Klein & Hubert / Dun foal jumping over a palomino yearling while playing. Nevada, USA.

Carol Walker / Lusitano horse, dun stallion pawing the ground, Portugal, May 2011

Carol Walker / Wild horses / Mustangs, band with dun filly drinking in waterhole, Pryor Mountains, Montana, USA

Carol Walker / Wild horses / Mustangs, palomino and red dun stallions rearing, Pryor Mountains, Montana, USA

Carol Walker / Wild horses / Mustangs, palomino and red dun stallions rearing, Pryor Mountains, Montana, USA

Piotr Naskrecki / Mayfly subimago, La Selva Biological Research Station, Heredia, Costa Rica

Carol Walker / Mustang horse, dun mare and blue roan foal nuzzling. Sand Wash Basin Herd Management Area, Colorado, USA.

Carol Walker / RF - Mustang horse, dun mare and blue roan foal nuzzling. Sand Wash Basin Herd Management Area, Colorado, USA. (This image may be licensed either as rights managed or royalty free.)

John Shaw / Soaptree yucca (Yucca elata) on gypsum dun, White Sands National Monument, New Mexico, USA. December

Juan Carlos Munoz / Dun Chonchuir Fort, Inishmaan, Aran Islands, County Galway, Ireland. May 2011.

Juan Carlos Munoz / Dun Aonghasa / Fort of Aongus hill fort. Inishmore, Aran Islands, County Galway, Ireland. May 2011.

Juan Carlos Munoz / Limestone pavement overlooking Galway Bay. Dun Duchathair / Black Fort Cliffs. Inishmore, Aran Islands, County Galway, Ireland. May 2011.

Juan Carlos Munoz / Limestone pavement overlooking Galway Bay. Dun Duchathair / Black Fort Cliffs. Inishmore, Aran Islands, County Galway, Ireland. May 2011.

Klein & Hubert / Dun mustang foal eating wildflowers, Rocky Mountains, USA. June.

Klein & Hubert / Fjord or Norwegian fjord horses - dun mares and foal trotting in grass meadow in fall, France

Klein & Hubert / Fjord or Norwegian fjord horse - dun mare standing in grass meadow in fall, France

Klein & Hubert / Fjord or Norwegian fjord horse - dun mare walking in grass meadow in fall, France

Klein & Hubert / Fjord or Norwegian fjord horse - dun mare portrait, France

Klein & Hubert / Fjord or Norwegian fjord horse - light dun trotting in grass meadow in fall, France

Klein & Hubert / Fjord or Norwegian fjord horse - light dun trotting in grass meadow in fall, France

Klein & Hubert / Fjord or Norwegian fjord horse - dun mare and foal in fall - portrait, France

Klein & Hubert / Fjord or Norwegian fjord horse - dun mare and foal in fall - portrait, France

Klein & Hubert / Fjord or Norwegian fjord horse - dun mare standing in grass meadow in fall, France

Klein & Hubert / Fjord or Norwegian fjord horse - dun mare in grass meadow in fall, France

Klein & Hubert / Fjord or Norwegian fjord horses - dun mares in grass meadow in fall, France

Klein & Hubert / Fjord or Norwegian fjord horses - dun mares in grass meadow in fall, one showing flehmen response,, France

Phil Savoie / Yellow dun mayfly (Heptagenia sulphurea) female, River Usk, Wales, UK, May.

Phil Savoie / Morning dun mayfly (Ephemerella excrucians) Bozeman, Montana, USA.

Phil Savoie / Pale morning dun (Ephemerella dorothea Infrequens) pair mating in flight, Madison River, Montana, USA.

Phil Savoie / Pale morning dun mayfly (Ephemerella Infrequens), female carrying egg ball she is about to deposit in river, Madison River, Montana, USA.

Phil Savoie / Pale morning dun mayfly (Ephemerella Infrequens), female carrying egg ball she is about to deposit in river, Madison River, Montana, USA.

Philippe Clement / Dun Telve broch near Glenelg, showing Iron Age drystone hollow-walled structure, Ross and Cromarty, Scottish Highlands, Scotland, UK, June 2017

Philippe Clement / Dun Telve broch near Glenelg, showing Iron Age drystone hollow-walled structure, Ross and Cromarty, Scottish Highlands, Scotland, UK, June 2017

Carol Walker / Head portrait of wild dun roan Mustang mare in Sand Wash Basin, Colorado, USA.

Carol Walker / RF - Wild dun Mustang stallion running in Adobe Town, Wyoming, USA. October. (This image may be licensed either as rights managed or royalty free.)

Klein & Hubert / Semi-wild dun tobiano Criollo newborn foal, only one hour, with dun mare in bog near forest, Tierra Del Fuego, Argentina.

Carol Walker / Wild Mustang, dun horse running near Adobe Town, Wyoming, USA.

Carol Walker / Wild Mustang, dun horse in dust, Sand Wash Basin Herd Area, Colorado, USA.

Carol Walker / Wild Mustang, dun horse in dust. Sand Wash Basin Herd Area, Colorado, USA.

Carol Walker / Wild Mustang, dun horse resting, Sand Wash Basin Herd Area, Colorado, USA.

Andy Sands / Mayfly (Ephemera danica) just emerged at waters surface in dun form, the first of two winged adult instars or developmental stages, Hertfordshire, England, UK, June.

Andy Sands / Mayfly (Ephemera danica) just emerged at waters surface in dun form, the first of two winged adult instars or developmental stages, Hertfordshire, England, UK, June.

Niall Benvie / Plastic duck in large pothole. Near Bridge of Dun, Angus, Scotland, December.

Niall Benvie / Plastic duck in large pothole. Near Bridge of Dun, Angus, Scotland, December. (This image may be licensed either as rights managed or royalty free.)

Carol Walker / Mustang / Wild horse, mare with foal, Wyoming, USA

Carol Walker / Lusitano horse, dun stallion prancing, Portugal, May 2011

Carol Walker / Sorraia horse, rare breed, portrait, Portugal

Carol Walker / Wild horse / Mustang, dun, Adobe Town, Wyoming, USA