We hope you will join us for the Roundtable Seminar Series event Friday, November 5th 2021 at 9:00 – 10:15 am ET where Drs. Alex Shaw, Amanda Rotella, Erez Yoeli, & Esther Herrmann will engage in discussion, moderated by Dr. Felix Warneken on the topic, “The Evolution of Reputation”. We’re looking forward to engaging with our members on important discussions happening in this area.

This series is an opportunity for HBES members to engage with experts on topics in the evolutionary behavioral sciences. Each virtual session will feature a panel of speakers answering questions from a moderator and live audience. Join us each month to discuss the big questions facing our field. Live sessions on the HBES ohyay space are open to registered HBES members only, but a recording will be made publicly available on Youtube after the event.

Please check your email for instructions to set up your ohyay account an access the HBES ohyay space that will be sent out on the week before the event.

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.

by Tom S. Roth & Iliana Samara

Maybe you still cherish vivid memories of your first date with your partner. Maybe you remember what they looked like, the sound of their voice, or even their scent! During first encounters, we are confronted with multiple sources of information, encompassing a range of modalities. It is well known that this information shapes our opinion about potential partners. However, what remains unknown is which of these sources is most important for partner choice. In an article that recently appeared in Evolution and Human Behavior, we examined this by exploring how attractiveness on different modalities relates to speed-date outcome.

A plethora of studies has investigated the role of visual attractiveness in partner choice. For example, when people go on a date with a visually attractive partner, they are more likely to seek another date with them. However, it has recently been suggested that when studying attractiveness we should look further than visual information, and include scent and sound as well. Even though some studies have investigated the attractiveness of scent and sound, their effect on actual partner choice remains unclear. Therefore, in our study, we combined multimodal attractiveness ratings with a natural paradigm to study partner choice, namely speed-dating. Speed-dates strongly reflect natural partner choice situations, and are therefore great tools to study partner choice in humans.

To study the role of multimodal attractiveness in partner choice, we invited a group of 67 heterosexual participants (34 women and 33 male), between the ages of 18 and 26 years old, to the lab. These participants were divided over 4 groups, that were each tested in a different time slot. Every group had approximately the same number of men and women. The experiment consisted of three stages: a sample collection stage, a sample rating stage, and a speed-dating stage.

In each group, we started by collecting samples. After arriving at the lab, we took a standardized portrait picture of each participant, recorded their voice while they were reading out a standardized Dutch text, and we asked them to bring a t-shirt that they had worn during the night. In this way, we had a sample that could be rated for visual, auditory, and olfactory attractiveness from each participant. In the sample-rating part of the experiment, participants rated the attractiveness of all the samples that we gathered from the opposite-sex participants on a 1 to 7 scale, with 1 meaning very unattractive and 7 meaning very attractive. Thus, the men rated the attractiveness of the picture, voice recording and t-shirt scent of the women in their timeslot, while the women rated the samples of the men in their timeslot. Finally, after completing all the rating tasks, participants took part in the speed-dating part of the experiment. Each participant had a maximum of 10 speed-dates, depending on the number of opposite-sex participants in their timeslot. At the end of each date, participants indicated whether they would like to go on another date with their partner, and how attractive they found their partner. Furthermore, people that both indicated that they wanted to meet their partner again, received each other’s contact information, so that they could meet up again after the experiment.

We investigated how well the ratings of the different modalities predicted the date outcome, in this case, how likely participants were to seek another date with their partner. We found that, as would be expected, visual information was the most important predictor of the willingness to date again for both men and women. Regarding the auditory information, we found that vocal attractiveness seemed to have a small positive influence on willingness to meet again in men, but not women. For olfactory information, we found the opposite pattern. There was no clear effect in men, but in women there was a surprising negative association. In other words, the more pleasant an odor was rated before the speed-dating paradigm, the less likely women were to indicate that they would go on another date with their partner.

This study is the first one that distinguishes between visual, auditory and olfactory attractiveness, and compares their importance in partner choice. In line with previous studies, we found that facial attractiveness was the strongest predictor for willingness to go on another date. This finding is not surprising, given that humans are visually well-equipped, and strongly inclined to pay more attention to visual information than other modalities.

When it comes to sound, auditory attractiveness seemed to be somewhat important for men, but not women. Nonetheless, this could be due to naturally occurring voice fluctuations during the date, for example people might tend to modulate their pitch when speaking to someone that they consider attractive. Importantly, this finding could be due to the noise in the room, as all group dates were hosted at the same time. This might have led to the voice of the partner being perceived as different compared to the recording.

Regarding the olfactory attractiveness, we found no discernible pattern for men, and a small but surprising negative association for women. This finding could be due to people wearing perfumed hygienic products before putting on the t-shirt (e.g., deodorant). Even though participants were instructed not to use such products before and while wearing the t-shirt, they might have wanted to mask their natural body odor. Therefore, it could be that the smell samples represent diplomatic odor (i.e., combination of natural body odor and hygienic products), thereby obscuring the effect of scent attractiveness on partner choice.

Visual attraction seems to be most important during first dates, while voice and scent seem to have little to no effect.

An interesting theoretical explanation for the fact that auditory and olfactory attractiveness seem to have little to no effect on mate choice compared to visual attractiveness could be that visual, auditory, and olfactory information become important in different stages of the relationship. In other words, upon meeting a new partner, visual information could act as a filter, ensuring that a sufficiently attractive partner is selected. Following this stage, auditory information and olfactory gradually might become more decisive for partner selection and maintenance. Future research can investigate whether this is indeed the case.

In sum, visual attraction seems to be most important during first dates, while voice and scent seem to have little to no effect. Thus, while attractiveness might indeed be multimodal, when it comes to first encounters it seems that the eyes have it.