Mating With Relatives? New Research Shows It Is Not a Big Deal in Nature

Wolves Mating Relatives

The study demonstrates that animals rarely attempt to avoid mating with relatives, a finding that was consistent across a wide range of conditions and experimental approaches. Wolves were among the species studied. Credit: Eric Dufour/Mostphotos

We usually assume that inbreeding is bad and should be avoided under all circumstances. But new research performed by researchers at Stockholm University, published in Nature Ecology and Evolution, shows that there is little support for this assumption.

The idea that animals should avoid mating with relatives has been the starting point for hundreds of scientific studies performed among many species. But it turns out the picture is more complicated.

“People assume that animals should avoid mating with a relative when given the chance,” says Raïssa de Boer, researcher in zoology at Stockholm University. “But evolutionary theory has been telling us that animals should tolerate, or even prefer, mating with relatives under a broad range of conditions for more than four decades.”

The study provides a synthesis of 139 experimental studies in 88 species spanning 40 years of research, settling the longstanding debate between theoretical and empirical expectations about if and when animals should avoid inbreeding.

“We address the ‘elephant in the room’ of inbreeding avoidance studies by overturning the widespread assumption that animals will avoid inbreeding whenever possible,” says Raïssa de Boer.

The study demonstrates that animals rarely attempt to avoid mating with relatives, a finding that was consistent across a wide range of conditions and experimental approaches.

“Animals don’t seem to care if their potential partner is a brother, sister, cousin or an unrelated individual when they are choosing who to mate with,” says Regina Vega Trejo, a researcher at Stockholm University and an author of the paper.

John Fitzpatrick

Dr. John Fitzpatrick, Wallenberg Academy Fellow, Department of Zoology, Stockholm University. Credit: Magnus Bergström/Knut and Alice Wallenberg Foundation

The study also looked at inbreeding avoidance in humans, comparing the results with similar experiments with animals.

“We compared studies that asked if humans avoid inbreeding when presented with pictures of faces that were digitally manipulated to make the faces look either more or less related to studies that used similar approaches in other animals. Just like other animals, it turns out that there is no evidence that humans prefer to avoid inbreeding,” says Raïssa de Boer.

“Our findings help explain why many studies failed to find clear support for the inbreeding avoidance and offer a useful roadmap to better understand how cognitive and ecologically relevant factors shape inbreeding avoidance strategies in animals,” says John Fitzpatrick an associate professor in Zoology at Stockholm University and the senior author of the study.

The findings will have wide reaching implications for conservation biology. Mate choice is increasingly being used in conservation breeding programs in an attempt to the success of conservation efforts for endangered species. What does this mean?

“A primary goal of conservation efforts is to maintain genetic diversity, and mate choice is generally expected to achieve this goal. Our findings urge caution in the application of mate choice in conservation programs,” says John Fitzpatrick.

Reference: “Meta-analytic evidence that animals rarely avoid inbreeding” by Raïssa A. de Boer, Regina Vega-Trejo, Alexander Kotrschal and John L. Fitzpatrick, 3 May 2021, Nature Ecology and Evolution.
DOI: 10.1038/s41559-021-01453-9

39 Comments on "Mating With Relatives? New Research Shows It Is Not a Big Deal in Nature"

  1. Clyde Spencer | May 3, 2021 at 3:02 pm | Reply

    Animals have little reason to avoid inbreeding. Nature is unforgiving with respect to recessive genes that decrease fitness or longevity, particularly if they cause death before reproductive age. Nature tends to weed out genes that negatively impact the survival of individuals. Animal mothers will sometimes kill and eat their offspring, presumably because they recognize a problem with them. On the other hand, wolf packs enforce abstinence on all except the dominant alpha male and female, which presumably carry the most fit genes.

    When new breeds of dogs are created, the objective is to obtain offspring as much as possible like the parents that carry the desired traits, without regard to relationship. Although, it is highly probable that the parents will be closely related. That does lead to some problems like hip-dysplaysia in some popular breeds; however, humans compensate for what would probably be fatal in a wild animal.

    On the other hand, humans go to great lengths to make sure that all their offspring survive as long as possible, even if they carry what are objectively defective genes and create a liability for both offspring and society in general. Also, there are issues of inheritance of property, wealth, and power, which get muddied by claims of survivors. Thus, there are ‘moral’ concerns established to discourage close breeding. Then there is the issue of jealousy, such as “launched 10,000 ships.” Humans are much more complex than animals and require self-imposed rules that allow us to live in some sort of relative harmony. Our incest taboos and laws are so severe that they probably create more emotional problems than they avoid.

