Pesachim 31b ~ A Dog's Sense of Smell

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”

— Williams H, Pembroke A. Sniffer dogs in the melanoma clinic? Lancet 1989; 333 (8640): 734.

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.

Berachot 43a ~ Musk and Ambergris

In a discussion about which blessings to say and when, the Talmud considers various kinds of fragrances.

ברכות מג, א

אָמַר רַבִּי חִיָּיא בְּרֵיהּ דְּאַבָּא בַּר נַחְמָנִי אָמַר רַב חִסְדָּא אָמַר רַב, וְאָמְרִי לַהּ אָמַר רַב חִסְדָּא אָמַר זְעֵירִי: כל הַמּוּגְמָרוֹת מְבָרְכִין עֲלֵיהֶן ״בּוֹרֵא עֲצֵי בְּשָׂמִים״ חוּץ מִמּוּשְׁק, שֶׁמִּין חַיָּה הוּא, שֶׁמְּבָרְכִין עָלָיו ״בּוֹרֵא מִינֵי בְּשָׂמִים״ 

Rabbi Chiyya, son of Abba bar Nachmani, said that Rav Chisda said that Rav said, and some say that Rav Chisda said that Ze’iri said: Over all the incense one recites: Who creates fragrant trees, except for musk, which is extracted from a living creature, and over which one recites: Who creates various spices.

The male musk deer Moschus moschiferus. Yes, the fangs are real. Image from here.

Musk, at least the musk that was one used in the perfume industry, is a secretion from a gland of the male musk deer Moschus moschiferus. However the term “musk”now includes a number of different chemicals which all share a common, distinct, and typical aroma. Today, the perfume industry almost exclusively uses synthesized compounds, which is certainly good news for the cute deer. In fact since 1979 trade in musk from several countries has been banned by the Convention on International Trade in Endangered Species of Wild Fauna and Flora.

In her exhaustive monograph on the subject, Cornelia Sommer notes that musk gland is found near the rear end of the musk deer which lives in the upper regions of Eastern Asia, though presumably once had a far wider natural habitat. Discovery and use of musk date back to ancient China and pre-historic India, and as we learn today, it is mentioned in the Talmud. In order to get access to the natural musk, the animal must be killed to remove the gland, also called musk pod. The pods which weigh about 70g, contain about 40% musk. They are dried, and the reddish-brown paste inside them turns into a black, granular material called musk grain. The aroma of the tincture, which is described for example as animal-like, earthy, and woody, becomes more intensive during storage, and only after considerable dilution does the obtained extract exhibit a pleasant odor.

Because natural musk is rare and expensive, chemists started to synthesize an alternative centuries ago. In 1890 the German chemist Albert Baur succeeded in synthesizing the first chemically defined substance with musk odor, which he patented and commercialized as “Musc Baur.” (Apparently, the discovery was happenstance. Bauer was actually tiring to find a better explosive when he chanced upon the synthetic compound.) Other members of this class of compounds, called nitro musks, were later synthesized and gained considerable commercial importance. Thousands of tonnes of synthetic musks are now produced each year.

Perfume in the urine of the deer, and the excrement of the whale

In his commentary on the Talmud, the great medieval exegete Rashi suggests that the musk can be found in the excrement of an animal (מן הרעי של חיה). And he was not wrong, since the musk deer excretes it in his urine to mark his territory and attract a female. Another animal whose excreta was prized in the production of perfume was the whale, or specifically the sperm whale. For centuries it produced a fantastically expensive substance called ambergris (from the old French meaning grey amber), which is produced in the intestines of the whale and excreted into the ocean. There is spends years bobbing about minding its own business, undergoing oxidation and photodegradation until it washes up on land. It has “a peculiar odour that is at once sweet, earthy, marine, and animalic.” Like musk it is used to produce perfume. In is classic novel Moby Dick, Herman Melville has an entire chapter on the mysterious ambergris.

