The Bloodhound's Amazing Sense of Smell

Holly is one of the best and brightest detectives on the Massachusetts State Police Force. She is an expert in her ability to assemble clues while tracking missing persons or hunting down criminals. How did she get to be so good at her job? That’s easy — she works like a dog.

As we learn in NATURE’s Underdogs, Holly is a bloodhound once slated for the death chamber. In her youth, she was so destructive that no family could manage her as a pet. Passed from home to home six times before her first birthday, she had little prospect of making it to adolescence. But then, someone entered her life who sensed that behind Holly’s troubled eyes was an animal with phenomenal natural abilities.

For the past 25 years, Larry Allen, a member of Barbour County Tactical Search and Recovery Teams in West Virginia, has been training bloodhounds for law enforcement agencies across the country. Rescuing problem dogs like Holly, he works them through their behavioral issues so that they may achieve their full potential as “gainfully employed” trackers. But Allen insists that training is a relatively small part of what makes these dogs so good at what they do. “The working ability of a bloodhound is 75 percent nature and 25 percent nurture,” he says. And the nature part of the equation resides in the animal’s exquisitely designed nose.

Holly now serves as an olfactory sleuth with the K-9 corps of the Massachusetts State Police.

Often called a nose with a dog attached, the bloodhound is so adept at scent tracking its trailing results is admissible evidence in a court of law. Its outstanding ability to read terrain with its nose is primarily due to a large, ultrasensitive set of scent membranes that allows the dog to distinguish smells at least a thousand times better than humans.

Researchers have estimated that a bloodhound’s nose consists of approximately 230 million olfactory cells, or “scent receptors” — 40 times the number in humans. Whereas our olfactory center is about the size of a postage stamp, a dog’s can be as large as a handkerchief — according to Allen, it is among the largest in canines. “The physical size of their olfactory area far exceeds most other working scent dogs,” he says. “The larger capacity combined with the desire to work makes them a very good tool.”

When a bloodhound sniffs a scent article (a piece of clothing or item touched only by the subject), air rushes through its nasal cavity and chemical vapors — or odors — lodge in the mucus and bombard the dog’s scent receptors. Chemical signals are then sent to the olfactory bulb, the part of the brain that analyzes smells, and an “odor image” is created. For the dog, this image is far more detailed than a photograph is for a human. Using the odor image as a reference, the bloodhound is able to locate a subject’s trail, which is made up of a chemical cocktail of scents including breath, sweat vapor, and skin rafts. Once the bloodhound identifies the trail, it will not divert its attention despite being assailed by a multitude of other odors. Only when the dog finds the source of the scent or reaches the end of the trail will it relent. So potent is the drive to track, bloodhounds have been known to stick to a trail for more than 130 miles.

A bloodhound’s outward appearance also adds to its tracking ability. Loose, wrinkled skin around the face helps trap scent particles and long, drooping ears that drag on the ground collect odors and sweep them into the nostril area. The dog’s long neck and muscular shoulders, which slope into its strong back, allow it to track close to the ground for miles on end.

For the past two centuries, these natural-born detectives have proven legendary in their role in law enforcement. One of the greatest sleuths in canine history was a Kentucky bloodhound called Nick Carter. His dogged persistence led to the capture and conviction of more than 600 criminals throughout his illustrious career.

Despite the technological advances of our current age, many experts agree that these canines are a greater asset to a police force than some of the best high-tech surveillance equipment. Their extraordinary ability to discern a cold trail has sent them on fruitful missions, following tracks over 300 hours old.

But there’s more to becoming a good police dog than simply an acute sense of smell. The dog must also have a predisposition to working with a handler, be eager to please, and have a strong play drive. “The bloodhound’s whole biological makeup enables it to track like no other dog. Such skills are gifts of nature,” says Aidan Woodward, the associate producer of Underdogs. “However, without the disciplined and focused assistance of the dedicated trainer, a novice bloodhound may not reach anywhere near the potential it could.”

Allen was able to provide the gentle discipline that Holly needed, though he initially had his doubts. “The first time I saw Holly, all I could think was, how am I going to make this puppy into a working dog in 12 weeks?” he recounts. “Little did I know that she would develop a love for the game within two weeks and go on to be one of the best trainees I have ever had. The more that I worked with her, the more solid she became and the more she became my partner.”

After 12 short weeks of training, Holly had polished her natural skills as a tracking dog and was ready to begin her new career with the Massachusetts State Police. “Parting with Holly was very difficult,” Allen admits. “Compare it to having your child get married and move to the other side of the world the next day.”

But he is thoroughly proud of all she’s accomplished. “The best part of working as a trainer with the dogs from rescue is watching them develop skills and confidence in themselves,” offers Allen. “The ultimate reward is having a dog that you trained be involved in saving a person’s life or tracking down a violent criminal.”

As for Holly, she has been given a second chance and a fresh new start in life. What could be more rewarding than that?

  • trish bowden

    Hope all is well with Holly and she is in good health and still working hard as ever with the Mass state troopers

  • kathy conner

    Do you have any suggestions for coon hounds?
    Are there any books or dvds I could use. I have a 10 month old puppy and have had her for a month.