    • SinaSinasina | May 3, 2021 at 10:21 pm | Reply

      I just find it odd that natural selection wouldn’t have influenced this behavior.
      Then again Lions certainly behave in a way that makes interbreeding unlikely..

      • Clyde Spencer | May 4, 2021 at 7:46 am | Reply

        Can you explain what you mean by lion’s behavior “makes interbreeding unlikely?” Lions live in a pride composed of several individuals, perhaps a couple of adult males and several adult females, plus young offspring. As far as I know, there is no social interaction to discourage any female in estrus from breeding with an available male. However, since the dominant male(s) will usually drive off other males he is not related to, it increases the likelihood of breeding with his daughters.

        • Male Lions who beat a lion with a pride of females will take over the pride and kill any cubs sired by the previous male. The lionesses go ito heat and the new male leader starts his own dynasty with non-familial members.

          • Clyde Spencer | May 4, 2021 at 8:24 am |

            Yes, a stray male may usurp an aging male dominating a pride. And, it is well known that he will kill the very young cubs. (The same is true with gorillas.) However, that doesn’t happen seasonally. It is usually several years before the ruling male declines enough to be at risk of being usurped. In the mean time, that old male has bred several times with his own daughters, and the new male doesn’t kill the females that are all closely related. This introduces new genes into the gene pool, but ignores the fact that for several years the gene pool was essentially stagnant. Then, with the new male, it has offspring with which it mates. Long term, gene diversity seems to convey survival value if the environment is changing. However, short term, it would seem that defective genes are weeded out quickly, but normal healthy genes, whatever the source, seem to work just fine. Where things get really interesting is when there is a mutation that conveys a significant advantage to survival, and that gene is propagated by inbreeding. From that, new species may evolve. However, without inbreeding, it would probably just die out.

    • “Our incest taboos and laws are so severe that they probably create more emotional problems than they avoid.” I disagree with this. Any person of even remotely healthy mind is not being harmed by these taboos and laws. I doubt that the masses emotional issues are coming from the fact that marrying grandma isn’t allowed. There is really nothing severe about these laws, in fact I will argue and say the laws need to be more severe. Too often children are molested or married off to family members because the laws are so lax. It’s too easy for these adults to get away with these incestuous behavior in regard to their own child under the safety of “culture” or “tradition”

      • Clyde Spencer | May 4, 2021 at 8:56 am | Reply

        Consider your choice of words — “molested” or “married off.” These are judgemental. In a normal, healthy family it is expected that there will be strong feelings towards each other, and living under the same roof creates an intimacy unlike that with strangers. Because there are both good biological and social reasons to discourage incest, the description of the action — molested — is purposely pejorative, even when both participants are consenting. It may well be that two siblings, or more commonly a man and his daughter, can have a caring relationship. However, it can’t be done openly and both are likely to feel guilt on several levels such as living knowingly in violation of the incest taboo, lying to others about it, and feeling unworthy. Even not committing incest, but feeling a sexual attraction may lead to the classic “Daddy Issues,” where a woman is attracted to an older man who reminds her of her father. I have known women whose first sexual experience was with an uncle, probably serving as a surrogate for the father, who was a ‘bridge too far.’ Both commission and non-commission of incest lead to psychological burdens. You would pile on to that with sending the man to prison (Rarely the woman unless it is for sex with a minor, particularly in a subordinate position, such as a student.) for an even longer period, and then the ‘victim’ would feel more guilt for her father or brother being in jail for participating in an act she may have enjoyed and desired.

        It is a complex problem. I agree that what we recognize as incest (although it varies with the culture) should be discouraged. However, I’m just suggesting that those like you can make the trauma inflicted by society even worse. In my opinion, it is a mistake to make the penalty far worse than the crime.

        In a perfect, simple world we wouldn’t have to deal with such issues. However, our world, is neither simple or perfect. Let’s not make it worse.

    • Actually when it comes to wolves, that’s not true. The alpha theory was formed by studying wolves in captivity, and the scientist that put it to reward has spent the rest of his career disavowing it.

      In the wild the reason there’s so much “alpha” preference is because packs consist of family groups. When there are conflicts in the family group, or it becomes large enough, members break off. This isn’t an option in captivity, so you see these alpha and omega relationships form, not unlike gangs or groups of humans in prison.

      In the wild a pack is much more fluid, and leadership is generally based on age, it’s not the constant violent struggle for dominance that’s been depicted. Though of course the corrections of a wolf parent are pretty intense by human standards. Likewise digs exhibit this behaviour, when in captivity, but frequently form more socially complex groups when they become feral and create their own packs.

      I find the claims about humans dubious (though it’s more likely the way it’s reported) because a facial recognition test is a very baseline test for whether we have a barrier to incest, and alone it would exclude any of the social behaviours that are important in human social bonds/interactions.

