An analysis of inbreeding studies suggests animals often keep it in the family

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Animals don't avoid mating with blood kin as much as previously thought. (Unsplash/Dušan veverkolog)

Animals aren't picky when it comes to mating with kin, according to researchers who say their findings challenge the widely accepted view that animals steer clear of inbreeding.

In a study published May 3 in Nature Ecology & Evolution, researchers examined the prevalence of inbreeding avoidance through a meta-analysis of 139 studies in which animals chose between making babies with an unrelated partner or a family member. It hinted that smaller studies not favoring the prevailing hypothesis were not published, suggesting that inbreeding may be even more common than the new findings indicate.

"Animals might want to avoid inbreeding because it can have negative effects in the offspring. For example, offspring resulting from inbred mating can be less fertile or have a shorter lifespan," said the study's lead author Raïssa de Boer, a postdoctoral researcher in zoology at Stockholm University. "That is why we commonly assume that animals avoid inbreeding."

There are a few different strategies that animals may use to reduce incestuous reproduction.

"For example, if animals live in very large populations or if one sex disperses from the [birth] site, then they will probably rarely encounter a relative," de Boer said. 

When animal species are likely to encounter family members during mating season, they may evolve ways to recognize kin and choose not to mate with them. And if related individuals do have sex, there are other ways to reduce inbreeding during or after the act. For example, mating duration between kin could be shorter or females may show preference for sperm of unrelated males over that of family members. 

There have been many studies looking at inbreeding avoidance in animals, de Boer said. "But because a lot of these studies were done with a single species, we were still lacking the big picture. Therefore, what we did was summarize all of these experimental studies using a meta-analysis to try to answer the question, do animals actually avoid inbreeding or not?"

de Boer and her colleagues pulled data from 139 experimental studies conducted over 40 years on a range of different animals such as primates, mice, birds, fish and insects. The studies evaluated interactions between individuals of varying levels of relatedness, and they looked at different inbreeding avoidance strategies, including choosing not to mate as well as those that occur during or after mating.

Overall, the analysis found only weak support for the hypothesis that animals avoid inbreeding. 

"What we find is contrary to what we usually expect. Animals don't really seem to care if they mate with a relative or not," de Boer told The Academic Times. "And we find that that result was pretty consistent across different experimental conditions and whether we looked at males or females." 

The analysis also included studies that examined mate choice in humans. In such studies, people rated the attractiveness of images that had been modified to look more or less like themselves, mimicking varying levels of kinship.

"Humans find pictures that look more or less like them equally attractive. So we find that humans didn't really differ from animals in that sense," she said. "This seems particularly unexpected because it's very ingrained in humans to be disgusted by inbreeding."

Through their analysis, the researchers found evidence for bias against publication of studies that refuted the idea of kin avoidance, which they said could be partly due to human aversion to incest

"Publication bias means that studies that are in line with a certain hypothesis are more likely to be published than studies that go against the hypothesis," de Boer explained, meaning that inbreeding avoidance could be even less common than the meta-analysis indicates.

According to de Boer, the research could have implications for management of endangered animals in the wild and captive breeding programs.

"In endangered species, you want to have as much genetic diversity as possible, but if they start inbreeding this will decline very rapidly," de Boer explained. "Because of human impacts, there are many animal populations that are fragmented. As [populations] become smaller, animals might be faced with a choice of mating with a related or an unrelated individual more often." 

"We always assumed that animals avoid mating with relatives even if they encounter them. But this study shows that they don't differentiate," she continued. "It's important to keep in mind that you can't rely on animals choosing an unrelated individual as their mate."

However, de Boer pointed out that most studies on inbreeding avoidance have been done in laboratory settings, so it's unclear how these findings apply to wild animals.

"Animals living in the wild have a very different life and face different challenges than animals in laboratories, and these differences could alter the costs and benefits of mating with a relative," she said. "Finding out whether or not animals in the wild avoid mating with relatives is a key next step that follows from our work." 

According to de Boer, the analysis challenges a widely held assumption that animals avoid inbreeding, but she says that the findings weren't "fully unexpected." Other researchers have previously proposed that animals may tolerate or even prefer to mate with related individuals in some scenarios, such as cases where animals have no other option but to mate with a relative.

"Maybe it's better to have offspring with a related individual than no offspring at all," she explained. 

It's also possible that mating with kin isn't always detrimental. Inbreeding has been shown to have negative impacts on offspring survival or boost the risk of diseases, but these effects haven't been evaluated in all species.

The study, "Meta-analytic evidence that animals rarely avoid inbreeding," published May 3 in Nature Ecology & Evolution, was authored by Raïssa A. de Boer, Regina Vega-Trejo and John L. Fitzpatrick, Stockholm University; and Alexander Kotrschal, Stockholm University and Wageningen University & Research.

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