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.

Franco Banfi / Sperm whale (Physeter macrocephalus) young male trying to copulate with a female. The calf of the female was in the way so the male is trying to chase it away. Dominica.

Dave Bevan / Female Giant wood wasp (Uroceras gigas) inserting ovipositor into tree stump to lay eggs, Carmarthenshire, Wales, UK, September.

Francois Savigny / Tiger male carrying Chital deer kill, Bandhavgarh NP, Madhya Pradesh, India

Tony Wu / Humpback whale (Megaptera novaeangliae) male with his penis extended. Vava'u, Tonga. South Pacific.

Yukihiro Fukuda / Japanese macaque (Macaca fuscata fuscata) rare white furred baby jumping, Jigokudani Valley, Nagano Prefecture, Japan. June.

Fiona Rogers / Wild mature male mandrill (Mandrillus sphinx) in gallery forest during dry season. Lope National Park, Gabon. June 2008.

Wild Wonders of Europe / E Haarberg / Chamois (Rupicapra rupicapra) male with erect penis in rutting season, Gran Paradiso National Park, Italy, November 2008

Karl Ammann / Male Bonobo {Pan paniscus} sitting, looking thoughtful. Lola Ya Bonobo Sanctuary, Kinshasa, DR of Congo, 2007

Doug Perrine / Humpback whale {Megaptera novaeangliae} with penis extended, Pacific (taken under NMFS research permit 882 issued by Hawai Whale Research Foundation)

Anup Shah / Hamadryas baboons display red bottoms {Papio hamadryas} C

Anup Shah / Male Chimpanzee sitting on tree buttress, Gombe NP Tanzania. Apollo