A new study suggests that dogs love tennis balls because of the smell of their owners.

Dogs are known for their love of tennis balls, but new research from the University of Portsmouth has found a possible reason for why dogs find them so appealing. The study was published in the journal Behavioural Processes and looked at a total of 18 dogs from two different breeds, who were trained to use three different toys.

Researchers found that both breeds had no preference for either toy until they were introduced to one particular toy. The toy that each dog chose was then placed in a room with their owner, who had also been separated from them by a screen. The results showed that both breeds spent more time looking at the room where they knew their owner was located when that room contained the toy they’d shown interest in. Dogs also spent more time sniffing at the air and pawing at the door when they could smell their owner through it.

This suggests that there is something about tennis balls that make them attractive to dogs, but what exactly it is isn’t clear yet. Lead researcher Dr Rachel Casey said: “Although we can’t say for certain what attracts dogs to tennis balls, this study suggests it could be an olfactory-based attraction similar to cats being attracted to catnip.