Animals played a key part in World War One - from transport to pest control they had important roles.
Both sides quickly realised that men on horses could not win the war in the trenches because of the muddy ground, so they were used for transportation instead.
Millions of horses were used in many different roles. They could be relied on to get food and equipment to the front line.
Meanwhile, strong ambulance horses carried wounded soldiers and artillery horses carried weapons, ammunition and other heavy loads.
Horses had a food ration of 20lbs of grain per day. In total, around 8million horses from all sides died during the war.
But the animals that were trusted the most and worked the hardest in the war were dogs. Medium sized dogs like German Shepherds were vital as they were agile and they were able to move unseen through the battlefield at night.
Among their roles included sentry dogs which stayed with one soldier or guard and were taught to give a warning sound such as growling or barking when they sensed a stranger.
As well as making excellent company for soldiers, dogs on the front line also made excellent rat catchers.
The conditions of the trenches were an ideal nesting place for rats as they fed on the rotting food as there was no proper way of getting rid of rubbish.
Despite all this, rats were sometimes helpful too. Many soldiers reported how rats sensed an oncoming attack from the enemy. They noticed that rats always ran away when this was about to happen. Rats would even sometimes warn and prepare the soldiers of enemy advances.
Pigeons also had a big part to play in communication in the war as they always flew home when released - so were used to send messages. They were far more reliable than man-made machinery to get messages from one military base to another and so pigeons were kept at military bases and headquarters.
One pigeon, called Cher Ami, meaning Dear Friend in French, became famous.
Used by US forces, Cher Ami managed to get 12 very important messages through one of the battles in 1918. It also saved the lives of nearly 200 soldiers during another battle. Cher Ami received a medal from the American government for her bravery.
There are many stories of animals that became companions and mascots to soldiers during World War One. Jimmy, The Sergeant, was a donkey born at the Battle of the Somme in 1916. He was wounded three times during the war. After the war, Jimmy helped to raise thousands of pounds for the RSPCA charity.