Some Greyhounds have a strong prey drive, having been bred to hunt and chase. For a few Greyhounds this urge can be difficult or impossible to overcome through training. Frequently, Greyhound adoption agencies require owners to keep them on-leash at all times, except in fully enclosed areas. A few agencies will also advise owners to keep their greyhounds muzzled around other dogs with which they are not familiar. While some owners disagree or ignore this advice, adoption groups like those linked here are generally very clear about this requirement. For the best interests of the breed in general they encourage owners to be cautious with Greyhounds because on the track they are trained to run, chase and bite regardless of weather or injury or individual desire, while at the same time breeders select dogs with a strong prey drive for breeding.