Today we will discuss the wonders of a dog’s sense of smell, prompted by this Mishnah:
פסחים לא, ב
חָמֵץ שֶׁנָּפְלָה עָלָיו מַפּוֹלֶת הֲרֵי הוּא כִּמְבוֹעָר. רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל אוֹמֵר: כל שֶׁאֵין הַכֶּלֶב יָכוֹל לְחַפֵּשׂ אַחֲרָיו
Chametz (leavened bread) upon which a rockslide has fallen is considered as though it has been eliminated, and it is not necessary to dig it up in order to burn it. Rabban Shimon ben Gamliel says: Any leavened bread that has been covered to such an extent that a dog cannot search after it is considered to have been eliminated.
The next question is of course, just how deeply buried a piece of chametz needs to be before “a dog cannot search after it”?
תָּנָא: כַּמָּה חֲפִישַׂת הַכֶּלֶב — שְׁלֹשָׁה טְפָחִים
It was taught -how deep will a dog search for something [using his sense of smell]? Three handbreadths
A tefach is a little more than three inches or around 8 cm (at least mine is) so if your chametz is accidentally buried at a depth of less than 9 inches, you have to dig it up and destroy it before Pesach. But any deeper, and it can stay where it is, because a dog will not search for it at that depth. Right?
Wrong.
But first, the bad news. There aren’t any scientific studies that have looked at a dog’s ability to detect buried chametz. I know this because I have looked. So we will need to use a surrogate measure of their sniffing ability. Instead of chametz, let’s look at their ability to detect something else. Buried human remains.
Search Dogs
In a 2003 study published in the Journal of Forensic Sciences, researchers collected fresh human tissue together with fresh animal remains “obtained from the meat department of a local grocery store” and buried them at various depths. They then tested the ability of four dogs (a Rottweiler, two German Shepherds and chocolate Labrador) with various degrees of training to find them. It is was a hard test for the dogs, and not all were able to find the samples. But one of the German Shepherds (a six year-old with five years of training) was able to detect skeletal human remains buried two feet underground. The trial was not repeated using loaves of bread, so we cannot know if this ability could detect chametz at that depth, but there is no reason to think otherwise. But two feet is just for beginners. In 2014, while searching for a missing teenager, dogs found human remains buried an astonishing fifteen feet underground.
Other studies have tested the ability of dogs to smell buried explosive mines in war zones. The success of the dogs depends on the weather, how much explosive the mine contains, and the depth at which it is buried. Some dogs could detect explosive buried 25cm beneath the soil. That’s three talmudic tefachim.
Dog’s can even smell cancer
“The idea of canines for disease detection can be traced back to 1989, when a woman noticed her dog constantly sniffing a mole on her leg that later turned out to be malignant melanoma”
Using their incredible noses, dogs have even been trained to detect the smell of cancer. “Since the first proof of principle study in 2004 showed that dogs could detect cancer at a better rate than chance, at least six follow up studies have confirmed these findings” wrote a team of neuroscientists in a 2014 paper published in the journal Cancer Investigation. “Through the use of blood, urine, feces and breath, it seems clear that dogs possess the ability to detect cancer in human bodily fluids.” They have been used to detect breast, prostate, ovarian, bladder and lung cancer, with varying degrees of success, as the table shows. The authors also note that a lot depends on the dog - and its handler. “Dogs that had been previously trained for other scent detection work, such as explosives, were highly effective at correctly identifying cancer, while those without any previous experience in scent detection generally performed worse. While some research has shown that varying dog breeds did not significantly alter performance, it should be further studied, especially due to the high levels of specialization attainable by breeds such as German Shepherds and Basset Hounds that are preferred by law enforcement.”
Summary of Canine Cancer Detection Studies. From Spencer W. Brooks et al. Canine Olfaction and Electronic Nose Detection of Volatile Organic Compounds in the Detection of Cancer: A Review, Cancer Investigation, 2005. 33:9, 411-419.
The Nose of an Average Dog
Later in this tractate the Mishnah reminds us how challenging it can be to make general rules about things like dough and chametz:
פסחים מח, ב
רַבִּי עֲקִיבָא אוֹמֵר: לֹא כׇּל הַנָּשִׁים וְלֹא כׇּל הָעֵצִים וְלֹא כׇּל הַתַּנּוּרִים שָׁוִין
Not all women [who are kneading the dough], not all wood, and not all ovens are the same
Rabbi Akiva’s point is that there are a lot of factors that effect the process of baking, and making general rules about the process is difficult. He may have said the same about the ability of a dog’s nose to detect things buried in the ground, or under a pile of rocks, or even inside a person. It depends on the dog, its trainer, and what exactly it is looking for. So while there may be some super dogs who could detect chametz buried to a far greater depth than three tefachim, the Talmud isn’t interested in rules for super dogs. Just your average one. And so it declared three tefachim to be the maximum depth of buried chametz that legally needs to be dug up. Any deeper, and you can leave it be. Just hope the dog doesn’t find it.