  • Carolyn Zadar

    How very blessed Holly was to have found such magnificent trainer and human being in Larry Allen. His genuine love toward her, Holly could certainly “smell” and became the great dog because of Larry. May he be blessed with all the goodness of life he deserves, the world is a better place because of him.

  • Barbara Meyers

    Thank you for another extraordinary program.

    I’m currently writing a book on the Human-Animal Bond which will include a chapter devoted entirely to Dogs & Horses in Law Enforcement and their human partners.

    I would like to contact Larry Allen to request an interview for the book.

    Would you be kind enough to pass this along to him or, if possible, provide me with contact information for Mr. Allen or the Barbour County Tactical Search & Recovery Team in W. Virginia?

    I will be happy to provide you and Mr. Allen with my bio, credentials, etc. upon request.

    With thanks,

    Barbara Meyers,
    Certified Grief Therapist~Animal Behaviorist~
    Human-Animal Bond Consultant
    Holistic Animal Consulting Center
    29 Lyman Avenue SI NY 10305
    718 7205548 bmeyers@si.rr.com

  • Taylor P.

    Can you e-mail me about, how dose a bloodhound track smell so well be for Oct 21, 08 thanks annd have a great day. By

  • ryan

    i am a corrctions officer and a bloodhound handler. i would like Mr. Allen’s contact info or if you could pass mine on to him for training of my dog and a few others thank you

  • Roberta

    I wish people wouldn’t buy these highly specialized dogs as pets. Holly was probably nuts with families because she wanted to track and they wanted her to play ball. Dogs are working animals by nature but put them in the wrong job and they will rebel. I have a lab mix and his job is to play ball. Needless to say, he is a perfectly well-behaved dog.

  • Mercy

    I would love to be able to contact Mr. Allen. I have a dog that is a Hurricane Katrina, Rita, and Wilma survivor that I could use some of his advice on. He happens to be a real good dog but there are a few behavioral problems that need to addressed. PLEASE HELP !!

  • sabrina

    It’s fascinating to learn about Holly’s gifted talents and existential turn around and the smart people who recognized her strengths and helped her to find her natural purpose.

  • Tyler Rios

    glad to hear there are other people so dedicated to blood hounds as me. i was wondering if u could give me sum tips on trainning my new bh? she is beautyful and very smart im hopeing to have her as a racoon an small game dog could u help?

  • Emily

    I’m really looking into getting into dog training and would love to get ahold of Mr. Allen as well. It would be much appreciated if you could give me his email or some way to contact him or even give him my email. Thanks.

  • Bill Stock

    Hello. I live in Acushnet, MA. I have a 2 year old bloodhound. I would love to get her into a tracking class. I would then like to volenteer her if/when needed. Is there such a class around here?? Is there a such thing? I would be greatful for any response. Thanks

  • Victoria


  • boonyamin adetoro

    this is very interesting and educative. please can you answer this question, between the dog and the rat which one has the best and greatest smelling capabilities? we are having this argument among friends here. thank you and stay blessed

  • bruce

    i had a coonhound, untrainable i think, who just like to track, so i bought 7 acres and he was happy as a lark; when i would bring him home after a day in the field he would love the fam and the house: he was run out!

  • Cpl S , Carty

    Larry the cape may county sheriffs department is looking for a bloodhound to replace maggie, Maggie is still working but she is 10 year old . give me a call on my cell the # is 609 425-0626

  • Melody Person

    My son has a female bloodhound that is 8 months old. She is often at my house when he works extremely long hours or weekends. She is very well behaved with one exception…she sleeps in the mudroom entry that is 5′ X 14′. She is eating my walls…. She just takes little nibbles all the time. It’s a great room for her with room to move around and look out he windows.

    I hate to crate her when I leave but that might be the only option. I’ve tried several recommended sprays but she simply licks the
    my last option.

    Her noise is always to the ground and she never……stops smelling around. I’ve heard dogs are attracted to the smell of sheet rock.

    Any suggestions would be greatly appreciated.


  • Linda

    Melody – Put the dog some place else. She doesn’t like being alone in that room. Crate her – plastic or wire. If that doesn’t work, then line the room with plywood so she can’t eat through the walls. I had to line one room with plywood so my Bloodhound would not eat the walls – but now she has another Bloodhound that she hangs out with so she is not lonely. Dogs don’t like little rooms even those with windows.

  • Ron Thompson

    Wow, some of these comments make me wonder about how people treat their dogs. We adopted a five year old coon hound and he has been one of the best pets we have had. Training him was an adjustment from prior bird dogs I have worked with but in the end we got it figured out and he is a great well behaved pet. He does not train like a bird dog does but all it took was a little research about the bread and understanding what makes them tick. No he isn’t going to play ball with me like my lab or brittany but we take him to the dog park and he has a blast tracking scents around the fence line and playing with the other dogs. He is terrific in the house and listens extremely well. The only real problem with him is he is led by his nose so I can’t let him off leash unless we are at the dog park. Overall he is one terrific pet.

    @Melody, have your son crate train her. It will be best for her and you. I’m not a big fan of crates and I minimize there use but all my dogs are crate trained because sometimes it is the only way to keep them safe and out of trouble.

  • kristin soares

    i needd help looking for a horse lost in the woods – any ideas?

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