Steroids Won't Help If You're a Loser

The more we study physiology and behavior across groups of animals, the more we find we have in common in the types of behaviors we express and the biological machinery of how our bodies influence what behaviors are expressed and when. But similarity does not mean the same. Sometimes seemingly small physiological differences can have big behavioral consequences.

A snuggly California mouse pair.
Photo from the Marler lab.

A lone wire-walking white-footed mouse.
Photo by the National Park Service.

Take the California mouse and the white-footed mouse, for example. Both are small grayish to brownish species of North American deer mice (the Peromyscus genus). But behaviorally, these species are quite different. One noticeable difference in their behavior is that California mice are very territorial and dominant towards intruders, whereas white-footed mice are more welcoming of other mice (or at least more ambivalent to their presence). These species also differ in how they respond to dominance challenges: California mice that win a challenge (by getting their opponent to show submissive behavior) are more likely to win future challenges (this phenomenon is called the Winner Effect). However, white-footed mice have the same probability of winning or losing a challenge regardless of whether they won or lost the previous one. Interestingly, the experience of winning causes the levels of testosterone (a steroid hormone) to surge in California mice, but not in white-footed mice. Could this difference in testosterone explain this difference in behavior between these two species?

Cathy Marler and her colleagues at the University of Wisconsin at Madison recently explored this question. They compared four groups of male mice: (1) California mice that won three challenges and had a saline injection after each win, (2) white-footed mice that won three challenges and had a testosterone injection after each win, (3) white-footed mice that won three challenges and had a saline (without hormone) injection after each win, and (4) white-footed mice that were handled by researchers three times (and had no dominance challenges) and had a testosterone injection after each time they were handled. Then they placed each mouse with a new challenger and measured what percentage of males in each group were dominant versus subordinate.


Most California mouse males that have been previously dominant and had
no hormone treatment win a future dominance challenge (black bar).
White-footed mice (grey bars) are only likely to win their next dominance challenge
if they had previously won AND had additional testosterone (W+T), but not if they
won without added testosterone (W+S) or had testosterone without wins (H+T).
Figure from Fuxjager et al., 2011, Proc. R. Soc. B.

Marler and her colleagues found that similar to the natural state, white-footed mice that won three times and were not injected with testosterone were less likely to win a later challenge than California mice that also won three times and were not injected with testosterone. But, white-footed mice that won three times and were injected with testosterone won their next challenge as often as the California mice did! Presumably, the California mice had their own natural surge of testosterone whenever they had a winning experience and the testosterone injections given to the white-footed mice after their wins mimicked this effect and increased their odds of winning later.

However, testosterone injections alone were not enough to increase the chances of winning: the white-footed mice that had testosterone injections without winning challenges were just as likely to lose their next challenge as those that had saline injections and won previous challenges. It’s the combination of winning experience paired with a surge of testosterone that is the winning formula. You can think of it this way: When (fictional) scientists gave frail Steve Rogers "Super Soldier Serum" to turn him into Captain America, it only worked because Steve Rogers was already a winner at heart. Had they given the serum to someone less remarkable, Michael Cera, for example, they just would have ended up with this:

Michael Cera's attempt to be Captain America. Photo by Gage Skidmore.

So what does this all mean? For one thing, it appears that white-footed mice have all of the necessary brain-wiring to show a Winner Effect in the same way that California mice do, but they don’t produce the testosterone surge to activate it. This opens up a bunch of new questions, like: Why don’t white-footed mice have a testosterone surge after winning? Is there some aspect of their lifestyle that would make such a testosterone surge costly? How does the body know to release testosterone in response to a social experience anyway? Other than testosterone, what makes a winner a winner?

In the end, the experience of winning is critical to the Winner Effect: Testosterone alone won’t help you win your next challenge. So don’t think steroids are gonna help you if you suck at your sport of choice. You have to already be a winner for extra testosterone to help you win… and at that point, who needs it?

Want to know more? Check these out:

1. Fuxjager MJ, Montgomery JL, & Marler CA (2011). Species differences in the winner effect disappear in response to post-victory testosterone manipulations. Proceedings. Biological sciences / The Royal Society, 278 (1724), 3497-503 PMID: 21490015

2. Oyegbile TO, & Marler CA (2005). Winning fights elevates testosterone levels in California mice and enhances future ability to win fights. Hormones and behavior, 48 (3), 259-67 PMID: 15979073