Question
So as many people I've watched the Lion King last weekend and have been reading a lot of theories about it online since then. One that stood out to me is that Simba and Nala are very close-related (they are cousins in the best case scenario, or maybe brother and sister or brother/sister in law). Now I've read that some lion species handle this issue in their ways. Some would just kick all the male cubs out of the tribe so they can build theirs, while in other cases the alpha male would just kill all the male cubs that aren't his. Now I'm curious about all this. Does this kind of close relationships have the same effects on lions as on humans? I know for a fact that it's not a problem with rats or sheep, which are mammals as well. I'm also not sure as to why killing male cubs solves the issue without making it harder for the tribe to grow. Thank you for reading this, I hope I posted in the right community.
Answer
The term is "inbreeding depression," which means the reduced fitness of a population as a whole due to inbreeding. The effect comes naturally from the way genetics works, and so just about every species could be affected by it if for some reason there was a small isolated population and thus a small amount of genetic diversity. In this sort of case, if an individual has an undesirable gene, this gene is much more likely to become widespread in the population, as the gene pool is very small.
A known example of this effect is with domesticated animals. When people selectively breed animals for specific traits, the way to do this is to choose a few individuals with those traits for breeding, but it's then easy to not have enough genetic diversity and have inbreeding depression. This is why dog breeds are commonly prone to various problems, like Dalmatians are highly prone to deafness, or many large breeds are prone to hip dysplasia. Domesticated rats are also affected by this, with much higher rates of cancer than wild populations.
As you said, this effect also applies to humans. A well known historic example is members of the European royal families being prone to hemophilia due to their tendency to only marry other royals, which is a very small population.
However, while inbreeding depression is a possible risk for any species, it's not a given that inbreeding will doom the species. There are known examples of species surviving despite low genetic diversity, or species even having reproductive strategies that tend to result in inbreeding despite these problems. One example is the cheetah, which has a very low genetic diversity throughout the entire species, and it is theorized some catastrophic event happened in the past that reduced the species to just a few individuals. Cheetahs managed to not go extinct at that time, but the result was a population with very little genetic diversity. Another example is the naked mole rat, one of the few mammal species that live in colonies much like ants or bees. These colonies don't intermix much, and so they become rather inbred.
As to how this all affects lions, right now the reality is because lions are quite endangered, with their populations quite fragmented, there are actually problems with lions and inbreeding depression. http://lionalert.org/page/inbreeding-depression-in-lions
But, in a more natural environment, not affected by humans, the way inbreeding is generally prevented is that male lions usually do not stay with the pride long enough for that to happen. Other male lions are constantly competing for their place in the pride, and so the rotation tends to happen quicker than the cubs can mature. Young male lions are not yet able to compete for the dominant male position, and so get chased off by the dominant male when they get old enough to start looking like competition.
As for the male lions killing cubs, it's a well known and pretty common phenomenon, but it's also not actually a prevalent an event as you might think, given how much it's discussed. It's also not to prevent inbreeding, as a male lion is more likely to kill cubs that are not related to it, as usually it happens when a new lion first takes over a pride. It's generally thought that it has to do with it not being to the male lion's advantage to put resources into cubs that aren't his, especially when you consider his time with the pride will only be a few years at best.
Answered By - Kai