Car rental companies today are reportedly charging for compact cars what they used to charge for their roomier, rugged counterparts. But it's not just the smaller version of cars Americans are developing a fondness for.

According to the American Kennel Club's rankings, last year's Top 15 most popular dog breeds included Yorkshire Terriers, Dachshunds, Shih-Tzus, Bulldogs, Miniature Schnauzers, Chihuahuas, Pomeranians and Pugs. And this year feels no different, with mini-pooches popping up in shoulder bags and grocery carts from coast to coast.

Why the sudden surge in appreciation for the compact canine? Actually, it's been more of a slow rise since the mid-'90s. Modern Americans live fast-paced lifestyles that often mandate more hours in the car and at the office than at home. In this case, a portable pooch may be a better match than, say, a 75-pound Boxer with more energy than a 10-year-old boy after a Red Bull float.

But don't be fooled: A pint-sized pooch — untrained — is tantamount to a two-ton tank of trouble. Take for example the following excerpts from letters I received this week:

"My Dixie! She is an extremely dominant, possessive, territorial 5-pound dog that my vet said is the most dominant Yorkie she has ever worked with. Most people giggle when they hear of this situation because she is so adorable and teeny. But last summer I was bitten so badly by her because Buddy, my Yorkie rescue, came near me, I required a trip to the emergency room and a healthy dose of antibiotics. Now Henry, the 1-year-old Shih-Tzu rescue, is on board, and frankly, I am completely beside myself with fear that she is going to seriously hurt one of them if they dare to even sit near me. These poor guys are terrified, and it breaks my heart to see them hiding when they should be sitting on the couch with me. I have to be so careful with Dixie that my heart is in my mouth almost all of the time when I am home."

From another reader:

"I have had Ginger, a 6-month-old female toy poodle, for four months. I already had a male toy poodle who is 3 years old. I am leaning toward possessive aggression and dog aggression. I think she has a split personality. She will jump in my lap and loves to be pet, but she gets into fights with my other dog several times a day. Today she growled and snapped at me when I tried to get her off the kitchen table. And later, when I broke up the fight with her brother, she bit pretty deep into my hand. I have a 2-year-old son I am concerned about. I bought Ginger to give my older dog company, but she is stressing me out."

Little dogs, big trouble. But that doesn't have to be the case. Neither Dixie nor Ginger was properly socialized or trained. People love the look and convenience of compact canines, but these dogs tend to get a free ride. Expectations are low and efforts at discipline are nonexistent because it's assumed the consequences, like the dog, will be minimal.

In April of 2007, National Geographic News and the journal Science printed the results of a study that asked the question: What makes a small dog small? They identified a variation in a single gene — IGH-1 — that acts as a key player in the suppression of growth in toy dogs. Lead researcher Elaine Ostrander of the National Human Genome Research Institute said, "The best way to describe the role of the gene is, it's like the 'reduce' button on a Xerox machine."

In other words, small dogs are still dogs — with all their varying personalities and temperaments, with all their instincts and teeth. We owe it to them as much as their large counterparts to give them the education they need to remain at the top of those lists.

Woof!

Dog trainer Matthew "Uncle Matty" Margolis is co-author of 18 books about dogs, a behaviorist, a popular radio and television guest, and host of the PBS series "WOOF! It's a Dog's Life!" Read all of Uncle Matty's columns at the Creators Syndicate website at www.creators.com, and visit him at www.unclematty.com. Send your questions to dearuncle.gazette@unclematty.com or by mail to Uncle Matty at P.O. Box 3300, Diamond Springs, CA 95619.

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