The flashiest examples of parasite manipulation are those that kill, dramatic behavioral changes that hasten host death and ensure transmission. But what about the quieter strategies? Some parasites benefit from particular endocrine environments and can manipulate host hormones for their own development. Using nearly 20 years of data from wild geladas infected with the larval tapeworm Taenia serialis, we show that–contrary to expectations of sickness responses–infected males have 65% higher estradiol, a 151% higher risk of losing reproductive access, and produce about 1.5 fewer offspring over their lifetimes. Androgens are maintained or elevated, hinting at compensatory responses in an ongoing evolutionary arms race. Subtle manipulation, major reproductive costs, and a rare window into endocrine warfare in the wild.
