F. McComas, USC
macaques (Macaca sylvanus), a primate species found in Northern
Africa, were found to have a naturally high white blood cell
count. U.Va. researchers think it is an evolutionary response
to the macaque's promiscuous mating behavior, especially the
female's. In their social system, males also take an extensive
part in caring for the young.
could be the key factor in immune system evolution, study suggests
A new study indicates that evolution
of the immune system may be directly linked to the sexual activity
of a species. Reported in last week's issue of Science, the comparative
analysis of 41 primate species demonstrates that the most promiscuous
species have naturally higher white blood cell counts -- the first
line of defense against infectious disease -- than more monogamous
findings strongly suggest that the most sexually active species
of primates may have evolved elevated immune systems as a defense
mechanism against disease," said principal investigator Charles
L. Nunn, a research associate in the biology
department. "We looked at animal species with a range
of mating behaviors and found a strong relationship between high
white blood cell counts and high promiscuity in healthy animals.
The more monogamous species have lower counts."
researchers compared 20 years of data on average white blood cell
counts for 41 primate species. The species studied represent the
major primate evolutionary groups and the full range of mating
behaviors. Some of the species are highly promiscuous, such as
the Barbary macaque, whose females may mate with up to 10 males
per day while in heat. Other species have varying levels of monogamy,
including some that mate with one partner for life. Data for each
species come from zoos and are composed of veterinary reports
of basal, or normal, white blood cell counts for healthy females.
"The implication of our finding is that the risk of sexually
transmitted disease is likely to be a major factor leading to
systematic differences in the primate immune system," Nunn
says. "This puts the evolution of sexual behavior in close
relation to the evolution of the immune system."
researchers also compared other behavioral and social factors
that might affect the animals' immune systems, including high
population density, which increases the risk of exposure to disease,
as well as exposure levels to soil-borne pathogens, namely fecal
contamination. They found that mating promiscuity affected white
blood cell counts far more than other disease risk factors.
"We expected to see a correlation between white blood cell
counts and various behavioral and ecological factors, but were
surprised to find that sexual activity appears to be the key factor
in how the immune system develops," said co-author John L.
Gittleman, U.Va. associate professor of biology. "This opens
up many new questions about behavior and the immune system."
researchers also compared mean white blood cell counts of humans
to the various primate white blood cell counts.
"Based on this comparison, humans are more similar to the
more monogamous primate species," Nunn said.
research is funded by the National Science Foundation and the
National Institute of General Medical Sciences, and was conducted
by three U.Va. scientists in the department of biology: Nunn,
who specializes in primates; Gittleman, who uses computational
methods to study evolution; and Janis Antonovics, who studies
sexually transmitted diseases.