A pair of studies led by Indiana University researchers provide new evidence that when it comes to evolution, the testes may play a key role. The research, led by Kimberly Rosvall, assistant professor in the IU College of Arts and Sciences' Department of Biology, finds that the testes -- or gonads -- have a greater impact than previously thought in evolution. The research was conducted in two subspecies of dark-eyed junco, a type of American sparrow.

The white-winged junco, or Junco hyemalis aikeni, is found in the Black Hills of South Dakota. The slate-colored junco, or Junco hyemalis carolinensis, is from the Appalachian Mountains in Virginia. The first is larger and more aggressive; the other is smaller and more docile.

The studies are published in the journals of Hormones and Behavior and of Integrative and Comparative Biology.

The first paper compares the subspecies in their expression of enzymes that make testosterone within the gonad. The second paper investigates how the subspecies' gonads differ in the expression of stress hormone receptor genes, which are known to lower testosterone.

"The majority of endocrinologists will tell you that testosterone-mediated traits such as physical appearance or behavior are regulated in a top-down fashion -- that the brain acts as the 'CEO,' telling the rest of the body what to do," Rosvall said. "But our data suggest that this CEO model is oversimplified, and that the workers in the testosterone 'factory' -- the gonad -- may actually play an important role in trait divergence."

Compared to the Virginia juncos, the South Dakota birds are not only larger and more aggressive, they also boast flashier plumage, including more white tail feathers and prominent white bars on their wings. All these traits are influenced by testosterone, which is widely recognized in biology to influence traits important in natural selection, such as survival and reproduction.

Within a population, some males produce more testosterone, others produce less. Over evolutionary time, average levels of testosterone -- and the traits that testosterone regulates, such as aggression, body size, and even parental care or sex drive -- may evolve to suit a given environment. The genomic mechanisms that allow testosterone levels to evolve, however, are not well understood.