The nose knows, and sometimes it circumvents the brain and affects behavior. When a female mouse isn’t ready to reproduce, her hormones silence her ability to sniff out male pheromones: She’s "odorblind" to males, according to a study published in Cell last week.
"Imagine this occurring in your visual system," Lisa Stowers from Scripps Research Institute tells Smithsonian. "If you just ate a big hamburger and then saw a buffet, you might see things like the table and some people and maybe some fruit – but you simply wouldn't see the hamburgers anymore.”
Stowers and colleagues focused on neurons in the female brain that are activated by hormones produced in the ovaries at various times during the reproductive cycle. Female mice are attracted to males during estrus, when they’re receptive to breeding, but they become indifferent toward males during diestrus, the period of sexual inactivity.
The team imaged neurons in the vomeronasal organ of the nose. These typically detect male odors – but not always, they discovered. The "gating" mechanism of these sensory neurons is inactivated in the presence of progesterone, a hormone produced in the ovaries during diestrus. Progesterone stops the neurons from transmitting the male odor smell to the female’s brain and as a result she can’t perceive the male odors that prompt certain behaviors.
In other words, the choice to perform a behavior is being regulated in the nose – with no thought from the brain. "I have never heard of anything like this before – in any sensory system," Stowers says in a statement. "The nose was making decisions and acting like an extension of the brain."
Until now, no progesterone receptors have ever been detected in vomeronasal organ neurons. However, even in the presence of progesterone, the neurons still responded to the scent of cat urine, a predator cue. This means that the hormone doesn’t disable the vomeronasal organ altogether: the neurons are selectively silenced by progesterone. When the hormone is detected, the signaling molecule that normally relays the odor to the brain is changed, but only male odor-specific neurons are susceptible to this change.
So, during diestrus, receptors in her nose block male-odor molecule signals from ever reaching her brain. These hormones diminish as the female ovulates so that she can smell potential mates again. And when ovulation ends, the cycle repeats.
"The nose is not simply a sense organ, passively vacuuming up all stimuli and transmitting that information to the brain," Stowers says in a press release. "Instead, hormonal signals from the body act directly on the nose to make a decision about what information to send to the brain. It is quite surprising that the nose, and not the brain, would be making such important decisions."
But they’re not sure why females get this male odor blindness: sure, male pheromones become temporarily irrelevant, but there’s no evidence that they would distract females from basic survival needs. Humans, by the way, can’t detect pheromones as far as we know.