Nick Turner / Men pulling on a rope in a tug of war competition, Cotswold Olimpicks, Chipping Campden, Gloucestershire, June 2011.

Andrew Parkinson / Dunlin (Calidris alpina) adult trying to pull a worm out of the wet sand, September, Shetland Islands, UK, September.

Tui De Roy / Flightless cormorant (Phalacrocorax harrisi), pair at nest with seaweed in beaks. Carcasses surrounding nest. Cape Douglas, Fernandina Island, Galapagos.

Kristel Richard / Two eagle hunters mounted on Mongolian horses in tug of war game over dead goat during the Buzkashi games, at the Eagle Hunters Festival, near Sagsai, Bayan-Ulgii Aymag, Mongolia. September 2014..

Kristel Richard / Two eagle hunters mounted on Mongolian horses in tug of war game over dead goat during the Buzkashi games, at the Eagle Hunters Festival, near Sagsai, Bayan-Ulgii Aymag, Mongolia. September 2014..

Kristel Richard / Two eagle hunters mounted on Mongolian horses in tug of war game over dead goat during the Buzkashi games, at the Eagle Hunters Festival, near Sagsai, Bayan-Ulgii Aymag, Mongolia. September 2014..

Kristel Richard / Eagle hunter mounted on Mongolian horse winning Buzkashi game after winning tug of war over dead goat, at the Eagle Hunters Festival, near Sagsai, Bayan-Ulgii Aymag, Mongolia. September 2014..

Kristel Richard / Two eagle hunters mounted on Mongolian horses in tug of war game over dead goat during the Buzkashi games, at the Eagle Hunters Festival, near Sagsai, Bayan-Ulgii Aymag, Mongolia. September 2014..

Kristel Richard / Two eagle hunters mounted on Mongolian horses in tug of war game over dead goat during the Buzkashi games, at the Eagle Hunters Festival, near Sagsai, Bayan-Ulgii Aymag, Mongolia. September 2014..

Kristel Richard / Two eagle hunters mounted on Mongolian horses in tug of war game over dead goat during the Buzkashi games, at the Eagle Hunters Festival, near Sagsai, Bayan-Ulgii Aymag, Mongolia. September 2014..

Florian Möllers / Giant Schnauzer x Hovawart dog and Maliois Herder x Dutch shepherd playing tug of war with Frisbee, Germany, September.

Florian Möllers / Giant Schnauzer x Hovawart dog and Maliois Herder x Dutch shepherd playing tug of war with Frisbee, Germany, September.

Steven Kazlowski / Polar bear (Ursus maritimus) pair of spring cubs play tug-o-war with a piece of whale remains during freeze up, 1002 area of the Arctic National Wildlife Refuge, North Slope, Alaska, USA

Nick Turner / Men pulling on a rope in a tug of war competition, Cotswold Olimpicks, Chipping Campden, Gloucestershire, June 2011.

Nick Turner / Men pulling on a rope in a tug of war competition, Cotswold Olimpicks, Chipping Campden, Gloucestershire, June 2011.

Petra Wegner / Cavalier King Charles Spaniel, bitch with puppy, blenheim, 12 weeks, playing tug-of-war with toy.

Petra Wegner / German Boxer with puppy, 3 months, playing tug-of-war

Petra Wegner / Kromfohrlander puppy, aged 10 weeks, playing tug-of-war with owner.

Petra Wegner / Kromfohrlander puppy, aged 10 weeks, playing tug-of-war with owner.

Petra Wegner / Kromfohrlander puppy, aged 10 weeks, playing tug-of-war with owner.

Suzi Eszterhas / Black-backed Jackal {Canis mesomelas} two pups playing 'tug of war' with a piece of meat, Masai Mara Triangle, Kenya

Petra Wegner / Blenheim Cavalier King Charles Spaniel, 5 months, playing tug-of-war

Jane Burton / Golden Retriever pups playing tug-of-war.

Petra Wegner / Australian Shepherd dog, playing with tug-of-war toy

David Tipling / Buzkashi, Mongolian tug of war, with a goat skin. Men on horseback with crowd watching in background. Eagle Hunters Festival, Bayan-Olgii, Altai Mountains, Western Mongolia. October 2008.

Jeff Foott / Tug-of -war game, ( Kukbar game) at the Eagle Hunters festival near Ulgii, Western Mongolia.