When presented with a choice of partners, they showed no overall preference for either males or females. When just a male was introduced into the cage, the modified males were far more likely to mount the male and emit a "mating call" normally given off when encountering females than unmodified males were.
However, a preference for females could be "restored" by injecting serotonin into the brain.The researchers have cautioned against drawing conclusions about human sexuality from the result.
The lazy takeaway from this, as seen in news sites, is that serotonin affects sexual orientation, with the suggestion that low serotonin might be the secret to the inexplicable condition known as homosexuality. I'm wondering whether a more plausible conclusion is that, with sexual selection being about competition amongst fit individuals, a prerequisite for having an active sexual preference is passing an internal test of subjective fitness, i.e., being aware that one has sufficiently high status to be picky. In other words, mice without functioning serotonin receptors perceive themselves as losers who will take anything that's warm and regard it, being more than they're entitled to, as a win.