Infidelity is easy to explain in males. By sleeping around, a guy can potentially impregnate more females and sire more offspring than if he just had one mate. But females cheat, too, even though a woman will only be able to have roughly one baby per year no matter how many male sex partners she has had.

One leading evolutionary hypothesis suggests that a female who mates with multiple males ensures the genetic diversity and quality of her offspring; having higher-quality offspring could theoretically give her more grandchildren later. A 17-year study, published in the June issue of The American Naturalist, challenges this hypothesis.

"This is one of the most careful and most robust studies to explore whether polyandry is adaptive in females," says Tommaso Pizzari, a University of Oxford biologist who was not involved in the research. "The answer is: not really."

Previous studies tested the "quality" hypothesis indirectly. In socially monogamous species, researchers would compare the legitimate and illegitimate offspring of cheating females by asking: Which offspring were larger? Which lived longer? But a better way to understand why female promiscuity evolved, says Jane Reid, a biologist at the University of Aberdeen in Scotland and author on the new study, is to determine whether those illegitimate offspring actually have more babies.

Reid and her team studied the isolated population of wild song sparrows on Mandarte Island in Canada. Like their mainland counterparts, these birds are socially monogamous. Males and females pair up for an entire breeding season or for several breeding seasons, and work together to feed the hatchlings and defend the nest. But they're not always faithful; blood tests showed that in this particular population, 28 percent of hatchlings were fathered by other males.

The researchers studied three generations of song sparrows (including more than 2,300 hatchlings) to see whether the cheating females' illegitimate sons and daughters would have higher reproductive success. It turned out that the bastard hatchlings actually fared worse than their legitimate siblings, producing about half as many offspring, on average. "It's not what people expected," Reid says.

David Westneat, a behavioral ecologist at the University of Kentucky, agrees. "It's essentially saying that there's no evidence for a hypothesis that's been very popular." But the theory is not dead yet, because the results need to be confirmed in other populations and other species. Westneat and Pizzari also expressed some concern over the fact that the study took place on an island—these small and isolated populations have a tendency to evolve in weird ways.

Nevertheless, the scientists agree that female promiscuity does not appear to have reproductive advantages for most of the female song sparrows on Mandarte Island. So what's the point of cheating, if it doesn’t help them to have greater evolutionary success?

Some theories blame it on the boys: Female promiscuity may be an ecological by-product of male promiscuity. If males are under strong selection to mate with multiple females, then in a closed environment females might be forced to mate with multiple males.

Reid is also investigating whether female promiscuity is a genetic by-product of the male version. If there are genes that encourage a male to be promiscuous, he might pass those genes on to his daughters, even though they may not benefit from the behavior. A recent study in zebra finches found support for this hypothesis. If it turns out that genetics have a heavy influence on feminine infidelity, Reid's team would like to try to pin down which genes are contributing to promiscuous behaviors.

38 Comments

1. 1. rhodinsthinker 11:13 AM 6/6/12

   The female in an EPC (Extra Pair Coupling) ran into one of the promiscuous males.

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2. 2. r0b3m4n 12:13 PM 6/6/12

   Gosh maybe their just horny. Or we could demonize men some more =:)

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3. 3. Derick in TO 12:48 PM 6/6/12

   Or maybe it's a way of hedging their bets, in case the regular male partner dies off.

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4. 4. Reader in Denver 01:17 PM 6/6/12

   Why don't they check the success of the illegitimate offspring of the males as well? I suspect they also have less mating success, and their offspring have less success.
   
   This certainly seems to be true with human children of fathers who have children both inside and outside of marriage. I have often thought about writing a book about it about historical figures. One could examine the likely prospects for the Schwarzenneger and Edwards illegitimate children. And how about the effect on the children of the primary coupling as well?
   
   And human beings are a lot more complex and take a lot longer and a lot more resources to raise than birds. So it seems the effect of actually not being evolutionarily successful would be more exaggerated.

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5. 5. lkw787 04:06 PM 6/6/12

   I think emotions have more to do with infidelity in humans. If a woman feels paid attention to and cherished and trusts that her mate is faithful she is more likely to be faithful (in my opinion.) I think that scientists who fail to take emotions into consideration when performing these studies are making a big mistake. Of course, economics also play into emotions...if a woman feels well cared for financially (and thus more secure) she is probably also less likely to stray (assuming being "taken care of" financially is part of what she wants in a mate.)