“Now this ambergris is a very curious substance, and so important as an article of commerce, that in 1791 a certain Nantucket-born Captain Coffin was examined at the bar of the English House of Commons on that subject. For at that time, and indeed until a comparatively late day, the precise origin of ambergris remained, like amber itself, a problem to the learned. Though the word ambergris is but the French compound for grey amber, yet the two substances are quite distinct. For amber, though at times found on the sea-coast, is also dug up in some far inland soils, whereas ambergris is never found except upon the sea. Besides, amber is a hard, transparent, brittle, odorless substance, used for mouth-pieces to pipes, for beads and ornaments; but ambergris is soft, waxy, and so highly fragrant and spicy, that it is largely used in perfumery, in pastiles, precious candles, hair-powders, and pomatum. The Turks use it in cooking, and also carry it to Mecca, for the same purpose that frankincense is carried to St. Peter’s in Rome. Some wine merchants drop a few grains into claret, to flavor it.

Who would think, then, that such fine ladies and gentlemen should regale themselves with an essence found in the inglorious bowels of a sick whale! Yet so it is.”

— Moby Dick chapter 93

The Elusive nature of Smell

In today’s page of Talmud, the rabbis consider the nature of smell.

״כֹּל הַנְּשָׁמָה תְּהַלֵּל יָהּ״ אֵיזֶהוּ דָּבָר שֶׁהַנְּשָׁמָה נֶהֱנֵית מִמֶּנּוּ וְאֵין הַגּוּף נֶהֱנֶה מִמֶּנּוּ? — הֱוֵי אוֹמֵר: זֶה הָרֵיחַ

“Let every soul praise the Lord” (Psalms 150:6). What is it from which the soul derives benefit and the body does not derive benefit from it? You must say: That is scent. Even over items from which only the soul derives benefit, one must recite a blessing and praise God.

According to the rabbis, this intangible sense leaves its mark not on the body, but on the soul. How remarkable it is that some fragrances are composed of substances found hidden deep inside animals that are rarely seen by us. And how much more pleasurable is it that now, no animals need be harmed in the making of this blessing.

Niddah 20a ~ Rabbis, Pheromones and the Scientific Method

Today the Talmud tells two stories of rabbis, Rav Elazar and Rava who each used their keen sense of smell to identify the origins of uterine blood.

נדה כ,ב

ואמאי קרו ליה מרא דארעא דישראל דההיא אתתא דאייתא דמא לקמיה דרבי אלעזר הוה יתיב רבי אמי קמיה ארחיה אמר לה האי דם חימוד הוא בתר דנפקה אטפל לה רבי אמי אמרה ליה בעלי היה בדרך וחמדתיו קרי עליה (תהלים כה, יד) סוד ה' ליראיו

There was an incident involving a certain woman who brought blood before Rabbi Elazar for examination, and Rabbi Ami was sitting before him.Rabbi Ami observed that Rabbi Elazar smelled the blood and said to the woman: This is blood of desire, i.e., your desire for your husband caused you to emit this blood, and it is not the blood of menstruation. After the woman left Rabbi Elazar’s presence, Rabbi Ami caught up with her and inquired into the circumstances of her case. She said to him: My husband was absent on a journey, and I desired him. Rabbi Ami read the following verse about Rabbi Elazar: “The counsel of the Lord is with those who fear Him; and His covenant, to make them know it” (Psalms 25:14), i.e., God reveals secret matters to those who fear Him. 

אפרא הורמיז אמיה דשבור מלכא שדרה דמא לקמיה דרבא הוה יתיב רב עובדיה קמיה ארחיה אמר לה האי דם חימוד הוא אמרה ליה לבריה תא חזי כמה חכימי יהודאי א"ל דלמא כסומא בארובה

The Gemara further relates that Ifera Hurmiz, the mother of King Shapur, sent blood before Rava for examination, [as she sought to convert and was practicing the halakhot of menstruation]. At that time Rav Ovadya was sitting before Rava. Rav Ovadya observed that Rava smelled the blood and later said to the woman: This is blood of desire. She said to her son: Come and see how wise the Jews are, as Rava is correct. Her son said to her: Perhaps Rava was like a blind man who escapes from a chimney, [i.e., it was a lucky guess…]

Available on Amazon. Results definitely not guaranteed.

