Testosterone in Male Chimpanzees lead to more parasites

Like many primates, male chimpanzees compete to establish dominance and get 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 defecate and collected fresh fecal samples. As they report in an article in BioPsychoSocial Medicine, they extracted two useful pieces of information from each sample: how much testosterone the chimp had and how many species of parasite were living in its gut.

Higher-ranking males had both more testosterone and more parasites, possibly because the testosterone suppressed their immune system. (The researchers used 1,700 hours of observation data to figure out dominance rankings.) So females may be impressed not only by a male's manliness, but by his ability to withstand the excess parasites in his gut.
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