Polyandry: What is the point?

[mynah]

I'm doing research for an article on mating systems in birds. Although most birds are monogamous (for life, or for the breeding season), many are polygynous (male mating with several females) and some are polyandrous (female mating with several males).

In monogamous species, almost everyone mates, and gene diversity is maintained. In polygynous species, only the fittest males mate, thus increasing male fitness, but with the risk of a decrease in gene diversity. More or less the same applies - with gender reversal - in polyandrous species, but there is a difference: Males have an unlimited amount of sperm to distribute, whereas females invest in a limited number of rather large, costly eggs. Why, then, are some bird species polyandrous?

[sman]

Robert Trivers (I believe) isolated the common denominator of sexual dimorphism as minimal investment in offspring, which is harmonious with much observation of the living world. Polyandry is definately the exception to the rule. I'm not posting with an answer, just curiosity.
Natural selection is a predictive theory, not just an explanatory one. If polyandry is observed, you can bet there's a benefit for it...even if it's not immediately obvious to us. I'm surprised more knowledgeable members of this forum havn't engaged this thread.

[Donk]

I've noticed that the local mallard ducks quite often form threesomes, with two males and a single female. Both males combine to keep marauding males away from "their" female. I would guess they both get better breeding chances by co-operating with each other than either would alone, but I'm open to other explanations.

[Galapagos]

From what little I have read on the subject, it may have something to do with parental investment and mode of fertilization. In cases of internal fertilization, males are guaranteed that their offspring will receive a certain amount of care, and the pressure is on them to take off. In cases of external fertilization, there is opportunity for males to start doing the productive work, and roles can be reversed more easily.

In "Animal Behavior" by Mark Ridley, Ridley says that he informally surveyed all of the organisms that have polyandrous mating systems, and that there was a clear correlation between mode of fertilization and mating system. This survey - besides not being a formal study- is also way out of date by now(my copy is 14 years old). Google scholar turns up some more hits for "polyandry external fertilization" and such, so that might be a good place to start if you have journal access.

Also, I happened to stumble upon this PBS page on Jacana polyandry on google, might be of interest:
Evolution: Library: Jacanas and Polyandry

[mynah]

Donk:
I hope for the females' sake these threesomes are as harmonious as they appear to be, as male mallards are notorious for mating with any ducks - not necessarily willing, female, of the same species, or even alive. They also do not form pair bonds. If these threesomes are indeed permanent or semi-permanent groups (rather than two repeat rapists and their victim), further study of this community is certainly be called for.

The sordid truth about mallards...

Galapagos:
Birds rather differ in their minimal investment from mammals in that the absolute dependence of the offspring on the mother (rather than on any parent) ends when the egg is laid, whereas young mammals depend specifically on their female parent for the duration of gestation and for milk after birth. This makes polyandry more likely to develop in birds than in mammals, although it doesn't actually explain how it came to be in the first place. One possibility I can think of could be a shortage of females in the past leading to selection for a breeding strategy that allowed each female to lay many more eggs than she could brood herself or with the help of only one male, but it is difficult to visualise conditions that would lead selectively to a shortage of females.

[mynah]

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[mynah]

I've thought of a way in which a shortage of females may occur... Just my own speculation and possibly a bit far out, but here it is:

In birds, females are the heterogametic sex, i.e. the one with two different sex chromosomes (as opposed to mammals, where it is the male). Having just one copy of the sex chromosome leaves the carrier vulnerable to a wider range of genetic problems than would otherwise have been the case. Could polyandrous species have suffered from an unusually severe load of sex-linked problems, leading to a markedly skewed male:female ratio? It would be interesting to know whether or not the ratio is skewed in polyandrous species. (It is in polygynous species, where conspicuous males encumbered with extravagant display feathers often fall prey to predators.)