Research on one species of African cichlid has revealed how social factors can sometimes make mating with a close relative evolutionarily advantageous.
Animals in the wild often avoid close kin as mates, as inbreeding causes harmful genes that might otherwise recede into the background to manifest in progeny more often. While animal breeders often practice inbreeding to cultivate desirable traits, they must then cull unfit offspring. However, recent theoretical predictions suggest that, at times, the benefits of inbreeding might outweigh the costs. Now evolutionary biologist Timo Thünken at the University of Bonn in Germany and his colleagues has discovered real-life evidence in support of these predictions. The scientists investigated the African cichlid Pelvicachromis taeniatus, a small monogamous fish that lives in the rivers and creeks of Cameroon and Nigeria. Males occupy caves, while females compete with each other for males.
“We initially wanted to investigate whether P. taeniatus avoid kin as mating partners, because it has been shown in other species that inbred offspring have disadvantages–for example, increased mortality,” Thünken said. “First, we conducted a female choice experiment,” he recalled. This involved aquariums with breeding caves for males and hiding places for rejected females. Against our expectations, females did not avoid brothers, but even preferred them,” Thünken told LiveScience. This proved true in 17 of 23 experiments.
Both parents in the species care for their young to protect them against predators, the researchers noted. This requires high levels of cooperation. Since kinship generally favors cooperation, Thünken and his colleagues theorized related parents did a better job of cooperating than non-kin. Their observations supported their ideas, finding that inbreeding pairs spent significantly more time accompanying their free-swimming young. They also discovered males of inbreeding pairs spent significantly more time guarding breeding caves and were half as likely to attack their mates. The researchers curiously found that inbreeding did not appear to lead to higher rates of harmful gene expression. However, Thünken and his colleagues noted inbreeding might affect traits they have not yet studied, such as thefertility of offspring.
The scientists plan to look next at the level of inbreeding in natural populations of the fish, the fitness consequences of inbreeding and the mechanisms of kin recognition in the species.
BACKGROUND: Negative effects of inbreeding on components of fitness (inbreeding depression) are well documented, but the extent to which social behaviour may mitigate such effects in animal societies is largely unknown.This is perhaps surprising, as social species exhibit a high potential both for inbreeding (typically living in kin-structured populations) and for cooperative behaviour to mitigate the negative effects of inbreeding. Recent evidence, from studies of non-social species, that the effects of inbreeding can indeed be mitigated by good environmental conditions lends strength to this possibility.
[…] AIM: To test for the first time whether cooperative behaviour can indeed mitigate the negative effects of inbreeding, using a long-term field study of the cooperatively breeding white-browed sparrow weaver as a model system. Sparrow-weaver parents are assisted with the rearing of their offspring by 0-12 helpers, which lighten parental workloads and increase the total rate at which the chicks are provisioned. The project will therefore investigate the extent to which helpers mitigate the effects of inbreeding on the fitness of both parents and offspring.
Isn’t it great, how humans are able to get extended family – parents, siblings, uncles, aunts, in-laws – to help raise our children? Social factors are extremely important for the success of human genes, more than for most species.
This is exactly the kind of research needed. Hopefully there will be more and more like this in the coming years as evolutionary biologists start to realize how badly understood endogamy is. I hope this project gets plenty of funding.
Scientists say a type of banded mongoose living in Uganda has one of the highest rates of incest of any mammal species, and the reasons behind that rate aren’t entirely clear. For the most part, mammals avoid incest and inbreeding, since it tends to cause a drop in fitness, with offspring of incestuous mating often suffering from health problems, a team of British and German researchers points out. Groups of banded mongoose living in Uganda’s Queen Elizabeth National Park are apparently an exception, the researchers say in their study published in the journal Biology Letters.
The researchers followed 14 mongoose groups resident in the park — an average group is a close-knit gathering of around 18 adults — and found most had dominant male and female members doing most of the mating, while animals on the fringes of the group seldom mated. The close-knit nature of the groups has resulted in an elevated rate of incest, the researchers found; 63.6 percent of newborn mongoose pups were the outcome of mating between two members of the same social natal group.
The high rated of incest may be linked to the dynamics of mongoose grouping, the researchers suggest. Newly formed groups have mortality rates three times as high as established groups, and any mongoose that attempts to move into a new group is generally turned away, often violently, they say. The result, they suggest, is that the animals have learned that with little opportunity for dispersal it’s safer to mate with a close relative within a group than risk death by venturing out into the world to find a mate.For mongooses, apparently, the genetic problems caused by inbreeding are acceptable as a less dangerous choice than leaving the pack in search of new mates.
Within the Uganda mongoose bands male mongooses were observed mating with their daughters, the researchers report, but there were no such mating observed between females and their sons. That’s not really a surprise, they say; males take much longer to reach sexual maturity than females, who have shorter lives and generally die before their own male offspring are old enough to mate with them.
This isn’t all that surprising. Every time such evidence comes out, everybody’s flabbergasted because they’re not paying attention to all the other cases. As genetic studies of animal mating patterns improve, we’ll likely find more and more cases like this.
Endogamy is a risky proposition, but everything in life is. The question for everything in evolution is: how risky, and for what reward? Endogamy and exogamy are just mating strategies. How often a species practices endogamy is related to the specific evolutionary circumstances of that species, including the social environment, which determine if and when mating with a close relative is worth the gamble. For some species the cost is too high. For others, it’s not. And for still others, it’s in between. I wonder where humans fall on that spectrum?