The polygyny threshold model is an evolutionary explanation of polygyny, the mating of one male of a species with more than one female. 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 genetics, territory, food supply, or other important characteristics are better than those of his competitors, even with two females on the territory.
The graphical depiction of the model presented in Gordon Orians' 1969 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 one female mate but who has defended more resources. The second curve is roughly the first curve shifted to the right some amount. The given shapes of the curve will change with other intrinsic factors like genetic quality and male paternal investment. It is important to note that the designation "female" and "male" here are oft accurate; however, in some mating systems the operational sex ratio leans towards females, who then have motivation to engage in resource defence 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 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 and thus the minimum environmental quality difference necessary to make bigamy beneficial for the female. Also important is the vertical line drawn from the intersection of the line with the bigamy curve to the monogamy curve above. This represents the fitness gain of a female who chooses monogamy over bigamy due to, here, resource holding differences.
Orians predicted that animals exhibiting resource defence polygyny, such as the fish Neolamprologus pulcher would fit to this model when living in successive habitats, where territory quality is very variable. Using the territory quality to decide whether to pursue a monogamous or polygynous mating relationship. This is shown in the red-winged blackbird by Pribil and Searcy (2001). Female red-winged blackbirds prefer to mate with males with territories over water and also unmated males. The females were given a choice between unmated males or previously mated males with the superior territories over water. In 12 out of 14 trials (86%) females chose the already mated male with the superior territory. [1]
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. [2]
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. [3] 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. [4] 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. [5] 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. [6]
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 reed warbler that showed the only cost of polygyny to these females to be higher mortality of nestlings that were belonging to the primary female. [7] Johnson, Kermott, and Lien conducted a study on the house wren ( Troglodytes 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. [8] 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 deduced that the costs could not be concluded to be the sole cause of this parental transition. [9] The numerous studies concerning polygyny costs show the different factors that not only cause these costs, but are also affected by these costs.
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Studies in human populations have demonstrated a decline of polygyny in highly stratified societies where rival wealth is concentrated by a small class of rich elite (economic inequality). [10]
The red-winged blackbird is a passerine bird of the family Icteridae found in most of North America and much of Central America. It breeds from Alaska and Newfoundland south to Florida, the Gulf of Mexico, Mexico, and Guatemala, with isolated populations in western El Salvador, northwestern Honduras, and northwestern Costa Rica. It may winter as far north as Pennsylvania and British Columbia, but northern populations are generally migratory, moving south to Mexico and the Southern United States. Claims have been made that it is the most abundant living land bird in North America, as bird-counting censuses of wintering red-winged blackbirds sometimes show that loose flocks can number in excess of a million birds per flock and the full number of breeding pairs across North and Central America may exceed 250 million in peak years. It also ranks among the best-studied wild bird species in the world. The red-winged blackbird is sexually dimorphic; the male is all black with a red shoulder and yellow wing bar, while the female is a nondescript dark brown. Seeds and insects make up the bulk of the red-winged blackbird's diet.
A lek is an aggregation of male animals gathered to engage in competitive displays and courtship rituals, known as lekking, to entice visiting females which are surveying prospective partners with which to mate. It can also refer to a space used by displaying males to defend their own share of territory for the breeding season. A lekking species is characterised by male displays, strong female mate choice, and the conferring of indirect benefits to males and reduced costs to females. Although most prevalent among birds such as black grouse, lekking is also found in a wide range of vertebrates including some bony fish, amphibians, reptiles, mammals, and arthropods including crustaceans and insects.
Behavioral ecology, also spelled behavioural ecology, is the study of the evolutionary basis for animal behavior due to ecological pressures. Behavioral ecology emerged from ethology after Niko Tinbergen outlined four questions to address when studying animal behaviors: What are the proximate causes, ontogeny, survival value, and phylogeny of a behavior?
The great reed warbler is a Eurasian bird in the passerine genus Acrocephalus.
The European pied flycatcher is a small passerine bird in the Old World flycatcher family. One of the four species of Western Palearctic black-and-white flycatchers, it hybridizes to a limited extent with the collared flycatcher. It breeds in most of Europe and across the Western Palearctic. It is migratory, wintering mainly in tropical Africa. It usually builds its nests in holes on oak trees. This species practices polygyny, usually bigamy, with the male travelling large distances to acquire a second mate. The male will mate with the secondary female and then return to the primary female in order to help with aspects of child rearing, such as feeding.
The lark bunting is a medium-sized American sparrow native to central and western North America. It was designated the state bird of Colorado in 1931.
A mating system is a way in which a group is structured in relation to sexual behaviour. The precise meaning depends upon the context. With respect to animals, the term describes which males and females mate under which circumstances. Recognised systems include monogamy, polygamy, and promiscuity, all of which lead to different mate choice outcomes and thus these systems affect how sexual selection works in the species which practice them. In plants, the term refers to the degree and circumstances of outcrossing. In human sociobiology, the terms have been extended to encompass the formation of relationships such as marriage.
A harem is an animal group consisting of one or two males, a number of females, and their offspring. The dominant male drives off other males and maintains the unity of the group. If present, the second male is subservient to the dominant male. As juvenile males grow, they leave the group and roam as solitary individuals or join bachelor herds. Females in the group may be inter-related. The dominant male mates with the females as they become sexually active and drives off competitors, until he is displaced by another male. In some species, incoming males that achieve dominant status may commit infanticide.
