All the Lassies were actually male dogs because female collies tend to "blow coat" (go through a massive hormone-induced shedding process) with each heat cycle. While males blow coat as well in reaction to a change in season, it is much less noticeable than what occurs with an intact female. By the time that spaying, which would reduce the dramatic shedding of the female, became commonplace, it had become tradition to use a male in the role. Additionally, it was believed that males, who often outweigh their female counterparts by as much as fifteen pounds, would look more impressive on film.