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Q. What do fondness for orgies, testicle size, and number of white blood cells have in common?

A. They all correlate.

So suggests research, published in Science November 10, by University of Virginia biologists, who were considering the relation between sexual promiscuity and the robustness of primate immune systems. Their conclusion? The more sex, the more immunity.

The new study doesn't mean sleeping around fortifies your immune system and grows your bangers. The correlation works only on an evolutionary scale. Dr. Charles Nunn and colleagues considered primates which as a class display a remarkable variety of sexual appetites ranging from the gibbon's monogamous lifelong pairings, to the randy and rambunctious Barbary macaque, whose females, when in estrus, will take on some ten males a day.

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The resources different primate species invest in disease fighting as measured by average number of white blood cells per unit blood varies widely, also. Nunn wanted to see if there was a connection between a species' promiscuity and the biological capital it invested in immunity.

Amour's armor

There's reason to imagine such a link sex, like any other biological interchange opens doors to infection. The more sex with different partners, the greater risk of catching a bug a rule that AIDS has brought home to humans after a lull when penicillin and other "magic bullets" seemed to keep sexually-transmitted disease at bay.

But there are other factors besides sex that come to mind at least a biologist's mind when talk turns infection vectors.

Shit, for instance. Feces don't just happen, they tend to fall. The more a species mucks about on the ground as opposed to, say, flying through the air or clambering in trees the more exposure to shit its own and that of other creatures and the toothier disease-fighting apparatus it needs. To be terrestrial is to be on guard.

Another correlate of infection is group size. The more gregarious an animal, the greater risk of picking up bacteria and viruses from his neighbors. When humans lived in isolated hunter-gatherer bands, for example, a band might get wiped out by infection, but there were no epidemics, which arise only with civilization's clustering of population.

So with three plausible reasons why primates would need more disease-fighting equipment promiscuity, terrestriality, and gregariousness the Virginia researchers crunched the numbers to see which one correlated best to the variations found in white- blood-cell counts.

Just to be sure about the promiscuity, Nunn also looked at testicle size since the more partners a species' females have, the more males compete to reproduce with rivals who have also left spermatic calling cards, the more copious ejaculate males need, and so the bigger their balls.

The results were clear only differences in promiscuity predicted the differences found in white blood cells. The model pegs humans as basically monogamous, with a soupçon's impulse toward promiscuity just about what human testicle size predicts.

What's the reason for the tight link between promiscuity and high white-blood-cell count? Nunn theorizes that infections from the ground and from neighbors tend typically to be hit-and-run jobs like the flu calling on the body to produce a lot of white blood cells on short notice. But many sexually-transmitted diseases stick around, burrow deep into the body's workings, and require to beat them back an immune system operating at a consistently higher baseline. Certainly many STDs fit that bill herpes, syphilis, gonorrhea. Nunn suggests that AIDS may not be the epidemiological exception, but that virulent sexually-transmitted disease are a long-standing problem for primates, at least, shaping our evolution.

Case closed? Not necessarily. Correlation isn't causation. The Virginia researchers took white blood cells as a marker of immune robustness, but they haven't yet shown that having more of them helps fend off sexually-transmitted infection. And maybe, suggest other biologists, the Virginians have it ass-backwards: the pumped-up sexual hormones that push a beast into promiscuity might themselves boost the immune system.

Whatever the direction of causation, the new research suggests that on an evolutionary scale, promiscuity and robust immune systems go hand-in-hand or shall we say "penis-in-orifice." But it still might turn out that to boost immunity, an orgy could be just what the doctor ordered.