This paper, by reviewing a number of books, articles, and research papers on the relationship between avian sexual displays/courtship behaviors to their relative hormones, aims to describe the current status of affairs in this field, pointing the areas where future research may be needed.
A Canadian scientist named Lisle Gibbs, who had been working on the ecology of Darwin's finches in the Galapagos Islands, began to take an interest in the new field of molecular evolution after he received his Ph. D (Gibbs et al., 1990). He decided to use DNA fingerprinting techniques on a handy species: the red-wings nesting in a marsh not far from Queen's University in Ontario. He and his co-workers mapped the territories of the males, noted the number of females that settled on each territory, and then counted the eggs and chicks that each nest produced (Gibbs et al., 1990). They then took tiny samples of blood from all the individuals on the site and typed their DNA. Birds, incidentally, make better subjects for this analysis than mammals if one is limited to obtaining only a small amount of blood, because their red blood cells, unlike humans' and those of other mammals (with the peculiar and completely inexplicable exception of camels) are nucleated. A human red blood cell looks like a doughnut, because the nucleus is absent after the first stages of cellular development.
Later, other researchers such as Emlen (1995) and Gowaty (1997) also suggested that because the nucleus is the part of a cell that contains the chromosomes and hence most of the DNA, bird blood is easier to use for typing than mammalian blood. When it is complete, the analysis yields dark-colored bands of dyed protein strung across a gel, or rubbery matrix that allows the proteins to pass through it. The different proteins are associated with different alleles, and because a chick can get alleles only from his or her parents, one can trace the parental source of each band in each chick's gel, given the assumptions outlined above.
When Gibbs lined up the gels, he discovered something startling: many, if not most, of the chicks in a given blackbird territory were fathered by a male other than the territorial one. The paper he and his co-workers published shows the map of the territories with two numbers for each nest: the number of chicks fledged from that nest, and the number sired by the territorial male (Gibbs et al., 1990). In the old days before DNA testing, these two numbers were assumed to be the same, Bray's troubling results notwithstanding. But Gibbs's results were too striking to ignore; the two numbers, far from being identical, were not even correlated. Males with four chicks on their territories might have fathered five others in the marsh, while males with ten chicks on their territories might actually have fathered only one. It wasn't just a matter of a couple of chicks here and there, it was as if the entire method for calculating reproductive success, that cornerstone of evolution, was shown to have a foundation of sand (Abramson and Pinkerton, 1995).
Incidentally, the data-gathering processes here also throw into relief one of the most pervasive myths many deal with in science, that one handles baby birds and then returns them to the nest at great peril, because the mother will smell a human on her offspring and reject it. Koenig, Haydock, and Stanback (1998) argue that most birds have a relatively poorly developed sense of smell, and biologists have in fact capitalized on this obliviousness of avian parents to make wide use of cross-fostering experiments, in which chicks are swapped among nests to determine the contributions of parental genes versus parental behavior to their development. In most cases, parents continue to feed an alien nestling as if it were their own.
To return to extra-pair copulations, however, it should be emphasized here that Gibbs's paper was not the earliest example of its kind. Other studies can be joined to it, and they have provided, within an astonishingly short time, a picture of animal relationships radically different from the one that existed earlier (Gaulin and FitzGerald, 1986). Hundreds, maybe even thousands, of papers published in scientific journals over the last fifteen years document extra-pair paternity in birds from every continent, every habitat. In fact, it is no longer enough to replicate a study of extra-pair paternity in yet another species.
A recent study pointed out that EPCs are now known to occur in every avian family (Brooke, Davies, and Noble, 1998). That means ducks, warblers, woodpeckers, wrens, orioles, the lot. This is that same group held up as a model of monogamy just a few short years ago. It was a real revolution, and it took place within less than a decade.
Two questions now emerge. First, why do people find this so shocking, not just a surprise, but a disturbing revelation about the animal world and potentially their own relationships? And second, what does this finding mean for the science of animal behavior and the notions about mating systems?
The answer to the first question is fairly obvious; people are shocked by the blackbirds because individuals identify with animals, particularly attractive or cuddly animals. Terms including adultery, infidelity, betrayal, cheating, fooling around, and more have been applied to findings like those of Lisle Gibbs in the popular press, and sometimes the scientific literature is not far behind (Eberhard, 1998). Birds have always looked so admirable in their industry during the breeding season, the way the male and female rush back and forth to their demanding brood of chicks. And there is something uncannily amusing about mere birds being placed in the same situation as millions of modern-day husbands, eyeing a child warily, making uneasy jokes about the milkman. It is tempting to go on to conclude that if warblers, robins, and other models of monogamy are doing it, it is essential to admit that extra-pair copulation, or adultery-or whatever term you prefer-is natural and expected, and maybe scientist should stop making such a fuss about it and resign to people's evolutionary heritage (Ellis and Ebertz, 1997).
