Females also keep harems of males (Polyandry)

Gough Moorhen a flightless gallinule (Author - A.J.Beintema) (CC BY 3.0)

Gough Moorhen a flightless gallinule (Author – A.J.Beintema) (CC BY 3.0)

Polyandry is where females associate with several males. In other words unlike in polygyny where males keep harems of females, here female takes up this role and keeps males in her harem. In recent years scientists have discovered many such cases in several species of birds, but prior to 1972 only two cases were known with certainty, one in the flightless gallinule and one in jacana. Interestingly all polyandrous species belong to two orders of Gruiformes (12 families covering bustards, cranes and rails) & Charadriiformes (14 families include waders, gulls, skuas, terns, auks and plovers).

In the bird world more than 90 percent species are socially monogamous (pairings remain intact during the breeding season, at least). Polyandrous species constitute about 1 per cent of all birds and differ from species with other mateship systems in the following ways:

Typically involves sex-role reversal — females are larger than males, more aggressive and more brightly colored — egg or clutch size is reduced, produce multiple clutches. Except in some sequential systems, the small, subordinate males perform all incubation behavior and provide all parental care for the chicks.

In these orders polyandry has evolved in combination with sex role reversal and assumption of incubation and parental duties by males. It is not clear why these two behavioral traits are related. The simplest forms of polyandry are in high latitude birds, but the best developed forms are seen in species found in the tropical regions.

Answer to the question, why this mating system exists in some species and not others is poorly understood. A new research, published in the journal Ethology, helps to explain how it evolves. The study’s author is Malte Andersson, professor of animal ecology at Göteborg University in Sweden. 

Three Steps

Andersson suggests that three evolutionary steps are crucial for polyandry to develop:

1.  Males take on care of the eggs.
2. Females become able to lay more eggs than a single male can accommodate.
3. Females compete with each other to lay clutches for several males.

Study suggests that male-only care of young can occur for a variety of reasons. For instance, egg production can become difficult for female in the times when food is scarce, so if males take over incubation duties, it is likely to boost the pair’s breeding success. 

The second step toward polyandry, increased female productiveness, can occur when breeding habitats are particularly rich in food, enabling females to lay clutches more rapidly. But the amount of chicks she can produce is limited by the number of eggs the parents can incubate.

In waders and shorebirds clutches are usually limited to a maximum of four eggs as they are large for the body size of the females. So instead of increasing the clutch size, a better option might be for a female is to have offspring with several different males. 

For waders like the red-necked phalarope, which gorge themselves on midge flies during a short but intense breeding season in northern countries such as Iceland, polyandry is an ideal solution. 

Males don’t face much difficulty in raising chicks alone, because they hatch with feathers on and within a few hours start scurrying around foraging for food. Chicks don’t require a lot of parental care—what they need is protection from the elements, shepherding and keeping them out of the way of predators.

The final step in the development of polyandry is for females to compete with each other over mates. 

Wattled jacana (Author - Charlesjsharp) (CC BY-SA 4.0)

Wattled jacana (Author – Charlesjsharp) (CC BY-SA 4.0)

Jacanas 

The most dramatic sex-role reversal is seen in jacanas, tropical freshwater birds. Females often dwarf the males, and conflicts over territories and mates can get particularly violent. 

In wattled jacanas (Jacana jacana), found in Central America, each female maintains a large territory and fiercely defends it against the intruding females. This area is further divided into smaller territories for her ‘harem members’, which normally include three to five males. She is free to mate with any one or all of the males and deposit her eggs in each nest that are constructed by her partners individually. Now it is the duty of males to incubate and look after the chicks, without knowing which of them is their own. In a study, DNA fingerprinting demonstrated that more than 40 per cent of jacana broods that males were tending included chicks fathered by a different male. However, when no extra males were available in a harem, all chicks were sired by the care-taking male. Normally a male jacana sits on the nest, watching the mother of the chicks he will raise, while she continues to mate with other males nearby.

