May Be Key Factor In Immune System Evolution, Study Suggests
9, 2000 -- A
new study indicates that evolution of the immune system may be directly
linked to the sexual activity of a species. A comparative analysis
of 41 primate species demonstrates that the most promiscuous species
have naturally higher white blood cell (WBC) counts -- the first
line of defense against infectious disease -- than more monogamous
species. The findings will be reported in the Nov. 10 issue of the
"Our findings strongly suggest
that the most sexually-active species of primates may have evolved
elevated immune systems as a defense mechanism against disease,"
says principal investigator Charles L. Nunn, a research associate
in the Department of Biology at the University of Virginia. "We
looked at animal species with a range of mating behaviors and found
a strong relationship between high WBC counts and high promiscuity
in healthy animals. The more monogamous species have lower WBC counts."
The researchers compared 20 years
of data on average white blood cell counts for 41 primate species.
The 41 species 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 ten males per day while in heat. Other species have varying
levels of monogamy, including some that mate with one partner for
life. The researchers found a direct correlation between WBC levels
and mating behavior. Data for each species come from zoos and are
composed of veterinary reports of basal, or normal, WBC counts for
"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."
The 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 WBC counts far more than other disease risk factors.
"We expected to see a correlation
between WBC 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," says co-author John
L. Gittleman, U.Va. associate professor of biology. "This opens
up many new questions about behavior and the immune system."
The researchers also compared
mean WBC counts of humans to the various primate WBC counts, and
found that humans are on par with the more monogamous primate species.
"Based on this comparison, humans
are more similar to the more monogamous primate species," Nunn says.
The research is funded by the National
Science Foundation and the National Institute of General Medical
Sciences, and was conducted by three U.Va. scientist 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.
Contact: Charles Nunn, (804) 982-5746
or Fariss Samarrai, (804) 924-3778