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

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

Mark Raycroft / Caribou (Rangifer tarandus) male, female, and calf, North America

Mark Raycroft / Woodland Caribou (Rangifer tarandus caribou) bull displaying to female, Newfoundland, Canada

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

Jan Wegener / Mallard (Anas platyrhynchos) couple courting, Berlin, Germany

Jan Wegener / Northern Pintail (anas acuta) pair flying, Vancouver, British Columbia, Canada

Jan Wegener / American Wigeon (Anas americana) pair with female in foreground, Vancouver, British Columbia, Canada

Roland Seitre / Dimorphic egret (Egretta dimorpha) standing on a branch, Ranomafana National Park, Madagascar.

Roland Seitre / Dimorphic egret (Egretta dimorpha) adult with chick preening, at nest, Ranomafana, Madagascar.

Doug Gimesy / White-cheeked gibbon (Nomascus leucogenys) female and two offspring, a male and female, sitting in tree. White-cheeked gibbons are sexually dimorphic with females having golden fur. Babies are born go...

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