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6. 6. ssm1959 04:29 PM 6/6/12

   Most studies of human infidelity also show a frequency somewhere in the 20% range. Interesting that a similar rate is seen in other pair bonded species. The rate is high enough and consistent enough to suggest that there must be more to the phenomenon than a genetic left over.

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7. 7. jgrosay 05:00 PM 6/6/12

   I guess it has been shown that men in the higher income and power positions tend having more lovers than others, some kind of women rewarding success in a way similar to the herds of deers where the dominating male impregnates many females, but an explanation is yet to be found in facts as the rapid spread in pre-historic times of advantageous genetic traits such as persistence in adulthood of the enzymes that allow bays to use the milk's sugars in groups that relyed on cows, sheeps and goats sine milennia other than a tendency in women to make their man horny. However, a recent UK study in unselected newborns showed a rate of genetical non-matching between the baby and their supposed male parent never higher than 2 or 3%. The subject is interesting, and knowing the old roman assertion "father always uncertain", and the comments by Jesse Bering that point that a women can become pregnant from a man she never met, by one of his partners bringing her sperm from a third man the woman having had previous sex with her last men passing sperm from opens an ocean of uncertainties.

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8. 8. RobLL 06:11 PM 6/6/12

   Would there be an advantage for isolated populations to get some genetic diversity when the chance arrives.

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9. 9. 1oldsarg 06:40 PM 6/6/12

   And studies in the UK or other Western societies bear the burden of Western mores. Other places, other ways. In some pre-technological societies where it thought that a woman assembles her child from parts of various men, that's what she attempts to do. The tribal comedian gives her baby a sense of humor, the best hunter, valor, the shaman, connections to the spirit world, etc. Whether or not we share that belief, it has worked for them for centuries.

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10. 10. mggordon 07:46 PM 6/6/12

    I have a vision of a botanist studying a sapling in a forest and saying to his apprentice, "I wonder if any trees are nearby?"
    
    Vestigial male behavior is to be expected in females simply because very little genetic difference exists. Thus a strong inherited instinct in men must also have expression, but perhaps only slightly less so, in women. It need not have ANY evolutionary advantage and could even be moderately disadvantagous.
    
    Going the other way; same thing -- evolutionary traits strongly bound to women are very likely to manifest in men even where no evolutionary advantage exists, such as the traits sometimes associated with homosexuality.
    
    This happens because men and women happen to be the same species. I suspect that may come as a surprise to some, a hypothesis that needs further testing ;-)

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11. 11. mggordon 07:47 PM 6/6/12

    Duh. RTFA.
    
    "Reid is also investigating whether female promiscuity is a genetic by-product of the male version. If there are genes that encourage a male to be promiscuous, he might pass those genes on to his daughters, even though they may not benefit from the behavior."
    
    Exactly so.

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12. 12. GG 08:11 PM 6/6/12

    Yeah, blame it on the guys. Classic!

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13. 13. jtdwyer 08:17 PM 6/6/12

    The most important statement made in this article is: "...because the results need to be confirmed in other populations and other species."
    
    Why are (at least the reports of) behavioral studies of a very narrow sample population so commonly characterized as having universal implications?

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14. 14. blindboy 10:28 PM 6/6/12

    I think they measured the wrong thing. Infidelity would be an advantage if it resulted in more total grandchildren than faithful partnerships, regardless of whether the parent was from a pair coupling or an extra-pair coupling. They should compare pairs in which some offspring result from extra pair coupling with pairs in which no offspring from extra pair coupling occur.
    The hypothesis could then be that a genetically based high libido was the real advantage and the infidelity simply a by product of that characteristic.
    

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15. 15. lawrencekanyan 05:24 AM 6/7/12

    can't there be a simpler explanation. What if after committing the female realizes that the male partner is incapable and finds someone else who can take better care of the offsprings so that have better chances of survival.

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16. 16. jtdwyer in reply to jtdwyer 06:41 AM 6/7/12

    Perhaps to explain a little more clearly, if the reproductive behavior of birds applies to other species, including humans, then why do different species have differing reproductive behavior strategies?
    
    If I recall, there are species of rodents who are strictly monogamous while their very close cousins strictly play the field. General characterizations of specific species' characteristics cannot be reliably extrapolated to other species.

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17. 17. blindboy in reply to jtdwyer 07:44 AM 6/7/12

    Fair enough jt but when the same behaviour occurs in a number of species it is reasonable to consider that similar factors may be in play. It is a bit irritating when a study of birds immediately becomes a discussion of human sexuality but it is quite possible that the factors driving a bird species towards infidelity are related to the factors driving a species of primate in the same direction.

