Monday, July 1, 2013

Fight or... Flehmen

The flehmen response—unlike the adrenaline induced responses for fight or flight—provides a wide range of animals with a means of receiving chemical information about their social world. Animals, including elephants, use the flehmen response predominantly to access information from other individuals related to their corresponding reproductive status. The response typically involves investigation of a urine sample from another individual or investigation of another individual’s genitals directly. Alternatively, animals may exhibit the flehmen response to analyze foreign or novel odors.

Flehmen responses usually operate through a curling back of the upper lip and subsequent inhalation of a scent. The scent is inhaled orally, bypassing the primary smell system reliant upon the nostril passages and instead directed toward the vomeronasal organ (VNO) located above the roof of the mouth and accessible via a duct (pictured below).

Duct leading to the VNO (aka Jacobson’s organ) within the roof of the elephant’s mouth

 In animals such as the Boer goat the curled back upper lip actually appears to physically obstruct the nostrils (Author: Böhringer Friedrich).

As with pretty much every other communicatory function, the superstar appendage that is the trunk plays a prominent role in the elephant’s flehmen response. The elephant will contact a substance with the protuberance on the tip of the trunk referred to as a finger. The substance is then transported by the trunk to the VNO at the roof of the mouth.

Try to imagine grabbing an object with your nose and you realize pretty quickly how starkly different we are from elephants. The elephant’s sense of touch and sense of smell are inextricably linked. Evolution had different plans for us, providing us with not one but two front limbs which—no longer required to assist us in locomotion—have been freed up for object manipulation. (Perhaps, the icing on the cake for us humans is that evolution managed to keep our nosy smell organ from becoming entangled within our highly adapted hands… although, the elephant would probably highlight this point as a rather distinct disadvantage.)

Bestowed by our ancestors with incredibly deft manipulation tools (hands), imagine how easy it’d be for us to perform the flehmen response on a sample of urine. I suspect we’d follow the elephant’s lead of directly transferring odorants instead of taking deep inhalations. Of course… we’d need a vomeronasal organ of our own to finish the job. Otherwise any chemical information from the urine would fall upon the vomeronasal equivalent of pinched nostrils.

As someone who’s never consciously received chemical information from urine, and who’s definitely never actively transferred urine by hand to the roof of his mouth, you can imagine my surprise when, while researching the VNO further, I discovered that I myself might very well possess my own VNO.

While many issues surrounding the human VNO are highly debatable, the existence of a VNO in human embryos is not one of them. Even in some adults, the same location (near the nasal septum) is occupied by a structure similar to the VNO found in human embryos. Reports vary widely on this second point. Still, as Michael Meredith notes emphatically in his critical review of the evidence surrounding the human VNO, structural evidence for the organ in no ways implies that the organ is functional.

The way I see it—emphasis on see—unlike elephants who rely more heavily on smell than sight, the primate tree has grown in a direction that favors visual signaling pathways in place of chemical ones. This sensory preference has left us with a vestigial VNO, a remnant of our ancient ancestors. Is it an old structure? Sure. But when considering its function or value, it’s likely to be about as wise as our wisdom teeth. And by that, I mean not very wise at all. Those are some poorly named teeth.

Meredith M (2001). Human Vomeronasal Organ Function: A Critical Review of Best and Worst Cases. Chem. Senses 26:433-445.

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