Donald M. Jones / Moose (Alces alces) bull lip-curling, Denali National Park, Alaska

Jeff Vanuga / Shiras moose (Alces alces shirasi) male, flehmen response / lip curling with mountains in background, Grand Teton National Park, Wyoming.

Jeff Vanuga / Bison (Bison bison) bull lip curling / flehmen response in mist, Yellowstone National Park, Wyoming, USA. September.

Donald M. Jones / White-tailed Deer (Odocoileus virginianus) buck lip curling, Montana

Hiroya Minakuchi / Chimpanzee (Pan troglodytes) showing fear, Ol Pejeta Conservancy, Kenya

Donald M. Jones / Bighorn Sheep (Ovis canadensis) pair of rams lip-curling during rut, Canada

Bruno D'Amicis / Apennine chamois (Rupicapra pyrenaica ornata) adult male flehmen / lip-curling during autumn rut. Endemic to the Apennine mountains. Abruzzo, Italy, November.

Bruno D'Amicis / Apennine chamois (Rupicapra pyrenaica ornata) adult male flehmen / lip-curling during autumn rut. Endemic to the Apennine mountains. Abruzzo, Italy, November.

Donald M. Jones / Elk (Cervus elaphus) bull lip curling in defensive display, northern Rocky Mountains, Canada

Donald M. Jones / Elk (Cervus elaphus) bull lip curling in defensive display, northern Rocky Mountains, Canada

Donald M. Jones / Bighorn Sheep (Ovis canadensis) ram showing teeth in defensive display, western Montana

Mark Raycroft / Moose (Alces alces) large bull lip curling by mountain pond, North America

Mark Raycroft / White-tailed Deer (Odocoileus virginianus) big buck lip curling in meadow, smelling for females who may be in estrus

Mark Raycroft / White-tailed Deer (Odocoileus virginianus) close-up of eight point buck lip curling, smelling for females who may be in estrus

Mark Raycroft / White-tailed Deer (Odocoileus virginianus) large buck lip curling in winter forest, smelling for females who may be in estrus

Mark Raycroft / White-tailed Deer (Odocoileus virginianus) buck lip curling as he smells for females who may be in estrus

Carol Walker / Andalusian colt {Equus caballus} lip-curling with grey mare, Longmont, Colorado, USA.

Donald M. Jones / Moose (Alces alces) bull lip curling, Wyoming

Hiroya Minakuchi / Chimpanzee (Pan troglodytes) showing fear, Ol Pejeta Conservancy, Kenya

Donald M. Jones / Rocky Mountain Elk (Cervus canadensis nelsoni) bull lip-curling during rut, Canada

Anup Shah / Sumatran Orangutan (Pongo abelii) female baby, named Sandri, curling lip, Gunung Leuser National Park, Sumatra, Indonesia

Donald M. Jones / Bighorn Sheep (Ovis canadensis) rams showing teeth in defensive display, western Montana

Jack Dykinga / Roosevelts elk / Roosevelts wapiti (Cervus canadensis roosevelti) curling upper lip in flehmen response, Prairie Creek Redwoods State Park, California, USA.

Sylvain Cordier / Baja California pronghorn (Antilocapra americana peninsularis) febuck curling lip. Baja California Desert National Park, Guerrero Negro, Baja California Sur, Mexico.