One would often notice predators making this strange grimace when smelling a urine patch and it is often thought that the animals in question is agitated and growling at the observer. A lot of photographers would also use photos of a lion flehmen as depicting a growling or snarling predator. This cannot be further from the truth.
Pondoro guides took this video on the 29th of November 2016 which perfectly depicts the typical facial grimace accompanying the action. Flehmen is a German word that describes the action of an animal analyzing a scent signal. It is practiced by most hoofed animals and predators.
Ducts in the mouth and nasal cavity opens up when pulling the lips back. This offers an organ called the vomero-nasal organ which is situated between the mouth and nasal cavities the opportunity to assess and analyze scent signals. The voter-nasal organ differs from conventional scent channels by having their own organs and nerve connection leading directly to the brain. Scent is delivered in liquid form, either as saliva after scent inspection or as urine itself.
This technique is very useful in decoding important information such as sexual status, dominance, identity and in particular the reproductive status of females where the brain will analyze the hormones in the liquid to see whether a female is worth pursuing for mating.
In hoofed animals the action is accompanied by a lip curl and extended tongue. In elephants the behavior is exhibited without any outward grimace or clues. Interestingly a hippo can do it under water.
I hope this blog post helps in explaining an often seen but misunderstood behavior. Look out for it on your next safari.