Monogamous pairing in animals refers to the natural history of mating systems in which species pair bond to raise offspring. This is associated, usually implicitly, with sexual monogamy.
Cooperative breeding is a social system characterized by alloparental care: offspring receive care not only from their parents, but also from additional group members, often called helpers. Cooperative breeding encompasses a wide variety of group structures, from a breeding pair with helpers that are offspring from a previous season, to groups with multiple breeding males and females (polygynandry) and helpers that are the adult offspring of some but not all of the breeders in the group, to groups in which helpers sometimes achieve co-breeding status by producing their own offspring as part of the group's brood. Cooperative breeding occurs across taxonomic groups including birds, mammals, fish, and insects.
Sturnus is a genus of starlings. As discussed below, the taxonomy of this group is complex, and other authorities differ considerably in which species they place in this genus, and in the species boundaries within Sturnus. The genus name Sturnus is Latin for "starling".
Mate choice is one of the primary mechanisms under which evolution can occur. It is characterized by a "selective response by animals to particular stimuli" which can be observed as behavior. In other words, before an animal engages with a potential mate, they first evaluate various aspects of that mate which are indicative of quality—such as the resources or phenotypes they have—and evaluate whether or not those particular trait(s) are somehow beneficial to them. The evaluation will then incur a response of some sort.
The sexy son hypothesis in evolutionary biology and sexual selection, proposed by Patrick J. Weatherhead and Raleigh J. Robertson of Queen's University in Kingston, Ontario in 1979, states that a female's ideal mate choice among potential mates is one whose genes will produce males with the best chance of reproductive success. This implies that other benefits the father can offer the mother or offspring are less relevant than they may appear, including his capacity as a parental caregiver, territory and any nuptial gifts. Fisher's principle means that the sex ratio is always near 1:1 between males and females, yet what matters most are her "sexy sons'" future breeding successes, more likely if they have a promiscuous father, in creating large numbers of offspring carrying copies of her genes. This sexual selection hypothesis has been researched in species such as the European pied flycatcher.
A biological ornament is a characteristic of an animal that appears to serve a decorative function rather than a utilitarian function. Many are secondary sexual characteristics, and others appear on young birds during the period when they are dependent on being fed by their parents. Ornaments are used in displays to attract mates, which may lead to the evolutionary process known as sexual selection. An animal may shake, lengthen, or spread out its ornament in order to get the attention of the opposite sex, which will in turn choose the most attractive one with which to mate. Ornaments are most often observed in males, and choosing an extravagantly ornamented male benefits females as the genes that produce the ornament will be passed on to her offspring, increasing their own reproductive fitness. As Ronald Fisher noted, the male offspring will inherit the ornament while the female offspring will inherit the preference for said ornament, which can lead to a positive feedback loop known as a Fisherian runaway. These structures serve as cues to animal sexual behaviour, that is, they are sensory signals that affect mating responses. Therefore, ornamental traits are often selected by mate choice.
Interlocus sexual conflict is a type of sexual conflict that occurs through the interaction of a set of antagonistic alleles at two or more different loci, or the location of a gene on a chromosome, in males and females, resulting in the deviation of either or both sexes from the fitness optima for the traits. A co-evolutionary arms race is established between the sexes in which either sex evolves a set of antagonistic adaptations that is detrimental to the fitness of the other sex. The potential for reproductive success in one organism is strengthened while the fitness of the opposite sex is weakened. Interlocus sexual conflict can arise due to aspects of male–female interactions such as mating frequency, fertilization, relative parental effort, female remating behavior, and female reproductive rate.
Polygyny is a mating system in which one male lives and mates with multiple females but each female only mates with a few males. Systems where several females mate with several males are defined either as promiscuity or polygynandry. Lek mating is frequently regarded as a form of polygyny, because one male mates with many females, but lek-based mating systems differ in that the male has no attachment to the females with whom he mates, and that mating females lack attachment to one another.
Social monogamy in mammals is defined as sexually mature adult organisms living in pairs. While there are many definitions of social monogamy, this social organization can be found in invertebrates, reptiles and amphibians, fish, birds, mammals, and humans.
In biology, paternal care is parental investment provided by a male to his own offspring. It is a complex social behaviour in vertebrates associated with animal mating systems, life history traits, and ecology. Paternal care may be provided in concert with the mother or, more rarely, by the male alone.
In behavioral ecology, polyandry is a class of mating system where one female mates with several males in a breeding season. Polyandry is often compared to the polygyny system based on the cost and benefits incurred by members of each sex. Polygyny is where one male mates with several females in a breeding season . A common example of polyandrous mating can be found in the field cricket of the invertebrate order Orthoptera. Polyandrous behavior is also prominent in many other insect species, including the red flour beetle, the adzuki bean weevil, and the species of spider Stegodyphus lineatus. Polyandry also occurs in some primates such as marmosets, mammal groups, the marsupial genus' Antechinus and bandicoots, around 1% of all bird species, such as jacanas and dunnocks, insects such as honeybees, and fish such as pipefish.
In animal behavior, resource defense polygyny is a mating strategy where a male is able to support multiple female mates by competing with other males for access to a resource. In such a system, males are territorial. Because male movement is restricted, female-female competition for a male also results. Males capable of maintaining a larger territory are said to have greater resource holding power. It is one of the three major types of polygyny, the other two being female defense polygyny and leks.