by Nicholas Grebe
One of the most basic decisions facing organisms of all kinds concerns how to spend limited resources. Given that energy and effort—which encompasses both physiological and psychological processes—draw from a finite budget, how should an individual partition its investments between the fundamental activities of life? For instance, upon reaching maturity, to what extent should an organism prioritize attracting and retaining mates, versus engaging in maintenance processes necessary to stay alive? We know this is a devilishly complicated question that depends on (at least) the evolutionary history, current environmental conditions, and developmental background of the organism in question. However, biologists are nevertheless keenly interested in identifying unifying mechanisms—conserved biological systems that may help explain why individuals differ so much in how they achieve evolutionary fitness outcomes.
One popular candidate mechanism stems from a physiological system common to all aerobic organisms. An inevitable consequence of creating cellular energy, whether you’re a nematode or a human, comes in the form of reactive oxygen species: by-products of oxygen metabolism that can damage DNA, cell membranes, and proteins. Naturally, organisms must counteract these “free radical” molecules, which they do via a number of elegant anti-oxidant mechanisms. (Interestingly, controlled amounts of reactive oxygen species help organisms carry out adaptive responses to events like infection or aerobic exercise. Problems arise when regulation of these molecules goes awry.) The dynamic balance between an individual’s production of reactive oxygen species and anti-oxidant neutralization—their state of oxidative stress—was the focus of our study.
Biologists have proposed that levels of oxidative stress, as a fundamental reflection of an organism’s state of energetic functioning, should be related to the expression of sexually selected signals. How and why this covariation occurs is a matter of serious debate, but let’s consider two simple alternative possibilities. On one hand, if an individual is better able to regulate oxidative stress, then perhaps that frees up a greater share of the energetic budget to spend on traits that are attractive to the opposite sex and/or beneficial for intrasexual competition: think brighter plumage, richer scent-marks, or more dominant personalities. Some even venture that these kinds of traits are attractive because they signal effective regulation of oxidative stress, though there could be other reasons for such an association. On the other hand, individuals who elect to devote more energy to sexual signaling, for whatever reason, might incur greater oxidative damage simply by virtue of expending more energy. These scenarios entail two opposing predictions: in the former, oxidative stress should be negatively related to indices of mating effort; in the latter, the two dimensions should be positively correlated.
Most of the background summarized above has concerned physical traits in non-human animals, but we wanted to investigate how oxidative stress might be implicated in human mating effort, broadly conceived. To do that, we explored associations between a biomarker of oxidative stress (8-OHdG, collected at two different time points per participant), psychological traits, and measures of health/athleticism in two Western university samples: one of 98 men, and one of 75 heterosexual couples. We focused on aspects of one’s personality and health that have established links to success in obtaining and keeping mates. Personality traits included social dominance (sample item: “I am quite good at convincing others to see my way”), non-submissiveness (“When other men/women cross the line with me, I’m not afraid to enter into a conflict with them”), and extraversion (“I often feel as if I’m bursting with energy”). Health traits included self-reports of athleticism, muscularity, and frequency of illness (all relative to others of the same age and sex). In our sample of couples, we also had partners rate each other on these same dimensions and found strong agreement in ratings, suggesting that these reports reflect apparent traits.
Our results were remarkably consistent in some aspects, but notably variable in others. In the consistent camp, we found that men’s 8-OHdG levels upon awakening—which we believe represents a relatively stable composite of oxidative stress—were negatively related to their social dominance, extraversion, and athleticism. However, on the variable side, these negative associations did not generalize to men’s oxidative stress several hours after waking, nor to women’s oxidative stress at any time. In fact, while not as consistent as patterns for men, women’s social dominance and athleticism were actually positively related to their oxidative stress levels.
So, we found some evidence consistent with each of the scenarios presented above. Regarding the former scenario, men who experience lower costs to energy production, in the form of oxidative stress, appear to invest more heavily in socially dominant and athletic traits. Regarding the latter, women who report greater investment in intrasexual competition, rather than paying a lower marginal cost, appear to simply generate more oxidative damage. Unexpectedly, however, these patterns were restricted to samples collected first thing in the morning. While providing preliminary data that speaks to some established questions, we think our study introduces just as many additional complexities, and it furthermore suggests we have much to refine in our understanding of oxidative stress and human mating effort.
A brief afterword: This blog-length outline presents a necessarily simplified narrative about the theory and findings regarding sexual signaling and oxidative stress. For instance, while this piece only briefly touches on sex differences, there are a number of interesting implications for oxidative stress – trait associations that differ between the sexes. Here, we think readers might be interested in a small window into our peer-review experience with this paper. While both referees found things to like in our original submission, one reviewer in particular was quite skeptical of our theorizing and interpretations. But, in a model of constructive criticism, this reviewer didn’t act to gatekeep publication of our study; rather, they pushed us to present a fuller picture of the diverse, even contradictory findings pervading the literature upon which we relied, and to embrace greater epistemic humility regarding what our findings can and cannot tell us. We did our best to heed this call, and we encourage readers to check out the full paper for a more complete discussion of this corner of scientific research. Maybe Reviewer #2 isn’t so bad, after all.