Norbert Wu / Devil Angler (Linophryne indica) fish, deep sea species showing large female with dwarf male attached

Suzi Eszterhas / Black Bear (Ursus americanus) 7 week old cubs playing in den. One cub shows brown color phase while the other shows black color phase

Donald M. Jones / Northern Pintail (Anas acuta) males flying with female in spring courtship display, Montana

Donald M. Jones / Bighorn Sheep (Ovis canadensis) female and male in winter, Jasper National Park, Alberta, Canada

Chien Lee / Stick Insect (Monticomorpha flavolimbata) pair mating in mountains, Bengoh Range, Ecuador

Donald M. Jones / Bighorn Sheep (Ovis canadensis) ewe and ram, western Montana

Michael Quinton / Dall's Sheep (Ovis dalli) ram, lamb, and ewe, Yukon Territory, Canada

Luciano Candisani / Green Anaconda (Eunectes murinus) female constricting male after copulation, after which she will consume him, Brazil

Alan Murphy/ BIA / Northern Bobwhite (Colinus virginianus) female and male, Texas

Brais Seara Fernandez/ BIA / Indian Peafowl (Pavo cristatus) female watching male performing courtship display, Castilla Leon, Spain

Donald M. Jones / Elk (Cervus elaphus) bull and harem of females and calves along river, Athabasca River, western Canada

Donald M. Jones / Bighorn Sheep (Ovis canadensis) rams, ewe, and lamb, western Canada

Rick Bowers/ BIA / Gambel's Quail (Callipepla gambelii) male at left with female, Arizona

Roland Seitre / Svalbard Reindeer (Rangifer tarandus platyrhynchus) male and females running, Spitsbergen, Svalbard, Norway

Roland Seitre / Svalbard Reindeer (Rangifer tarandus platyrhynchus) pair, Spitsbergen, Svalbard, Norway

Thomas Marent / Gelada Baboon (Theropithecus gelada) male and females huddling for warmth, Simien Mountains National Park, Ethiopia

Alan Murphy / Northern Bobwhite (Colinus virginianus) female and male, Texas

Alan Murphy / Eastern Bluebird (Sialia sialis) male and female, Texas

Alan Murphy / Eastern Bluebird (Sialia sialis) male and female, Texas

Anup Shah / Cheetah (Acinonyx jubatus) chasing Impala (Aepyceros melampus) female, Masai Mara, Kenya

Donald M. Jones / Bighorn Sheep (Ovis canadensis) ram and ewe during rut, Montana

Donald M. Jones / Bighorn Sheep (Ovis canadensis) ram and female, western Montana

Vincent Grafhorst / African Lion (Panthera leo) pair mating, Khutse Game Reserve, Botswana

Vincent Grafhorst / African Lion (Panthera leo) male and female courting, Khutse Game Reserve, Botswana

Michael & Patricia Fogden / Black-faced Impala (Aepyceros melampus petersi) male, females, and young drinking at waterhole, Etosha National Park, Namibia

Piotr Naskrecki / Bearded Weevil (Rhinostomus barbirostris) species displays interesting sexual polymorphism, Costa Rica

Pete Oxford / Chachi Tree Frog (Hyla picturata) pair, Cotacachi-Cayapas Reserve, northwest Ecuador

Pete Oxford / Chachi Tree Frog (Hyla picturata) pair, Cotacachi-cayapas Reserve, northwest Ecuador

Pete Oxford / Chachi Tree Frog (Hyla picturata) pair, Cotacachi-Cayapas Reserve, northwest Ecuador

Konrad Wothe / Fiddler Crab (Uca maracoani) waving, Caroni Swamps, Trinidad, West Indies

Tui De Roy / Great Frigatebird (Fregata minor) courting males vying for female's attention, Genovesa Tower Island, Galapagos Islands, Ecuador

Tui De Roy / Great Frigatebird (Fregata minor) recently formed pair, female inspecting male in courtship display with extended gular pouch, Genovesa Tower Island, Galapagos Islands, Ecuador

Norbert Wu / Old-world Chameleons, male and female preparing to mate, Africa

Mitsuaki Iwago / Australian Sea Lion (Neophoca cinerea) male approaching a female to mate, Australia

Shin Yoshino / African Lion (Panthera leo) male and female, Masai Mara National Reserve, Kenya

Mitsuaki Iwago / Ostrich (Struthio camelus) male and two females running, Serengeti National Park, Tanzania

Mitsuaki Iwago / African Lion (Panthera leo) couple fighting during mating, Serengeti National Park, Tanzania

Mark Moffett / Strawberry Poison Dart Frog (Oophaga pumilio) group showing color variation from different islands, Bocas del Toro, Panama

Bernard Castelein / Dimorphic egret (Egretta dimorpha) in white phase, flying, Ankarafantsika National Park, Madagascar

Paul D Stewart / Illustration of Cowslips (Primula veris) from 'Darwinism' by Alfred Wallace 1889. In 1862 Darwin reported how Primula plants have two forms of flowers, pin and thrum eyed, differing in the height and...

Paul D Stewart / Portraits of Cowslip (Primula veris) flowers, pin and thrum eyed, showing heterostyly ie stigma at different positions. This was noted by Darwin and shows a mechanism to avoid self fertilisation.

Willem Kolvoort / Dimorphic Egret (Egretta dimorpha) on edge of Aldabra lagoon with coral 'mushrooms', Natural World Heritage Site, Aldabra 2006

Willem Kolvoort / Dimorphic Egret (Egretta dimorpha) on edge of Aldabra lagoon with coral 'mushrooms', Natural World Heritage Site, Aldabra 2006

Willem Kolvoort / Dimorphic Egret (Egretta dimorpha) and Crab plovers (Dromas ardeola) on edge of Aldabra lagoon with coral 'mushrooms', Natural World Heritage Site, Aldabra 2006

Willem Kolvoort / Dimorphic Egret (Egretta dimorpha) and Crab plovers (Dromas ardeola) on edge of Aldabra lagoon with coral 'mushrooms', Natural World Heritage Site, Aldabra 2006

Sam Hobson / Standard Radolfzell rings and colour-rings for northern goshawk (Accipiter gentilis) ringing. Two different sizes as males and females are sexually dimorphic. Berlin, Germany. March 2014.

Visuals Unlimited / Leafhoppers (Cicadellidae) mating, while a small leafhopper crawls on the back of the male. Male and female of this species are dimorphic. Upland rainforest near Archidona, Ecuador

Nick Upton / Sexually dimorphic St. Mark's fly / March fly (Bibio marci) pair mating; males have much larger eyes and clear wings, Wiltshire garden, UK, May.