    • Kole Visinand | May 4, 2021 at 9:29 pm | Reply

      The only thing I find serious issue with here is the but about wolves. The idea of an ‘alpha’ the way himans think of it was a scientific mistake that was cemented in the cultural zeitgeist before it could be corrected. Wolves are far more cooperative than previously assumed, and a pack is often sinply a ‘family’ of parents and offspring, or a group of ‘friends’ of a few couples that work together to hunt, often splitting off into more groups as more pups are born.

    • As others have mentioned, wolf packs are just family units, similar to how humans organize themselves into families.

      > wolf packs enforce abstinence on all except the dominant alpha male and female, which presumably carry the most fit genes.

      This would be like saying that humans enforce abstinence in a household on all except the alpha father and mother in the house, presumably because they have the “most fit genes” (which, of course, sounds absurd).

  2. I wonder if humans are studying animals to become animals or not become animals !!!

  3. Steven Smithy | May 4, 2021 at 5:17 am | Reply

    This is great news!

  4. TrollStomper | May 4, 2021 at 5:42 am | Reply

    And thus the progressive push for normalizing incestuous relationships within society based on science. Because we’re all just a bunch of dumb animals anyway, right?

    • Kole Visinand | May 4, 2021 at 9:24 pm | Reply

      What about the right? I could argue that conservatives push insest with the Bible; just saying. (Adam and Eve’s descendents, Lot and his daughters, etc.). In fact in some ways it’s worse, since science can be changed, but religious texts can’t. In both cases, interpretation is important, and what people choose to do with that information equally so.

  5. I think the big problem is we, as scientists, can see the potential dangers of inbreeding. But thinking on an evolutionary level there is no disincentive to not inbreed.

    Even if it is detrimental to your offspring, genes don’t think. They don’t evolve immediately and drastically. In this way, genes and “evolution” are blind. Our genes don’t understand pros and cons of different types of breeding, they have the information they store and that’s it. Humans, with our brains and ability to think can recognize the issues with inbreeding… But in the tree of life, DNA leads to proteins, these proteins lead to series of events that culminate into reproduction. And we are just a product of eons of this evolution.

  6. Daezie Leigh K. L. | May 4, 2021 at 6:58 am | Reply

    Truly a well thought out and accurate, informative comment Sir. Kudos

  7. Daezie Leigh K. L. | May 4, 2021 at 6:59 am | Reply

    ** I meant Clyde’s. ^.^! ^^^

  8. Anastasia Beaverhousen | May 4, 2021 at 7:44 am | Reply

    Wolves also don’t have science class to teach them why their baby is deformed and humans did inbread back in the day thats how we learned it didn’t work 🤔 this is the worst article I’ve ever read! Is it a test to see how dumb humans are?

  9. Clyde Spencer | May 4, 2021 at 8:09 am | Reply

    “… humans did inbread [sic] back in the day thats [sic] how we learned it didn’t work …”

    And what was the lesson learned back in the day when small hunter-gatherer tribes had little opportunity to breed outside of with those in the tribe they were closely related to? That the tribes became extinct? And, ultimately all humans died out?

    While there is the classic example of those carrying the recessive gene for hemophilia, marrying those closely related allows the defect to be expressed and shortens the life span and reduces the quality of life. However, please explain why two healthy, closely related individuals with no recessive genes with negative impacts, who have exceptional, desirable qualities, such as a high IQ, would necessarily have defective offspring. Even regression about the mean only suggests that the children of such a couple may not be as bright as the parents, but would still be brighter than average.

    If you accept the Bible as being true, then how do you explain where Adam and Eve’s children found others to mate with who they weren’t closely related to? That story suggests that inbreeding was the norm “in the day.”

  10. Apparently the researchers never read up on european monarchies like Charles II.

  11. Genetically, small Zebra groups can no longer range freely due to farming and human habitation. Their young are being born with very different stripes than the adults.

  12. Jerzy Kaltenberg | May 4, 2021 at 2:57 pm | Reply

    it is possible that homo sapiens attitudes to inbreeding are function of the fact that inbreeding reliably produces individuals with lower IQ, sometimes fairly profound mental retardation. Given a choice of prospective mates, women are less likely to choose ones less fit, fitness being at least partly a function of success/social status / wealth.

    As population size grew, social status and fit became more important. We are a species which relates to the environment through our social bonds. What was once fairly unremarkable in small tribes/clans/populations is now rare, because our ability to function in society has replaced (for most of us) dependence on small, tightly knit family groups.

