Ben Gillett / Slow motion clip of a female Appaloosa horse grooming a male, Bristol, England, UK, May.

John Chan / Two male Southern Pacific rattlesnakes (Crotalus oreganus helleri) fighting during mating season. This behaviour is known as a "combat dance", and is used to determine dominance and the right to mate...

Yashpal Rathore / Bengal tiger (Panthera tigris tigris) dominant male (T29) cooling off in a waterhole Kanha National Park, Central India. Camera trap image.

Yashpal Rathore / Bengal tiger (Panthera tigris tigris) dominant male (T29) and resident female (T27) cooling off in a waterhole Kanha National Park, Central India. Camera trap image.

Yashpal Rathore / Bengal tiger (Panthera tigris tigris) female (T27) acting cautiously as dominant male (T29) approaches watering hole Kanha National Park, Central India. Camera trap image.

Yashpal Rathore / Tiger (Panthera tigris tigris) dominant male tiger approaching 'Beja pani' waterhole, Kanha National Park, Central India. Camera trap image.

SCOTLAND: The Big Picture / Orca whales (Orcinus orca) pod surfacing together, Shetland, Scotland, UK. April.

Ross Hoddinott / Banded demoiselles (Calopteryx splendens) male, roosting among grasses, silhouetted against the sun reflected in the water, Lower Tamar Lakes, Cornwall, UK. July.

Ross Hoddinott / Banded demoiselle (Calopteryx splendens) male, resting on grass head, Lower Tamar Lakes, Cornwall, UK. June.

Ross Hoddinott / Common blue butterflies (Polyommatus icarus) basking in the morning light, Vealand Farm, Devon, UK. July

Espen Bergersen / Aerial shot of Killer whales (Orcinus orca) near the surface, showing mating behaviour including a male Killer whale displaying penis, Troms, Norway, August.

Espen Bergersen / Aerial shot of Killer whales (Orcinus orca) near the surface, showing mating behaviour, Troms, Norway, August.

Nick Upton / Pair of White storks (Ciconia ciconia) mating on their nest, Knepp Castle Estate, Sussex, England, UK, April. This is the first recorded instance of White storks nesting in the UK for several hundreds...

Nick Upton / Pair of White storks (Ciconia ciconia) arranging nest material on their nest, Knepp Castle Estate, Sussex, England, UK, April. This is the first recorded instance of White storks nesting in the UK for...

Nick Upton / Pair of White storks (Ciconia ciconia) arranging nest material on their nest, Knepp Castle Estate, Sussex, England, UK, April. This is the first recorded instance of White storks nesting in the UK for...

David Gallan / Cameratrap video of a male Superb lyrebird (Menura novaehollandiae) courting female on display mound, Australia, July.

David Gallan / Male Satin bowerbird (Ptilonorhynchus violaceus) courting female on suburban street verge, bower filled with blue plastic, Australia, October 2018.

John Chan / Male Anna's hummingbird (Calypte anna) cleaning its beak on a twig, Southern California, USA, January.

Michael Hutchinson / Pair of Marsh fritillary butterflies (Euphydryas aurinia) courting on a Buttercup (Ranunculus), Devon, England, UK, June.

Michael Hutchinson / Pair of Marsh fritillary butterflies (Euphydryas aurinia) courting on a Buttercup (Ranunculus), Devon, England, UK, June.

Michael Hutchinson / Pair of Marsh fritillary butterflies (Euphydryas aurinia) mating, Devon, England, UK, June.

Michael Hutchinson / Close-up of a pair of Marsh fritillary butterflies (Euphydryas aurinia) mating, showing tips of abdomens, Devon, England, UK, June.

Michael Hutchinson / Close-up of a pair of Marsh fritillary butterflies (Euphydryas aurinia) mating, shot tilt downs, Devon, England, UK, June.

Michael Hutchinson / Male Marsh fritillary butterfly (Euphydryas aurinia) displaying to a female on a Buttercup (Ranunculus) flower, Devon, England, UK, June.

Klein & Hubert / African lion (Panthera leo) portrait, South Africa

Klein & Hubert / Mustang stallion kicking out after winning fight. Rocky Mountains, USA, June.

John Chan / Male California least tern (Sternula antillarum browni) presenting fish offering to female during mating, Huntington Beach, California, USA, May.

John Cancalosi / American toads (Anaxyrus americanus), several males competing to mate with female, Maryland, USA, April.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri), two males, one brooding young chick, the other incubating hatching egg. Atka Bay, Antarctica. August. Series 4/4.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri), two males, one brooding young chick, the other incubating hatching egg. Atka Bay, Antarctica. August. Series 3/4.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri), two males, one brooding young chick, the other incubating hatching egg. Atka Bay, Antarctica. August. Series 2/4.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri), two males, one brooding young chick, the other incubating hatching egg. Atka Bay, Antarctica. August. Series 1/4.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri) breeding colony, males huddling on sea ice during polar night, incubating eggs, ice shelf in background. Atka Bay, Antarctica. July 2017.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri) colony huddled during polar night, males incubating eggs. Atka Bay, Antarctica. July 2017.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri) colony, males huddling whilst incubating eggs. Atka Bay, Antarctica. July.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri) colony, males huddling whilst incubating eggs. Atka Bay, Antarctica. July.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri) males huddling whilst incubating eggs,under full moon during polar night, Atka Bay, Antarctica. July.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri) male examining dead chick on ice. Atka Bay, Antarctica. August.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri) male feeding chick aged a few days. Atka Bay, Antarctica. August.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri), group of three males incubating, one with begging chick. Atka Bay, Antarctica. August.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri) egg hatching on father's feet. Atka Bay, Antarctica. August.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri) males in breeding colony incubating eggs, huddling during winter storm. Penguins on outside repositioning themselves to gain protection from weather. Atka Bay, A...

Stefan Christmann / Emperor penguin (Aptenodytes forsteri) males in breeding colony huddling during winter storm. Atka Bay, Antarctica. July.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri), pair mating. Atka Bay, Antarctica. May.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri) breeding colony, mainly males brooding eggs, huddled together on sea ice. Atka Bay, Antarctica. June.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri) pair practising incubating with a snowball. Atka Bay, Antarctica. May.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri) pair, female laying egg. Atka Bay, Antarctica. June.

Stefan Christmann / Emperor penguin (Aptenodytes forsteri), pair transferring recently laid egg from female to male. Atka Bay, Antarctica. June. Sequence 2/2.