Cheating cheetahs prosper
Where's the evolution?
Genetic variation is a key ingredient of evolution. Natural selection acts on the genetic variation present in a population to remove those variants that fail to produce offspring in a particular situation and spread those variants that are particularly good at producing offspring. A population with no genetic variation (in which every individual is genetically identical) cannot evolve in response to environmental or situational changes. If, for example, a genetically uniform population were exposed to a new pathogen and did not carry the gene versions necessary to fend off the disease, the population could face complete extinction. On the other hand, a population with high levels of genetic variation is much more likely to include at least a few individuals carrying the gene versions that provide protection from the pathogen and, hence, to evolve in response to the new situation instead of going extinct. A population with low genetic variation is something of a sitting duck vulnerable to all sorts of environmental changes that a more variable population could persist through.
And unfortunately, those are exactly the circumstances faced by cheetahs today. As a species, cheetahs have famously low levels of genetic variation. This can probably be attributed to a population bottleneck they experienced around 10,000 years ago, barely avoiding extinction at the end of the last ice age. However, the situation has worsened in modern times. Habitat encroachment and poaching have further reduce cheetah numbers, consequently snuffing out even more genetic variation and leaving cheetahs even more vulnerable to extinction.
Nevertheless, conservation biologists may have reason to hope that this genetic variation will not drain away as quickly as was thought now that researchers from the Zoological Society of London have laid bare female cheetahs' cheating hearts. The scientists have found that not only do female cheetahs bear single litters with multiple fathers, but those fathers are rarely near neighbors. Females seem to mate with individuals from far-flung regions, meaning that the cubs' fathers are only distantly related to one another. Furthermore, female cheetahs don't even return to the same males year after year: consecutive litters from the same mother all had different sets of fathers. In short, female cheetahs' mating habits wind up getting genetic information from lots of different fathers into the next generation and that helps preserve genetic variation!
In fact, the impact of multiple matings on genetic variation may help explain how the trait evolved in cheetahs in the first place. Biologists hypothesize that in an unpredictable environment, like the Serengeti, having variable offspring would have been advantageous to a female cheetah. Even if several of her cubs were killed by a new disease, succumbed to a novel environmental stress, or just didn't have what it took to make a living in the Serengeti, a female with a variable litter could still hope that one of her cubs would have "the right stuff" to survive (shown in Scenario 1 below). Biologists refer to this as "bet-hedging" not putting all your eggs (or in this case, cubs) in one basket. On the other hand, a female with a more genetically uniform litter might not have any of her cubs survive their dicey environment (Scenario 2 below). In this case the female who mated multiple times and had variable offspring would pass her genes on to the next generation, while the female who mated singly and had a more uniform litter would not. Over time, if this imbalance persisted, natural selection would favor females with genetically variable litters and hence, females who engaged in multiple matings. So perhaps, what we humans perceive as promiscuity is actually an adaptation that allows female cheetahs to increase the odds of having at least one cub survive to pass on her genes.
While it's certainly possible that the multiple mating strategy spread because of its impact on the genetic diversity within litters, biologists have also come up with two main alternative hypotheses to explain why multiple mating is common in cheetahs:
Figuring out which of these three hypotheses (if any) is the right one will require further research. But regardless of what we learn about the evolutionary origins of multiple mating in female cheetahs, its evolutionary consequences today are clear. The fact that female cheetahs bear young with many different fathers helps preserve what little genetic variation the species has left and could even buy us some time in our efforts conserve these endangered animals.
Discussion and extension questions
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Cheetah photo courtesy of John White; Photo of Dada Gottelli in the field provided by Dada Gottelli
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