Testosterone Tradeoff

Testosterone Tradeoff

Although it’s present in both sexes, testosterone is commonly known as the “male” hormone. That’s because males have more of it, and it strongly influences male sex traits and mating habits. Now, a study in birds shows that extra testosterone can give males a leg up in the mating game.


Feathered Casanovas. I'm Bob Hirshon and this is Science Update.

Would you die younger for a better sex life? That’s what extra testosterone does for birds called dark-eyed juncos. North Dakota State University biologist Wendy Reed and her colleagues found that when young male juncos were treated with extra testosterone, they attracted older, more fertile females, had more extramarital sex, and fathered more offspring than untreated males. But Reed says that attracting more mates also seemed to attract more predators.


They also had lower immune function than control males, and so they paid a cost of that in actually lower survival rates.

Reed says that factors like these may keep the junco population’s natural testosterone levels in check. I'm Bob Hirshon, for AAAS, the science society.

Making Sense of the Research

In order to thrive, organisms have to pull off all kinds of balancing acts. Too much or too little of anything, whether it's fat, vitamins, or salt, can be unhealthy or even deadly. Biological functions like blood pressure, heart rate, and body temperature have to be kept within an optimal zone. Behaviors need to be moderated too; animals must judiciously divide their time between foraging for food, looking for mates, hiding from predators, playing, and resting.

Balance is not just for individual organisms. Evolution puts pressure on entire species to keep certain traits within an ideal range. For example, animals that are larger than average may have a physical advantage over their competition, but those that grow too large may become clumsy and slow. Birds that attract mates with colorful feathers must trade off the advantage of getting to reproduce with the drawback of being more obvious to predators.

In this case, there are two opposing evolutionary pressures acting on testosterone levels in juncos: On one side, having more testosterone makes male birds more attractive to mature females at an early age. So from that angle, it would seem that the more testosterone you have, the better your chance of passing on your genes to the next generation. On the other hand, too much testosterone is linked to poor health, bad parenting, and more attention from predators, all of which would decrease your reproductive prospects in the long run.

It's important to note that although every male junco will have a slightly different amount of testosterone than the next one, the birds with too much testosterone in this experiment weren't found in nature. They were created artificially, by injecting normal, young males with extra testosterone over a period of time. So it's telling that juncos with that much testosterone aren't really out there, despite the hormones' short-term advantages. The implication is that the junco genes that control testosterone levels have evolved to operate within certain limits. Even if a mutation occurred that caused males to have too much testosterone, those males would not be as successful in passing on their genes. Eventually, they would probably die off, restoring the natural range of testosterone to its comfortable middle ground.

Now try and answer these questions:

  1. For dark-eyed juncos, what are the advantages of having more testosterone?
  2. What are the disadvantages?
  3. Explain how, over time, evolutionary pressures may keep junco testosterone levels at a moderate level.
  4. Reed found that high-testosterone male juncos fathered more offspring, but were neglectful parents. Why might this reduce the high-testosterone males' overall reproductive success?
  5. Suppose that extra testosterone simply made male juncos die younger but father more offspring, and that these offspring were just as healthy and well-cared for as the offspring of other, normal-testosterone males. If a mutant gene emerged that caused males to have extra testosterone, would it become more or less common over time? Explain.

You may want to check out the May 26, 2006 Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: microbes on Mars, cell phones tracking weather, the relationship between brain size and intelligence, humans controlling sharks and cockroaches, and the pros and cons of having lots of testosterone.

For Educators

From the National Geographic News Article Lizards Explain Survival of the Not-So-Fittest, students can learn how environmental conditions and behavior affect natural selection, and why it's not always just the fittest that survive.

In the Access Excellence activity Not Just a Bag of Beans, students investigate evolutionary biology. They use a simulation to determine the types of natural selection possible and then demonstrate that variation exists in a population.

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