are restricted, and (7) polygyny should evolve more readily among species in which clutch size is strongly influenced by factors other than the ability of the adults to provide food for the young”. Gathering empirical support for this theory was slow but was first provided by Pribil and Searcy (2001). They studied red-winged blackbirds in Ontario, Canada. They argued the polygyny threshold model required that there be evidence that (i) females paid a price for polygyny, (ii) female choice is influenced by male or territory quality, and (iii) if offered a superior male or territory a female would mate polygynously even though unpaired males were available. Condition (i) was demonstrated by Pribil (2000) by manipulating harem size and showing that females in harems of size 1 had higher reproductive success than those in harems of two. Conditions (ii) and (iii) were tested Pribil and Searcy (2001). In this study female red-winged blackbirds were offered nesting platforms over land with unmated males and nesting sites over water with mated males. The sites over water are consider superior because of the greatly reduced mortality of young in these sites. Pribil and Searcy (2001) observed a strong preference for the superior quality sites despite the presence of a mated male. Pribil (2000) and Pribil and Searcy (2001) provide strong support for the polygyny threshold model, at least in this population of red-winged blackbirds.
Impact: 7 The polygyny threshold model offered a simple evolutionary basis for polygynous mating systems and for the variation seen in mating status of males. Despite the theoretical appeal of this model it is worth noting the 30-year interval between the original proposal and the design and execution of a critical experiment.
References Orians, G., 1969. On the evolution of mating systems in birds and mammals. Am. Nat. 103, 589e603. Pribil, S., 2000. Experimental evidence for the cost of polygyny in the red-winged blackbird Agelaius phoeniceus. Beyond Behav. 137, 1153e1173. Pribil, S., Searcy, W.A., 2001. Experimental confirmation of the polygyny threshold model for red-winged blackbirds. Proc. R. Soc. Lond. B 268, 1643e1646. Verner, J., Willson, M., 1966. The influence of habitats on mating systems of North American passerine birds. Ecology 47, 143e147.
1969 The polygyny threshold model The concept Many birds are monogamous but some species exhibit polygynous mating systems. Gordon Orians (1969) developed early ideas about bird mating systems into a formal algorithm to predict when a female might accept a polygynous pairing rather than a monogamous pairing. This algorithm incorporated aspects of the quality of male territory and is called the “polygyny threshold model”.
The explanation Orians (1969) suggests that mate choice may evolve if (i) acceptance of one mate precludes mating with another and (ii) rejecting a mate will likely be followed quickly with another choice of mate. Males that control access to nesting sites may vary in the quality of these sites. Thus, one aspect of female choice among birds would be the relative quality of the sites controlled by different males. However, if a male has already paired with a female then a second possible mate must take into account (i) the reduced time the male may have for care of the young and (ii) the increased demand for resources on a site with two nests. Orians argues that if two males have identical quality sites but one male is paired and one is not, then a female should always prefer the unpaired male. However, it is possible that if the nest site quality is sufficiently different, then a paired male on a high quality site may be preferred to an unpaired male on an inferior site. The difference in environmental quality required to make such a decision possible is called the polygyny threshold. The term was first used by Verner and Willson (1966). Orians (1969) expands upon the ideas of Verner and Willson and makes seven different predictions from his model, “(1) polyandry should be rare, (2) polygyny should be more common among mammals than among birds, (3) polygyny should be more prevalent among precocial than among altricial birds, (4) conditions for polygyny should be met in marshes more regularly than among terrestrial Conceptual Breakthroughs in Evolutionary Ecology ISBN: 978-0-12-816013-8 https://doi.org/10.1016/B978-0-12-816013-8.00031-4
© 2020 Elsevier Inc. All rights reserved.
73
j
74
Conceptual Breakthroughs in Evolutionary Ecology
environments, (5) polygyny should be more prevalent among species of early successional habitats, (6) polygyny should be more prevalent among species in which feeding areas are widespread but nesting sites are restricted, and (7) polygyny should evolve more readily among species in which clutch size is strongly influenced by factors other than the ability of the adults to provide food for the young”. Gathering empirical support for this theory was slow but was first provided by Pribil and Searcy (2001). They studied red-winged blackbirds in Ontario, Canada. They argued the polygyny threshold model required that there be evidence that (i) females paid a price for polygyny, (ii) female choice is influenced by male or territory quality, and (iii) if offered a superior male or territory a female would mate polygynously even though unpaired males were available. Condition (i) was demonstrated by Pribil (2000) by manipulating harem size and showing that females in harems of size 1 had higher reproductive success than those in harems of two. Conditions (ii) and (iii) were tested Pribil and Searcy (2001). In this study female red-winged blackbirds were offered nesting platforms over land with unmated males and nesting sites over water with mated males. The sites over water are consider superior because of the greatly reduced mortality of young in these sites. Pribil and Searcy (2001) observed a strong preference for the superior quality sites despite the presence of a mated male. Pribil (2000) and Pribil and Searcy (2001) provide strong support for the polygyny threshold model, at least in this population of red-winged blackbirds.
Impact: 7 The polygyny threshold model offered a simple evolutionary basis for polygynous mating systems and for the variation seen in mating status of males. Despite the theoretical appeal of this model it is worth noting the 30-year interval between the original proposal and the design and execution of a critical experiment.
References Orians, G., 1969. On the evolution of mating systems in birds and mammals. Am. Nat. 103, 589e603. Pribil, S., 2000. Experimental evidence for the cost of polygyny in the red-winged blackbird Agelaius phoeniceus. Beyond Behav. 137, 1153e1173. Pribil, S., Searcy, W.A., 2001. Experimental confirmation of the polygyny threshold model for red-winged blackbirds. Proc. R. Soc. Lond. B 268, 1643e1646. Verner, J., Willson, M., 1966. The influence of habitats on mating systems of North American passerine birds. Ecology 47, 143e147.