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18. 18. jtdwyer in reply to blindboy 07:48 PM 6/7/12

    Is there some basis for presuming that bird behavioral factors apply to humans?

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19. 19. bernardpalmer 12:17 AM 6/8/12

    blindboy said "The hypothesis could then be that a genetically based high libido was the real advantage and the infidelity simply a by product of that characteristic."
    
    Sounds reasonable to me.
    Here's an excerpt from 'What is the Primary Fundamental Right?'
    
    "Undressed fertile looking women in men's glossy magazines in the early sixties and seventies combined with low taxes probably had a lot to do with the success of the American economy prior to the wholesale adoption of big government in the nineties. Male sexual desire apparently promotes entrepreneurism which is the back bone of Capitalism."
    http://www.primaryfundamentalright.org/index.php?pageName=pfrWhatIs
    
    Always struck me as interesting that the words 'Slut' and 'Lust' use the same letters.

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20. 20. lpwagner 04:26 PM 6/8/12

    Nipples!

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21. 21. lpnetherton in reply to jgrosay 05:24 PM 6/8/12

    This makes just as much sense as the original article's conclusions. It's totally nonsense to put human morality into sexual behavior of animals, including humans.

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22. 22. JimStag 11:19 PM 6/8/12

    Years ago I read that female promiscuity in mammals could be measured by (indirectly, by observing) male testicle size. That highly promiscuous females cause males of the species to evolve / adapt to produce higher sperm counts, so that their DNA triumphs in a statistical battle for fertilization in the face of increased competition. By this benchmark, it was surmised (at the time) that human females must exhibit relatively high promiscuity - relative to other mammals - based on human male testicle size / overall body size ratio.

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23. 23. blindboy 05:15 AM 6/9/12

    Amongst primates and possibly other mammals, the greater the size difference between males and females, the less monogamous the species. Humans would seem to fiti neatly into this continuum being slightly sexually dimorphic and less than perfectly monogamous.

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24. 24. hcc2009 10:22 AM 6/9/12

    This study itself might be fine, but to draw any kind of conclusion from it about humans, or even mammals, is pure rubbish. And rubbish that I've seen repeatedly coming out of the "evolutionary psychology" discipline. Its practitioners seem to be on a mission to support tired old notions of western gender roles and the importance of the nuclear family. If these guys had taken even one class in cultural anthropology they would know that the nuclear family is a fantasy even in humans. And to infer something from birds? Please.

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25. 25. hcc2009 in reply to mggordon 10:26 AM 6/9/12

    Not surprising at all. Stephen Jay Gould wrote about this in, among other essays, "Male Nipples and Clitoral Ripples." His point is that there are some things you get solely from being a human being, whether you need them or not. While I would argue with the implication that the human clitoris is as useless as male nipples, still his point is a good one.

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26. 26. hcc2009 in reply to JimStag 10:28 AM 6/9/12

    Good one. And cows? Ha ha bulls have gigantic testicles. Most herd animal females are highly promiscuous -- I still recall our pony mare "smiling" at every male that went by -- which probably accounts for all the fighting among males. So yes it all kind of "hangs" together. Still making wild inferences like these is hardly scientific.

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27. 27. hcc2009 in reply to blindboy 10:31 AM 6/9/12

    Hard to see any particular correlation there. Among primates the size difference is greatest in gorillas and they are more monogamous than their cousins the partying bonobo with nearly identical sizes. Also herd animals mostly have near-identical sizes and they are highly promiscuous. Oh well.

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28. 28. zampaz 01:04 PM 6/11/12

    JT;
    As is common practice in the Scientific Community when referring to "Human Sexuality" we shall use the code words;
    "Birds and Bees." The former generally relating to the female of the species, the latter the male. Unless random use is used for further obfuscation purposes.
    Are you sure you're old enough to be reading these
    "Scientific" topics?

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29. 29. TangoVera in reply to r0b3m4n 05:02 PM 6/11/12

    Gosh, that wouldn't make any difference if they already had a mate.

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30. 30. GreenMind 05:50 PM 6/11/12

    I think there are some assumptions made in the article that may lead to unwarranted conclusions.
    
