Eye color varies from brown to blue, and occasionally eyes of differing color occur; this is usually seen with merles. The ears of the border collie are also variable — some have fully erect ears, some fully dropped ears, and others semi-erect ears (similar to those of the rough collie). The border collie is widely considered to be the most intelligent dog breed. The border collie ranks 1st in Stanley Coren's The Intelligence of Dogs, being part of the top 10 brightest dogs. Although the primary role of the border collie is to herd livestock, the breed is becoming increasingly popular as a companion animal. Although the primary role of the border collie is to herd livestock, the breed is becoming increasingly popular as a companion animal. In the UK, there are two separate registries for border collies. The International Sheep Dog Society encourages breeding for herding ability, whereas the Kennel Club (UK) encourages breeding for a standardised appearance.