A couple of days back I posted a link to Ed Yong’s synopsis of the paper just released by Nature purporting to show that modulating serotonin levels in mice could have an effect on their sexual orientation. In short, low- or no-serotonin producing mice no longer discriminated sexually between males and females, mounting cagemates of either gender with equal vigor. There’s really no point in re-summarizing the study as Scicurious has provided an excellent recap.
Here’s a serotonin deficient mouse mounting first a male, then a female:
As the paper’s authors themselves noted, one alternative explanation for the findings is that the loss of gender discrimination was a byproduct of a serotonin-regulated increase in sexual behavior rather then a change in sexual preference per se. Although the study showed that no serotonin mice didn’t show increased sexual behavior towards solo females, they mounted close to 90% of the females in that condition, not leaving a lot of room to show an increase.
Furthermore, mice rely on pheromones to determine who is female and who’s not. The depletion of serotonin might have had some effect on the peripheral olfactory system such that the mice couldn’t distinguish the males from the females. The study did include a control condition which showed mice’s olfactory systems were still at least somewhat functional (they could distinguish between sesame oil and air, and could still identify fox urine.) But maybe the difference between physiological levels of pheromone requires a level of sensitivity in the olfactory system that was lost in the serotonin-depleted animals. I’m not convinced that this explanation has been sufficiently ruled out.
Finally, on a more speculative note, serotonin is known to play a large role in regulating forms of social behavior including aggressiveness and social dominance, in a wide range of species from mice to humans. These animals were completely depleted of serotonin which should lead to significant changes in their social behavior. Past research has associated low serotonin with increased aggressiveness in mammals. Could it be that when male mice mount other males, they are doing something other than “getting it on”? (Male mice with normal levels of serotonin sometimes do mount male mice. Why’s that?).
Although the study’s findings are just the kind of topic that the mainstream media loves (anything involving sex), and they’ve responded with salacious headlines in kind, it seems prudent to interpret the results cautiously and await further work.