If a female jacana gets predated or dies females from next door will swallow up her territory and take on males that come with it. If the male is sitting on a clutch of eggs, the new female will destroy them and if there are chicks they will be killed because they’re not hers. She will force the male to sit on a new clutch that she lays. This has been documented in research by Stephen Emlen and Natalie Demong (1989). In this experiment Demong and Emlen found that removing females from a territory resulted in nearby females attacking the chicks of the male in most cases, evicting them from their nest. The males then fertilized the offending females and cared for their young.

This infanticidal behavior can be compared to that of male lions when they take control of a new pride and polygynous primates such as langurs. 

Every day female takes a round of her territory and makes sure that it has not been taken over by any intruder. In the process, she also visits each of her mates and solves their disputes that often take place between the adjacent territory holders. Whenever, there is such situation, she intervenes first from one side, then from the other. These females have insatiable hunger for males and are always on the lookout for more.

Advantage of the system

Polyandry is beneficial for female jacanas. “If a large, dominant female can maintain a harem,” Emlen says, “her reproductive success is enhanced by having lots of males raising young for her. But the males may pay the cost. There thus is sexual conflict over the preferred mating system. Females benefit from pairing polyandrously; males, however, would avoid the cost of cuckoldry if they paired monogamously.” Why does the male jacana tolerate this behavior by his mate? In heavily populated and highly competitive habitats with limited space for nests, some males never get the chance to reproduce at all. “I guess you could say the males are lucky to be seduced and abandoned, considering the alternative,” Wrege, a co-author, says. “At least they’re adding something of their own to the gene pool.”

Spotted sandpiper (Actitis macularia)

Clashes between female spotted sandpipers, a North American wader, can get real nasty, when a female tries to usurp harem of males, from other female. They may try to puncture rival’s eye or break her leg. During this, males usually just sit there, waiting impassively for a winner to emerge.

In spotted sandpipers males are usually smaller, less aggressive and tend the nest and young. Larger polyandrous females fight for territories and mate with more than one male. Males that mate with the same female set up smaller territories within her territory and defend them against each other. Males tend to have more of the pituitary hormone prolactin than females. Prolactin promotes parental care, which may explain how the role reversal develops each season. Unlike other species where courtship performances are done by males, here it is the female that performs courtship displays. Females that are looking for mates over a wide area may do this up and down considerable lengths of shoreline. Interested males remain on the territory while uninterested males are chased away.

Red-necked phalarope (Phalaropus lobatus)

Red-necked phalarope

Red-necked phalarope

Female dominance in the mating game is also reflected in the size and appearance of polyandrous birds. For instance, females of red-necked phalarope, a small migratory wader that breeds in the Arctic regions of North America and Eurasia, are the ones that actively seek to attract a mate, they are larger and have more brightly colored plumage than males. 

In three phalarope species the typical avian sex role reversal has been observed. The females pursue and fight over males and defend their mate from other females until the clutch is complete and the male begins incubation. The males perform all incubation and chick-rearing duties, while the females search for other mates. If a male loses his eggs to predation, he may re-pair with his original mate or a new female to try again. If it is too late to start new nests or the breeding season is nearing its end, females begin their southward migration, leaving the males to incubate the eggs and look after the young.

Emu (Dromaius novaehollandiae)

As with many other Australian birds, such as the superb fairywren, unfaithfulness towards the partner is the norm for emus too, despite the initial pair bond. In these birds also it is the male that takes up the task of parenting. Once the male starts brooding, the female usually wanders off, and may mate with other males and lay in multiple nests;

Emu            (pix SShukla)

Emu (pix SShukla)

thus, as many as half the chicks in a brood may not be fathered by the incubating male, or even by either parent, as emus also exhibit brood parasitism. A phenomenon where one animal relies on others to raise its young. This system relieves the parasitic parents from the investment of rearing young or building nests for the young, enabling them to spend more time on other activities such as foraging and producing further offspring. Bird parasite species mitigate the risk of egg loss by distributing eggs amongst a number of different hosts.

