Like many male primates, male chimpanzees compete to establish dominance—and decide who gets more access to females. Higher-ranking chimps have more testosterone, which is associated with aggression. But testosterone isn't necessarily healthy. For example, it suppresses the immune system. Researchers watched 22 male chimps in Kibale National Park in Uganda, waited for them to poop, and collected fresh fecal samples. From each sample, they extracted two useful pieces of information, they describe in BioPsychoSocial Medicine: How much testosterone the chimp had and how many species of parasites lived in his gut. Higher-ranking males had both more testosterone and more parasites, possibly because the testosterone suppressed their immune system. So females may be impressed not only by a male's manliness, but by his ability to withstand the excess onslaught of parasites in his gut.
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