Animals were still used in cavalry charges, especially on the Eastern Front, early in the war. But mostly, animals were used for the same things they were always used for in war. In particular, draft animals, including mules and horses, were used to pull heavy weapons and equipment. Some animals, however, were used for innovative purposes. Carrier pigeons were used to carry messages between the trenches. With the advent of poison gas warfare, canaries, long used in mines to determine whether there was breathable oxygen, were employed to test for the presence of poison gas. Cats and dogs were used to hunt down the ubquitous rats that plagued the trenches. Dogs were used as sentries, as pack animals, by military police, and to detect and carry supplies to wounded soldiers.