A typical day at work for Cesar Millan might include putting on his running shoes and taking a fourhour jog with 40 dogs--large and small, young and old. Amazingly, most of those dogs belong to him. The rest are at the heart of his work; they are troubled dogs sent to Millan to learn good behavior. Obviously Millan, star of the TV show "Dog Whisperer" and author of the bestselling book Cesar's Way,has a special gift for working with these animals. Growing up on a farm in Mexico, Millan knew he wanted to work with dogs. His first job, at age 15,was helping a veterinarian  .He was so good at calming scared dogs and handling all kinds of situations that people started calling him "el perrero", Spanish for "the dog boy".Since then, he has built a rewarding career around his favorite animal. "My grandfather taught me at an early age not to work against nature,"he explains. In nature, _ . They form a group and follow one leader. Millan's specialty is teaching people to be pack leaders for their dogs. Studying dogs on the farm where he grew up,Millan realized they need lots of exercise to be calm. He explains his approach,"Exercise and discipline first,and then affection!"He says a lot of people get it backwards because they don't realize what dogs really need. Therefore, while he trains dogs, he teaches owners to understand that their pets need rules. He's helped famous people like Oprah Winfrey and movie star Will Smith and everyday people too. To find the right career, Millan encourages kids to do what they enjoy. "A lot of people don't realize I've been working with dogs for more than 20 years--long before my TV show or book. Success followed me because I was following my dream of being the best dog trainer in the world." By saying "dogs are pack animals"(in Para.4),the author means that  _  .
dogs like to live with people dogs are easier to be trained few dogs can be leaders dogs live in groups