From BBC
"A 2013 study of 70 dogs of 37 breeds found that pooches circle before they poop, seemingly to align their spines with Earth’s north-south axis. The placement of 1,893 stools (and 5,582 pools of urine) were analysed and when the dogs stopped spinning, they avoided east-west and pointed their noses pole-ward. The effect was seen when Earth’s magnetic field was stable, but disappeared when solar activity caused it to fluctuate. Just like migratory birds and salmon, it seems that dogs can sense Earth’s magnetic field, but instead of using it for navigation, they use it for defecation! Why that is, nobody knows!"