Most modern dog breeds, including chihuahuas, have recent wolf DNA from interbreeding, a new study finds.

A new study reveals that nearly two-thirds of modern dog breeds, including chihuahuas, carry detectable wolf DNA, likely from interbreeding within the last few thousand years rather than ancient ancestry. Analyzing thousands of dog and wolf genomes, researchers found small amounts in most breeds—around 0.2% in chihuahuas—while wolfdogs like Czechoslovakian and Saarloos breeds have up to 40%. Some hounds and sight hounds also show notable levels, but Saint Bernards had none. Scientists suggest village dogs may have facilitated gene flow, especially as habitat loss brought wolves into contact with stray dogs. Wolf DNA may influence traits like independence, suspicion of strangers, and territorial behavior, though breed traits don’t predict individual behavior. The findings, published in PNAS, highlight ongoing genetic ties between wild wolves and domestic dogs.