Comparative studies have studied testosterone levels and related them to mating systems and aggression, but very few studies have attempted to relate testosterone to fitness, that is, the combination of lifetime reproductive success and survival, in the wild or experimentally. Over nine breeding seasons, Wendy Reed (North Dakota State University) and her colleagues followed a group of dark-eyed juncos, small mountain songbirds found throughout North America. They injected males with elevated levels of testosterone and found that they had shorter lives but that they were very successful at siring more offspring — even with females who were mated with other males. “The surprising result was that testosterone-treated males had a higher overall fitness than control males,” write the authors in a study in the May issue of American Naturalist.
This led to the question of why don’t juncos naturally have higher levels of testosterone? Testosterone-treated males produced more offspring, but they were smaller, and smaller offspring had lower postfledging survival. Older, more experienced females preferred to mate with older males and realized higher reproductive success when they did so. While young males treated with testosterone increased their ability to attract older females, it resulted in poor reproductive performance. “Although testosterone increased male fitness, as measured by lifespan and number of offspring, the extended effects on offspring and female mates were generally negative and may ultimately constrain the evolution of higher testosterone levels in males,” conclude the authors.
May 9, 2006
Original web page at Science Daily