This conclusion, however, is precisely what this paper argues against. The birds aren't cheating, they are just doing what they do, and they did not invent the rules about the pair bond between a male and female, people did. It isn't cheating if there are no rules to break. If individuals try and use their behavior as a model or justification for people's own, they not only run the risk of making decisions about their morals on very shaky grounds, researchers might miss what is interesting and vital about the animals' own behavior.
This is not to say that people cannot see a great deal about the influence of social expectations on scientists by examining their work on animal behavior. For instance, some of the literature on EPC is interestingly divided as to whom it portrays as the active party. Initially, there seemed to be two approaches, neither one particularly favorable to females (Ellis and Ebertz, 1997). Either males were roaming around and taking advantage of hapless females waiting innocently in their own territories for the breadwinner males to come home with the worms, or else females were brazen hussies, seducing blameless males who otherwise would not have strayed from the path of moral righteousness.
Bray's "female promiscuity" label is just one example (Bray, Kennelly, and Guarino, 1975, p. 188). A paper published in the prestigious journal Nature refers to young in warblers as "illegitimate," as if their parents had tiny avian marriage licenses and chirped their vows (Qtd in Koenig, Haydock, and Stanback, 1998, p. 83). That some scientists in modern society take this view should come as no surprise.
The problem is, however, that scientists sometimes allow their prejudices to influence their interpretation of what they see the animals doing, and analysis of extra-pair copulation is a prime example (Orians, 1969). Another mating system-related case illustrating the influence of bias comes from a 1972 paper on the Tasmanian native-hen (Gallinula mortierii), a bird with a rather complex set of relationships between the sexes, including what appears to be polyandry, multiple males associated with a single female. The paper refers to this behavior as "wife-sharing," but the term "husband-sharing", disturbing as it might be, is almost never used (Qtd in Hrdy, 1999, pp. 78, 90). Making the males the active parties may reduce the likelihood of noticing what the females do, of seeing things from their point of view.
In response to the second question, the most productive approach to this revolution in mating systems is rather to explore what extra-pair copulations mean for both males and females and what environmental factors might favor or disfavor them. This avenue has indeed been taken by many researchers, particularly recently. Several papers from the 1990s evaluate costs and benefits of EPCs to both sexes. In one of these, published in 1998, Marion Petrie and Bert Kempenaers remark, "In most bird species it is likely that females control the success of a copulation attempt and of transfer of sperm," a statement that would have been unlikely to appear in print a couple of decades ago (p. 53). Other researchers working alongside with Petrie, such as Jennions (2000), Doums and Moller (1998) are trying to determine whether the population density of individuals within a species, the tendency for a species to migrate rather than to breed and winter in the same place, or some other aspect of a species' ecology may influence the opportunity for extra-pair mating (Petrie, Doums, and Moller, 1998; Jennions, Petrie, 2000).
It is likely that males and females derive different benefits from such mating, because of the different sorts of limitations on male and female reproductive success. Settling on a territory with one male and mating with another can allow a female to increase the genetic diversity of her offspring. In an unpredictable environment, having chicks with as many different characteristics as possible increases the likelihood that at least some will survive and reproduce themselves, a rather literal version of not putting all your eggs in one basket (Gowaty and Buschhaus, 1998). This may be particularly true if circumstances do not allow many females to be associated with males possessing especially favorable characteristics; at least some of the offspring a female produces can be sired by the best male around, assuming there is some way of distinguishing this hero from the rest (Davies, 1999). Variations on this good genes idea abound, and another study by Petrie and two co-workers found that in species where more genetic variation exists, extra-pair paternity was more common, an intriguing and heretofore unexpected connection (Petrie, Doums, and Moller, 1998). Mating with more than one male can also ensure against infertility or low sperm counts in a given mate, or make a female's chance of re-mating higher if she loses her first mate because she has already established a bond with another male.
