They are the truly forgotten dead. Sixteen million animals “served” in the first world war – and the RSPCA estimates that 484,143 horses, mules, camels and bullocks were killed in British service between 1914 and 1918.

Some died before they reached the western front: of 94,000 horses sent from North America in 1917, 2,700 drowned when their vessels were sunk by submarines. Trench dogs hunted for rats in the trenches. Others carried messages. The German army alone employed 30,000 dogs. In a canine echo of War Horse, dogs were recruited from animal shelters, and when that supply ran out, from the general public. “I have given my husband and my sons,” wrote one English woman, “and now that he too is required, I give my dog.”

In no man’s land, dogs did jobs humans could not, such as taking supplies to the wounded so that they could treat themselves; and “mercy dogs” would stay with dying soldiers to keep them company. Such stories bear witness to the loyalty of animals. Dick, a black retriever messenger dog, was wounded in action but recovered enough to resume his duties. He developed a limp, grew weaker, and had to be put down. A postmortem showed that he’d been working with a bullet lodged in his chest and a shell splinter close to his spine.

Other animals acted as canaries in the mine. One South African unit had a baboon called Jackie with sharp hearing, who would tug at men’s sleeves if he detected enemy advances. Slugs were used when it was discovered they would visibly demonstrate their discomfort in the presence of mustard gas in smaller quantities than humans could sense, allowing soldiers to don their gas masks in time.

Facebook Twitter Pinterest Dogs are used by the British army to pull a machine gun in 1915. Photograph: Universal History Archive/UIG via Getty Images

Some animals may have been grateful for more placid roles. At the vast military hospital at Netley on Southampton Water, where thousands of shell-shocked troops were treated – including the poet Wilfred Owen – donkeys were employed to calm men suffering from PTSD. On ships, dogs, pigs and even magpies became animal spirits to deflect the stress of war.

Around 100,000 pigeons served, too. One delivered a message from a US battalion trapped behind enemy lines: “Our artillery is dropping a barrage directly on us. For heaven’s sake stop it”. The birds’ work was so important that they were protected by the Defence of the Realm Act, which criminalised any attempt to kill or maltreat them. It was an ironic implementation of animal rights to mirror the first conservation law in Britain, introduced by the 8th century saint, Cuthbert, who declared that the eider ducks of the Farne Islands should be protected; ironic because during the war, huge rafts of eiders at rest in the North Sea were used by the RAF for target practice.

Whales were used for the same purpose. It was the first time cetaceans had been seen and photographed from the air. One account noted, “In the half-lights, these huge monsters bore a strong resemblance to a submerged U-boat, and, as the rule in war was, ‘When in doubt, bomb’, a good many of them were killed by our aircraft.” Meanwhile, 175,000 whales died in the South Atlantic to furnish rifle oil, fuel for trench stoves and oil to protect against trench foot. Germany culled dolphins and seals for their oil.

Whales were also co-opted to deal with food shortages: as Michael Freemantle notes in his book The Chemists’ War, Lever Brothers had worked out how to hydrogenate whale oil to make it fit for human consumption. Most terribly, these placid animals were processed into munitions themselves as their bodies yielded glycerine for bombs.

Facebook Twitter Pinterest The British cavalry passing the remains of Albert Cathedral, after the second Battle of the Somme, August 1918. Photograph: Hulton Archive/Getty Images

It wouldn’t be until later in the century that humans employed cetaceans in war, but even in the postwar period, consciences were stirring. In a 1924 essay entitled The Impudence of Flags: Our Power Resources and My Elephants, Whales and Gorillas, HG Wells, prophetic author of The War of the Worlds, wrote: “The dwindling world fauna of this planet is in urgent need of international game laws and a supernational game-keeper. Species of whales are being exterminated because the ocean is no man’s land.”

In the fitful periods of peace in the last century, there was no armistice for animals, and monuments to their efforts are few, London’s Park Lane memorial being a noble exception. One hundred years after the first war of the anthropocene, itself a perversion of nature and fought for the Earth’s resources, these non-human casualties remain as an indelible stain on our conscience.

• Philip Hoare is an author. His latest book is RisingTideFallingStar