When it comes to wooing females, it’s not just the size of a male gazelle’s antlers or the boldness of his personality that counts—it’s the number of parasites he’s harboring. All Grant’s gazelles (Nanger granti) have intestinal worms, but according to a study presented in Princeton, New Jersey, at the Animal Behavior Society meeting last week, those males able to defend land and a harem begin their reign with a relatively low number of parasites. A male spends his days in the Kenyan savanna in a small bachelor group until he’s able to race faster and leap higher than other males, becoming a top dog. As he makes this transition, his testosterone levels rise. The hormones, in turn, suppress his immune system, making him more susceptible to acquiring worms. His risk of infection rises further with every female that joins his harem, because they also carry the parasites and deposit the eggs on the grasses he eats. Over time, because of his increasingly heavy parasite load, the male can no longer chase off competitors; he loses his harem and becomes a lowly bachelor again, until he’s shed the worms. Then the cycle begins anew. Failing territorial males treated with a deworming medication were able to regain their vigor—demonstrating the parasites’ key role in the males’ journey from bachelor to top male to bachelor again. “It’s the first time to see a complete cyclical picture where behavior modifies an animal’s risk of acquiring a parasite, and how that infection, in turn, modifies the animal’s behavior,” the study’s lead author Vanessa Ezenwa, a disease ecologist at the University of Georgia, Athens, told the meeting.