    First, the author seems to assume that the population is at equilibrium genetically. I'd like to know about the genetics of promiscuity in these birds. After all, evolution requires not merely variation and selection (both of which seem to be present) but the variation also has to be heritable. I'd like to know what proportion of the offspring of the promiscuous females are also promiscuous, and likewise what proportion of the offspring of the non-promiscuous ones. Given the extreme selection pressure against promiscuous behavior, it seems unlikely that the population is at a equilibrium genetically. Selection may have "used up" all the existing genetic variation, and further evolution would depend on new mutations (or immigrants with variants) that prevent promiscuity.
    
    Second, it is possible that migration is bringing genetic variants permitting promiscuity into the population from elsewhere where it is not at such a large disadvantage.
    

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31. 31. Sylvie Degiez 10:11 PM 6/11/12

    Only with extreme caution can we establish a link between animal behavior and human behavior. Yet not all humans are equally human: a few renounce the amazing opportunity only we have, to override instinct by rational analysis. For those, their lifestyle make them more akin to animals as they are ruled by instincts mostly. For others their choices makes them more akin to the sentient and analytical beings of our future.
    Anyway can someone tell me what is the purpose of such study?
    What are the applications of the possible findings?

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32. 32. GreenMind in reply to Sylvie Degiez 01:31 PM 6/12/12

    The purpose of this study, like most scientific studies, is to see if our hypothesis is right. As the article says, for a long time scientists have thought they understood why females are promiscuous, because the offspring may be more diverse. This study shows that the hypothesis is wrong, at least for these birds for the duration of the study.
    
    We can apply this study to humans very easily, by saying, "No, we don't really know why women are promiscuous."
    

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33. 33. GreenMind 01:44 PM 6/12/12

    Another factor not covered by this study is the fact that it is limited in time. Perhaps females really do benefit from having diverse offspring, but only once in a great while, such as when a disease hits the population. When that happens, diversity may be so critical to survival that it instantly (in a year or two) boosts the genes that permit promiscuity, or reduces the genes that cause fidelity.
    
    I'd also like to know if the study has enough data to be able to say whether it is really true that males boost their reproduction using promiscuity. I can think of reasons why it might not, such as if the effort to mate with females who are not their mates detracts from the effort to feed the offspring of their mate. Or perhaps the mate of those strange females somehow recognizes that those offspring are not his, and does not feed them, and therefore they are not fed enough to thrive.
    
    I think it's time to go look at the original article.

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34. 34. lvzee 09:20 PM 6/14/12

    I thought the best evolutionary strategy for females was to be relatively promiscous, but if possible, avoid letting any partner know that there were others. This enables them to accept gifts and protection from more than one mate. This also offers insurance in the event that a mate moves on or is killed.

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35. 35. estermazda 05:54 AM 6/19/12

    This is probably linked to dominance-induced behaviors,( which can take many forms in humans: fashion, big cars, stage, anything to get everybody's attention ) and concerns both sexes and is inherent to any social arrangement where several members of the same species interact. The passive or active mode of dominance is always beneficial for those involved, since equals are condemned to fight each other.

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36. 36. 4_2Discover 09:54 AM 6/19/12

    "...having higher quality offspring..." Uh, how is this known simply by cheating on a partner. Stuff happens as in the entropy department. Ergo, does quantity imply quality. Methinks not. Anyone else?

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37. 37. 1godsbutler 01:03 PM 6/25/12

    NOTE: We, as a species, shouldn't forget that natural trait called the "Motivational Triad" which is within us all, from the microbe to the mammal; There are 3 stages of this phenomenon... 1st: Pleasure or Seeking such, 2nd: Avoiding pain, and 3rd: To use or exert the least amount of energy. This piece of data should fit quite nicely with this nonplussed noun of nature, which I would call... evolutionary infidelity.-Dane Dino Butler

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38. 38. sdlawrence 09:06 PM 9/26/12

    "...a better way to understand why female promiscuity evolved...is to determine whether those illegitimate offspring actually have more babies."
    
    The study is completely flawed based on that one circular premise. Evolution is a balancing act of quantity versus quality, among other things. Bigger does not necessarily mean better, and smaller does not necessarily mean worse. Likewise, less or more offspring is not a meaningful metric unless you can explain, in specific context, why one might be preferable to another. Merely saying "more offspring" represents a "reproductive advantage" is meaningless, because "advantage" is ill-defined, or worse yet, defined a question-begging circular premise.
    
    Sea turtle infant mortality is responded to by gajillions of sea turtles that all hatch at once - with millions fo offerings to the destructive gods. So there's your argument for sea turtle polyandry, but even if it is correct, it does not necessarily explain polyandry of other animals.

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