Engaging in mating with males other than the one whose territory she shares may also have costs for the female. Although little-studied in birds and other nonhumans, sexually transmitted diseases are a risk that increases with every sexual partner, and bird sex may be particularly nasty in this regard. Unlike mammals, most birds lack a penis and the genito-urinary and digestive systems all empty into a common chamber, called a cloaca (this structure is also present in reptiles and amphibians, as well as some fish) (Gowaty and Buschhaus, 1998). Fertilization is accomplished by contact between the birds' cloacal openings, with sperm being released by the male into the cloaca of the female in a hurried operation that, at least to a human observer, doesn't look like much fun. The process was referred to as a "cloacal kiss" by some overly poetic naturalist many years ago (Qtd in Birkhead and Moller, 1992, p. 37). In any case, this set-up means that in addition to the usual exposure to the bacteria, viruses, and other germs associated with the genital tract, sex involves contact with the digestive and urinary systems, and it is known what hotbeds of microorganisms those can be; this is why you wash your hands after using the toilet. Many scientist have wondered if animals with a cloacae run a greater risk of sexually transmitted diseases than those without, but to my knowledge this question has never been explored.
This cost also applies to males, of course, but females have other risks as well. The major one is considered to be the loss of a male's help in offspring care if, from his perspective, there is some chance that the young have been sired by another male. This risk appears to vary among species and in different circumstances, but may have shaped part of the inherent conflict that occurs between the sexes. In addition, looking for some action on the side takes time and energy, and many birds are already working flat out during the breeding season, finding food, chasing away predators, and keeping the chicks warm. Taking time from any of these activities may mean less well-fed offspring that might not successfully compete with other birds when they grow up, a potentially large risk indeed.
In just the last few years, studies examining these costs and benefits for the sexes have proliferated (Jennions and Petrie, 2000). The proportion of offspring sired by extra-pair males appears to vary from 0 (real, true, till-death-do-us-part monogamy is still seen among snow geese and some sea birds) to a whopping 90 percent, the latter in a species of fairy wren (a group of birds in the genus Maluru), brilliantly colored tiny birds from Australia (Jennions and Petrie, 2000). Do species with showy ornaments like these have a greater tendency toward EPC? What about variation in paternal care? Can males adjust the amount of energy they expend on offspring depending on the likelihood that they share genes with all of the young in the nest? What characteristics are preferred in extra-pair partners, and do those differ from the traits preferred in more long-term mates?
Answering questions like these will keep scientists busy for many years to come. Scientists now refer, somewhat awkwardly, to a species' social mating system (for example, males and females appear to have a single partner, hence social monogamy) as distinct from its genetic mating system (in fact, many young are the product of extra-pair mating, hence genetic polygamy), recognizing that these are often not the same in a single population of animals. It is important to remember that these are categories created by people, not birds, and it cannot be expected that animals fit neatly into them.
The news about extra-pair copulation shook the world of behavioral ecology, and with luck it will shake some stereotypes down with it, in particular some male-centered stereotypes. In a paper, feminist scientist Patty Gowaty (1997) pointed out several biased practices in the study of animal behavior that a less male-centered viewpoint helps to shatter, such as examining female behavior only with respect to males, or seeing females only as either virgins or whores. With characteristic bluntness, she also suggested that some ideas about female behavior in animals "would seem to have more to do with the nature of some men's minds than most females' lives" (Gowaty, 1997, p. 97).
Two ideas have been longstanding in the literature about animal mating, particularly if invertebrates are included: females often mate with more than one male, and polyandry is rare. Those two notions seem diametrically opposed, and yet they both seemed to be supported in nature. Multiple mating by females is an undeniable empirical observation, but it has been difficult to explain, in part because polyandry, the long-term association of a female with more than one male, both seemed theoretically unlikely to evolve and has been observed in only a handful of species. Polyandry looked implausible because at first glance it seemed detrimental to males and nearly pointless for females to maintain more than one mate; two males cannot get a female any more pregnant than one, whereas multiple females can each be inseminated by a single male.
And yet-females of a lot of different species, including jacanas and now it seems red-winged blackbirds and many others, mate many times, with the same or different males. Often those types of mating are solicited by the female herself, in animals ranging from butterflies to rhesus monkeys. If females are coy and passive, if polyandry has no benefits, how does one explain this? From the animals' perspective, or more correctly from the perspective of evolution, it does not matter what the system looks like. The only thing that matters is leaving genes, and if mating with several males helps, a female will try to do so. In either case, the next interesting part of sexual relationships comes about because what benefits a female is not necessarily what benefits a male, and vice versa.
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