Nick Upton / Flesh fly (Sarcophaga sp.) "bubbling" regurgitated stomach contents in order to increase the concentration of its food by evaporating excessive water content. Wiltshire, UK, September.

Nick Upton / Flesh fly (Sarcophaga sp.) "bubbling" regurgitated stomach contents in order to increase the concentration of its food by evaporating excessive water content. Wiltshire, UK, September.

Ashley Cooper / Ashram's bio-gas plant, that is fed with food waste and manure and fuels the kitchens and the Ashram's vehicles. Muni Seva Ashram,Goraj, near Vadodara, India,

Ashley Cooper / Ashram's bio-gas plant, that is fed with food waste and manure and fuels the kitchens and the Ashram's vehicles. Muni Seva Ashram in Goraj, near Vadodara, India. December 2013.

Ashley Cooper / Ashram's bio-gas plant, that is fed with food waste and manure and fuels the kitchens and the Ashram's vehicles. Muni Seva Ashram in Goraj, near Vadodara, India. December 2013.

Ashley Cooper / Ashram's bio-gas plant, that is fed with food waste and manure and fuels the kitchens and the Ashram's vehicles. Muni Seva Ashram in Goraj, near Vadodara, India. December 2013.

Andy Rouse / Great crested grebe (Podiceps cristatus) feeding feather to chick, aiding digestion, Cardiff, Wales, UK, March.

Markus Varesvuo / Two Black grouse (Tetrao / Lyrurus tetrix) females feeding on little stones to aid food digestion, Suomussalmi, Finland, February.

Markus Varesvuo / Male Black grouse (Tetrao / Lyrurus tetrix) feeding on small stones to aid digestion, Inari, Kiilopaa, Finland, January.

Markus Varesvuo / Male Black grouse (Tetrao / Lyrurus tetrix) feeding on little stones to aid digestion, Suomussalmi, Finland, February.

Anup Shah / Silvered / silver-leaf langur (Trachypithecus cristatus) feeding on the charcoal from a burnt tree trunk to aid digestion. Bako National Park, Sarawak, Borneo, Malaysia.

Nick Upton / Paper wasp (Polistes dominulus) bubbling regurgitated nectar in morning sunshine to speed digestion, Greece, August.

Pascal Tordeux / Mainframe used for the central controlling and monitoring of the process of anaerobic digestion in a cattle farm, Spain, January 2013.

Suzi Eszterhas / Chimpanzee (Pan troglodytes) reaching for leaf to eat with red colobus monkey meat (leaves used aid in digestion of meat) tropical forest, Western Uganda

Nick Upton / Face fly / Autumn house fly (Musca autumnalis) bubbling its stomach contents to speed digestion, Wiltshire garden, UK, April.

Nick Upton / Noon fly / Noonday Fly (Mesembrina meridiana) 'bubbling' stomach contents, perhaps a form of external digestion in flies. Wiltshire, UK, May.

Nick Upton / Lance fly (Dasiops sp. possibly Dasiops spatiosus) 'bubbling' regurgitated stomach contents in order to increase the concentration of its food by evaporating excessive water content, Wiltshire, UK,...

Fiona Rogers / Verreaux's sifaka (Propithecus verreauxi) feeding on sand and soil to aid digestion, Berenty Private Reserve, Madagascar, October

Anup Shah / Two Verreaux's sifakas (Propithecus verreauxi) feeding on sandy soil to aid digestion, Berenty Private Reserve, Madagascar, October

Fiona Rogers / Ring-tailed Lemur (Lemur catta) eating soil and sand to get important minerals to aid digestion. Berenty Private Reserve, Madagascar. Oct 2008.

SINCLAIR STAMMERS / Purple stinger jellyfish {Pelagia noctiluca) digesting eels in bell, Mediterranean

PREMAPHOTOS / Terrestrial flatworm / Planarian, crawling across rainforest floor, Trinidad Note - Digestive enzymes secreted from mouth, located in centre of body, to begin external digestion

Jane Burton / Yellow Labrador dog, regurgitating food after eating grass.

Igor Shpilenok / Kamchatka Brown bear (Ursus arctos beringianus) eating grass after hibernation to initialise digestion, Kronotsky River, Kronotsky Zapovednik Reserve, Kamchatka, Russia.

Suzi Eszterhas / Brown-throated Three-toed Sloth (Bradypus variegatus) biologist, Rebecca Cliffe, feeding harmless red dye to sloth for digestion study, Aviarios Sloth Sanctuary, Costa Rica

Suzi Eszterhas / Brown-throated Three-toed Sloth (Bradypus variegatus) biologist, Rebecca Cliffe, feeding harmless red dye to sloth for digestion study, Aviarios Sloth Sanctuary, Costa Rica

Ugo Mellone / Hooded grebe (Podiceps gallardoi) ingesting a feather to aid digestion, Strobel plateau, Santa Cruz, Patagonia, Argentina. Cropped.

Markus Varesvuo / Black grouse (Lyrurus tetrix) male eating stones to aid food digestion in gizzard. Suomussalmi, Finland. February.

Jussi Murtosaari / Capercaillie (Tetrao urogallus) male eating stones / gastroliths to aid digestion. Joutsa, Central Finland. January.

Doug Gimesy / Carolyn Donovan (certified emergency critical care veterinary nurse from the Peninsula Vet Emergency & Referral Hospital) counts the faecal pellets from a koala (Phascolarctos cinereus) in care and be...