In emus males become broody as soon as their mates start laying and may begin incubation even before the clutch is complete. From this time on, they do not eat, drink, or defecate, and stand only to turn the eggs, which they do about ten times a day. They develop brood patches, a bare area of wrinkled skin which is in intimate contact with the eggs. Over the course of eight-week incubation period, they lose a third of their weight and survive on stored body fat and on any morning dew that they can reach from the nest.

Ruff (Philomachus pugnax)

The level of polyandry in the ruff is the highest known for any avian lekking species and for any shorebird. More than half of female ruffs mate with and have clutches fertilised by, more than one male and individual females mate with males of both main behavioural morphs more often than expected by chance. In lekking species, females can choose mates without risking the loss of support from males in nesting and rearing chicks, since the males take no part in raising the brood anyway. In the absence of this cost, if polyandry is advantageous, it would be expected to occur at a higher rate in lekking than among pair-bonded species.

Tasmanian nativehen (Tribonyx mortierii)

A flightless rail, Tasmanian nativehen (also written as Tasmanian native-hen or Tasmanian native hen) (Tribonyx mortierii) is one of twelve species endemic to the Australian island of Tasmania. Their breeding structure can be monogamous or polygamous usually polyandrous. Among these birds each group contains a single breeding female that mates with all the males in the group. The unusual social structure based on groups who stay together is not known to occur in many other species; the high ratio of male chicks to female chicks hatched is thought to be a possible explanation for this.

White-winged trumpeter or Pale-winged trumpeter (Psophia leucoptera)

Pale-winged trumpeter or White-winged trumpeter; (pix SShukla)

Pale-winged trumpeter or White-winged trumpeter; (pix SShukla)

White-winged trumpeter, also known as the Pale-winged trumpeter, a species of large South American bird, smaller than a crane but bigger than rails, engages in simultaneous polyandry with a different twist.

Found in the southwestern Amazon rainforest of Brazil, northern Bolivia and eastern Peru, the bird forms group in which only dominant female lays eggs, while the male members, three to four, struggle to copulate with her and in the process often interrupt each other’s attempts. Though the female responds to the attempts of all the males, the dominant one succeeds in copulating most often.

After the copulation is successfully completed all males help raise the brood that hatches from her single clutch of eggs. Other females in the group seem to be those that have not reached sexual maturity. It is possible that the other females of the group might be engaging in the sexual activity with some of the subordinate males, if they managed to do it out of the sight of dominant male and female, but they apparently do not lay eggs.

Tinamous

Spotted Nothura (Author - Dario Sanches) (CC BY-SA 2.0)

Spotted Nothura (Author – Dario Sanches) (CC BY-SA 2.0)

Tinamous is an order of birds known as Tinamiformes. It comprises of a single family Tinamidae containing 47 species. In most tinamou species, males practice simultaneous polygyny and the females sequential polyandry. This is not invariable; ornate tinamous (Nothoprocta ornata) form stable pairs, and spotted nothuras (Nothura maculosa) are monogamous when young and polygamous when older. There are larger numbers of females than males; for example, the variegated tinamou (Crypturellus variegatus) has a female to male ratio of 4:1.

A tinamou female lays several eggs that male incubates while the female departs to seek another mate. Large species lay one egg every 3–4 days, while the smaller ones on consecutive days. The females lay eggs in multiple nests throughout the nesting season.

There may be as many as 16 eggs in a clutch, a consequence of several females laying in the same nest. As the male grows older and becomes more mature he attracts more females and may have the eggs of up to four females under him. The variegated and ornate tinamous males have single-female nests, consequently only one or two eggs per nest. This may result from food shortage in their ranges and the consequent ability to care for only one or two chicks.

Leave a Reply

Your email address will not be published. Required fields are marked *


five × = 45

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>