Males keep harems of females (Polygyny)

Marsh Wren (Author - tgreyfox) (CC BY-SA 2.0)

Marsh Wren (Author – tgreyfox) (CC BY-SA 2.0)

Polygyny is believed to be the primary mating system in animals. It means one male mates with more than one female while each female mates with only one male. According to an estimate 2 per cent of all birds indulge in polygyny, whereas in North America, 14 of 278 breeding songbird species (11 of which nest in marshes or grasslands) are polygynous. Among these birds usually it is the female that takes over the responsibility of parental care.

If we analyze the contribution in the process of procreation we will find that males make a lesser investment in the embryos that result from the fusion of egg and sperm cells. Reason is simple. The sex that produces the larger reproductive cells (eggs) is the female, and the one that produces the smaller (sperm) is the male. The variation is especially prominent in birds, since the sperm is microscopic and the egg (relatively) enormous. Thus, male puts proportionately little effort into any single embryo, while the female has a great stake in each one, since she can produce relatively few eggs in her lifetime. In this situation of limited opportunities, females must therefore exercise care in choosing the fathers of their limited number of young.

Since males make very little contribution with vast resource of sperms at their disposal they become much less choosy and attempt to have as many mates as possible. Evolution also favors this behavior as it leads to leaving a maximum number of offspring. A male that mates with a weak or otherwise unfit female loses a small part of his reproductive potential; a female making a similar mistake may sacrifice all or almost all of hers.

Since most birds are monogamous, it is expected that both parents must help in raising the young if the adults are to have much chance of leaving any genes to posterity. Now the question is under what circumstances, then, can polygyny occur? One hypothesis is that polygyny occurs where males hold territories that vary greatly in the quality of resources. In the situation females will be forced to choose superior males, which mean those occupying high-quality territories. When those males already have mates, females have a choice. They can either select a male that holds an inferior territory, or they can become the second mate of one of the superior males. If the disparity between high and low-quality territories is great enough, it is better to go for the latter strategy. Logic is even if there is little or no aid from a male holding a resource-rich territory there are still better chances of producing surviving offspring than the full cooperation of a male with an inferior territory. The male with a superior territory will benefit by increased reproduction, as will the second female.

Red-winged Blackbird male (Author - Walter Siegmund) (CC BY-SA 3.0)

Red-winged Blackbird male (Author – Walter Siegmund) (CC BY-SA 3.0)

This is what often found in nature. Female Marsh Wrens (Cistothorus palustris), a small North American songbird, near Seattle, Washington, sometimes mate with already-mated males, even when bachelors are available. The number of females mated to each male depends upon the amount of vegetation growing in the males’ territories, presumably an indicator of the availability of insect food. Studies of Yellow-headed Blackbirds (Xanthocephalus xanthocephalus), Red-winged Blackbirds (Agelaius phoeniceus), Dickcissels (Spiza americana), Lark Buntings (Calamospiza melanocorys) and Indigo Buntings (Passerina cyanea) also show clear relationships between various aspects of territory quality and the likelihood that a male holding a given territory will have more than one mate.

Polygyny is not always associated with territoriality. For instance, many seed-eating species of African weavers (relatives of House Sparrows) found in savanna have enormous resources and their males are also not territorial, seemingly because defending an area does not increase their access to food. Females of these birds apparently do not need help from males to raise the young. Since weavers nest in colonies, the need for a partner in nest defense is also minimal. The female is thus free to opt for any male to father her offspring, despite his other attachments. Here, as in situations where males are territorial, polygyny is related to the availability of resources – in this case their superabundance rather than their uneven distribution.

Monogamy is most common in birds; polygyny is rare compared to mammals. If we talk in terms of evolution polygyny in birds might have evolved because many females do not require male support to care for their offspring, so males are free to invest in multiple females. Nevertheless, male parental care is frequently found in many polygynous territorial species, leading to female competition for male assistance. Most often, males seek a second female to impregnate, once the first one has laid eggs. Strongly polygynous or monogamous species display increased female-female aggression. Many factors — territory size, habitat quality, predator density and nest spacing — contribute to female aggression. Many times they have been observed fighting for resources from the male, such as food and nest protection. The female disadvantages of mating with an already-mated male can be overcome with ample resources provided by the male, resulting in female choice.