  13. Muhammad Abdulla Kashmiry | May 4, 2021 at 3:53 pm | Reply

    News from Alabama

  14. This article is written to promote a reaction and only presents one side of the equation then draws conclusions that are unsupportable. The fact that there may be either no difference or even a tendency to reproduce with a sibling or close relative does not preclude that the result of such behavior is damaging for the offspring. Nature’s way of dealing with this is simple, eventually the damaged line will not survive. However there is more than enough evidence to the contrary, that cross breeding with unrelated individuals strengthens the gene pool. So humans have rightly created taboos in society and practice cross breeding with domesticated animals to avoid having to live with the resultant damaged offspring. When the chances of a result are 1/1000 or 1/1M it looks great in s scientific article but who wants to BE the ‘1/’. I give this author a rating of 1* and a big boo for taking an interesting scientific study and presenting unsupportable and reactionary conclusions.

  15. The deformities in the Hapsburg line…

    • Clyde Spencer | May 4, 2021 at 4:41 pm | Reply

      There is a difference between something that can happen and will happen. Inter-marrying with close relatives that have defective genes will result in problems with the offspring. However, people who marry completely unrelated people sometimes have children with genetic defects. Either way, it is a tragedy. However, it is almost impossible to predict ahead of time if unrelated people will suffer the same problems generally expected (but doesn’t always happen) for inbreeding. I think that what the author of this article is trying to demonstrate is that the dire predictions for inbreeding may be an overreaction — another urban myth.

  16. This could be misleading. While animals are doing bad things, the effects may not be obvious initially. It could even be the natural process. However if humans could do better, we should not learn from bad behavior from others.

  17. Patrick Brasel | May 4, 2021 at 5:42 pm | Reply

    Jesus, Clyde. That was good, but I still didn’t like it.

  18. Alabama approves

  19. Kole Visinand | May 4, 2021 at 9:07 pm | Reply

    Even so, animals inbreeding often creates genetic problems. A great example are white tigers, the product of humans selfishly inbreeding them for a certain look. All white tigers are inbred, and the orange offspring of white tigers are tossed due to their health problems not being offset by their white coloring which is desirable to some humans for its ‘rarity’. The least unhealthy a white tiger can be is to be crosseyed, which all of them are, but most have far worse health complications than that. In essence, it is important when dealing with animals in captivity, where nature can’t fix itself, to avoid inbreeding. That may mean choosing their mates for them, apparently. Also, to anyone reading this, thank you, and I hope that together we can push back against cruelty to tigers and stop glamorizing things like ‘Sigfried and Roy’ or more recently, ‘Tiger Kings’.

  20. Tilly Foster | May 4, 2021 at 9:07 pm | Reply

    Nothing says lovin’ like marrying your cousin.

  21. No evidence that humans avoid inbreeding. Hmm my friend says your brothers cute I vomit in my mouth. I say to her, your brothers cute she vomits in hers. Till this happened I didn’t even think if my brother was cute I just assumed he wasn’t and never questioned it. And I’m assuming this isn’t a rare event considering the odds of being friends with soemone else who’s as rare as I am science says not possible.

  22. Jordan belfort | May 5, 2021 at 2:10 am | Reply

    One big fat no, is this comment hook/comment bait? Have u ever heard about habsburg family? And long chins and retardations? Its not even debatable.. Where to report for comment bait? I mean if u are checking me for spam i can check u for comment bait?

  23. Genetic mutations in offspring making them less adapted to the environment is the consequence of sustained inbreeding. So, does it make sense for DNA to produce a genetically inferior host body that’s less likely to be selected by a mate for further reproduction or dies off due to health complications? I say inbreeding is counterintuitive given this perspective. However, if we take the good with the bad then one of such mutations or defects must be significant enough as to be an adaptation to an environmental selecting agent that would otherwise eradicate the species.

  24. Because Drosophila melanogaster mate with next of kin, homo sapiens should too?
    Cannot Stockholm U study something of benefit to man?
    We are all dumber now having read this article.
    This study should be up for a Darwin Award!

  25. JoAnn Leichliter | May 5, 2021 at 2:36 pm | Reply

    Wolves, as those who have observed them closely over the years can tell you, actually have strong incest avoidance. This has been observed in many captive wolves, too, although (undetstandably) it is less absolute.

  26. I’m trying to understand what was the need to compare humans? No matter how you alter a photo most people would avoid inbreeding if they’re aware, as humans have a more complex ability to think in comparison to other species. Just seems like an odd topic to research to me since there’s no reason anyone should knowingly go down that road

  27. Curiousityman17 | May 6, 2021 at 3:40 am | Reply

    Isn’t the Cheeta population experiencing problems partially due to Inbreeding?

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