Sexual promiscuity in primates may lead to a stronger immune system, according to a recent study headed by Dr. Charles Nunn, a University postdoctoral biology research associate.
After years of studying white blood cell counts of various monkeys, Nunn and other University scientists have determined that having many sexual partners increases a monkey's chance to fight disease.
Their research focused on 41 different species of monkeys, ranging in promiscuity from the monogamous white-handed gibbon, to the libidinous Barbary macaque, whose females mate with up to 10 different males every day while in heat.
The scientists were interested in three factors that influence disease risk - social, ecological and sexual factors.
Animals living in big groups are exposed to more diseases. In very social species, some diseases tend to show up more often, but there was no increase of white blood cell (WBC) count, which would indicate a stronger immune system. This happens because species learn to evade disease risks, Nunn said. Animals in large groups can avoid the sick members of the group.
Ecological migration patterns also affect disease risk. Monkeys that swing in trees are exposed to different diseases from the ones that walk on the ground because they are in contact with different environments. Again, species learn to evade disease risks by changing their normal ranging patterns so they don't walk where they defecated.
Nunn and his team were surprised to find that while these social and ecological aspects affect risk factors, they apparently do not influence WBC count. Only sexual factors appeared responsible.
One possible explanation is that in social and behavioral situations, the ability to avoid disease is possible. However, in many species females have no choice of a mate; more aggressive males simply go about their business in a random fashion.
Symptoms of many sexually transmitted diseases are latent, causing difficulty in distinguishing members of a community that suffer from STDs.
But when fighting STDs, abstinence isn't always an option because if a species doesn't engage in sexual behavior, it doesn't reproduce and it dies out rather quickly.
There must be some other way to defend against disease. The immune system evolves to permit sex without killing the species.
Another justification for implicating sexual factors is that the body's defense mechanism is closely related to the infection strategy used by the disease.
"Sexually transmitted diseases live forever ... they tend to hide from the immune system," said Nunn. Such diseases tend to have periodic eruptions that come and go. On the other hand, maladies such as the flu get into the body, reproduce extremely quickly and pass on before being wiped out by the immune system. "Their purpose is to get from me to you," said Nunn.
Because promiscuous species are more exposed to STDs, their immune systems are better at fighting off those diseases. This correlates with the higher WBC count in these monkeys.
"This paper raises a lot of questions" about the workings and evolution of the immune system, said Dr. John Gittleman, an associate professor of biology and co-author of the study. The sheer breadth of the research required extensive cooperation among the three researchers.
Nunn added that "none of us would have done this research individually ... there are new hypotheses to test" as a result of this collaboration.
Nunn, Gittleman and Biology Prof. Janis Antonovics, the third co-author of the study, take a specifically evolutionary view of how and why the immune system in these primates has evolved and which factors and disease strategies affect this evolution. In the future, they will investigate the factors that lead to the evolution of diseases. Researchers also want to explore characteristics and factors that affect the evolution of the immune systems in other species.
"We're very interested to determine whether this pattern extends to other groups," Gittleman said.
University researchers were interested in how diseases work, from the initial infection through the disease's eventual defeat by the immune system, he said. They will look at which diseases occur in which monkeys and how that relates to where they live and what they do, on a day to day basis as well as sexually.
Future research at the University may include other species, such as bats, which are also highly promiscuous. In addition, researchers will be looking into diseases caused by bacteria and viruses, how often and why they show up in other species, and how this affects the variation and evolution of immune systems in more species.
Unfortunately, until this research is proven to be applicable to humans, there's no way to tell if promiscuity could help your chances of fighting off disease.
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