Cost and benefits for males

Cost

Due to the fact that one male sires all of the offspring in polygynous systems there is less genetic diversity. While being the only father to all offspring is extremely beneficial to a male’s fitness, it increases the chances of passing on of genetic disorders through inbreeding. In addition to this it is difficult for males to monopolize many females at once, leading to extra-pair copulations (EPC) (in monogamous species one individual mates outside of the pair). Since they are not being watched properly by the breeding males they mate with other males unnoticed. These breeding males also have short tenure, and it is common for groups of males who do not have harems to attack a breeding male in order to gain reproductive access to his females.

Benefits

The largest advantage for males in polygyny is the increased fitness and reproductive success of the lone male as he fathers all the offspring. Being the sole owner of a harem is highly advantageous for the male, he has much higher chances of his progeny surviving, which means he is passing on his genes to more individuals.

Cost and benefits for females

Cost

Since one male sires all the offspring there is less genetic diversity in the community, which is not good for the progeny. Females sometimes face infanticide, which is when an incumbent male is overthrown and a new male takes over his position and kills all of his current offspring, as he has not fathered them. Since the females no longer have offspring to nurse, they will go into estrous sooner, which allows the new male to mate with the females earlier.

Benefits

Unlike in males, extra-pair copulation (EPC) is gainful for females as it gives them more mate choices and ensures increased genetic diversity in the community. EPCs illustrate sexual conflict, a situation in which one behavior is advantageous for one sex, but disadvantageous for the other.

Great reed warbler (GNFDL) (photo -- Marek Szczepanek)

Great reed warbler (GNFDL) (photo — Marek Szczepanek)

Great reed warbler (Acrosephalus arundinaceus)

The great reed warbler is one of the few species that is polygynous and has a system of harem, where males make available resources, such as nest protection and varying levels of parental care, to the females in the harem. Females in the harem are able to breed at the same time, indicating that harem size and the number of male offspring are related. The most important factor when determining male fitness is the order in which he arrives to the territory. Males that arrive early usually get good nesting sites, improving their odds for attracting more females. Furthermore, a greater song repertoire is linked with an increase in harem size and increased male fitness as females have a preference for mating with males having a more wide-ranging song repertoire. It is also likely that broad song repertoires are an additional cue for a good mate, in conjunction with male territory size and quality. An extensive song repertoire develops with age, and older males are more likely to dominate better territories, giving a plausible reason as to why females prefer older males. Although highly debated, female choice in the great reed warbler may be explained by the good genes theory, which says females select males that seem to have genetic advantages, which increases offspring quality. False paternity and decreased offspring survival are two factors which might contribute to a decrease in male fitness.

Evidence of female choice

Female choice is the act in which any female chooses her mate on the basis of attractiveness of his qualities. This is very common in polygynous system. Here females choose males on the basis of secondary sexual characteristics, which may indicate access to better and more resources. For instance, great reed warbler females show preference for males with larger song repertoires because this indicates that they are older and may have better nesting territories. Female Coquerel’s sifaka (Propithecus coquereli), a diurnal, medium-sized lemur, mates with the winners of battles for the harem because the male has shown that he is stronger than another, potentially offering more protection from predators and passing on healthy genes to the offspring. Female red-winged blackbird (Agelaius phoeniceus) exhibits aggression towards other females upon intrusion into the harem, usually heightened around breeding season. This demonstrates that the females are protecting their male from intruding females, suggesting they are preventing female access to a desirable mate. However, one of the most obvious pieces of evidence for female choice in polygynous mating systems is extra-pair copulations, which demonstrates that a female is selecting a male, other than the breeding male, as a mate rather than or in addition to continuing to mate with the dominant breeding male.

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