Rhinos do it. Horses and hedgehogs do it. Even tigers and our beloved house cats do it. Snout lifted, they curl and pull the top lip up resulting in what looks like the goofiest “smile” ever. It’s called the flehmen (FLAY-men) response, a behavior you’ll see when an animal comes across a smell that requires deep consideration. When an animal curls its lip up like that, the nostrils are constricted. Air inhaled through the mouth enters openings in the roof of the mouth, called the Jacobson's organ. There, scent particles are drawn over a special patch of sensory cells, and finely detailed information is relayed to the brain. Think of it as smelling in high-definition. [caption id="attachment_125672" align="aligncenter" width="683"] With a scrunched up face, tongue out, this Sumatran tiger is processing a lot of scent molecules.[/caption] What is the animal learning from the scent? A male will check a female’s urine for pheromones to determine if she’s in estrus. In other instances, the aroma might identify boundary territory—including if the animal "knows" the owner. [caption id="attachment_125673" align="aligncenter" width="1024"] This Malayan tapir is just starting to flehmen. Step one: lip curl.[/caption] Humans like to look at things as they go about their business, animals pick up scents we can only imagine. It’s not uncommon for animals at the Safari Park and the San Diego Zoo to do the lip-curl when keepers put spices, perfumes or other scented liquids in an exhibit. Where we might stare at something to let our brain process it, scent-oriented animals go full-flehmen. Jacobson's organ for the win!   Wendy Perkins is a staff writer for San Diego Zoo Global.