Each rabbi had inhaled the odor of the sample of blood and and concluded that it was “the blood of desire.” Rashi explains that the women from whom the sample came had sexually desired her husband to the degree that it had prompted a uterine bleed (שנתאוית לבעלה וראתה הדם מחמת תאוה). Leaving aside the question of whether menstrual bleeding and sexual desire are somehow related, the question we will focus on is this: could these blood samples really have smelled different? And that brings us to the topic of the day: pheromones.

Pheremones and Menstrual synchrony

“Pheremones are chemical signals that have evolved for communication with other members of the same species.” That is how the Oxford University zoologist Tristram Wyatt, defines them, though he is an admitted sceptic when it comes to their existence in us. The earliest claim that they might exist in humans was made back in the 1970s in those famous studies of synchronized menstruation in women who lived together in the same college dormitory.

The landmark study was published by in 1971 the American physiologist Martha McClintock, who studied the menstrual cycles of young women living on the leafy campus of Wellesley College in Massachusetts. McClintock analyzed their mensrtual diaries, and noted “a significant increase in synchronization among roommates and among close friends.” She was quick to note that although this was a preliminary study, “the evidence for synchrony and suppression of the menstrual cycle is quite strong, indicating that in humans there is some interpersonal physiological process which effects the menstrual cycle.” Just as Rava and Rav Elazar had claimed.

Lots of hypotheses were proposed to account for this menstrual synchrony. Perhaps the young women were eating similar foods, or were influenced by the same weather, or were subject to the same daily stresses of exams. Perhaps the phases of the moon were responsible? Or perhaps they were influenced by the presence of men near the campus? There was another possibility too. Perhaps there was a pheromonal signal between the menstruating women. That might be the cause. Maybe a chemical signal between the women caused the synchrony, rather than it occurring as the result of an alignment of each woman’s cycle to some external environmental signal.

Only there was a big problem. Over the decades scientists tried to replicate McClintock’s findings, but they couldn’t. Twenty years passed and four studies couldn’t replicate the findings. Thirty years passed and new experiments failed to find evidence of menstrual synchrony. And now, almost fifty years since the original study the scientific consensus is that at best there is no evidence for it; at worst, it has been thoroughly discredited. Here is how a 2014 review in the Journal of Sex Research summarised the field:

An appreciation of the likely patterns of ovarian cycling throughout much of human evolutionary history (until the 20th century) coupled with data on the extraordinary variation within and among contemporary women in cycle length quickly leads to a nagging doubt regarding the likelihood of MS sensu stricto. Add a good dose of probability theory and the fact that reasonably well designed studies have failed to support the MSH, and one is left wondering why so much attention has been given to searching for elusive mechanisms and constructing convoluted evolutionary scenarios…

So much for menstrual synchrony. But now let’s get back to the question of pheremones. Since a pheromonal mechanism of synchronization is the only plausible mechanism to account for synchrony, and since synchrony doesn’t occur, then maybe it follows that there are no pheromones that modulate the length of the human menstrual cycle. To test this, Jeffrey Schank, a psychologist from the University of California at Davis painstakingly reviewed “all the studies directly or indirectly related to pheromone modulation of the menstrual cycle,”though he noted that “this is a very small literature of eight studies spanning 25 years.” All eight studies had serious methodological flaws that you can read about here, and Schanks concluded that when taken together, “these results cast doubt on the existence of pheromones that modulate the length of menstrual cycles.”

For example, consider a study by Kathleen Stern and (you guessed it…) Martha McClintock, published in the very prestigious journal Nature in 1998. They claimed that that odourless compounds “collected from the armpits of women in the late follicular phase of their menstrual cycles accelerated the preovulatory surge of luteinizing hormone of recipient women and shortened their menstrual cycles. Axillary (underarm) compounds from the same donors which were collected later in the menstrual cycle (at ovulation) had the opposite effect: they delayed the luteinizing-hormone surge of the recipients and lengthened their menstrual cycles.” Wouldn’t that support the suggestion that humans produce compounds that regulate a specific neuroendocrine mechanism in other people without being consciously detected as odours? That is, after all, the classic definition of a pheromone.

Well no. In the first place the study results were a trend but were not statistically significant. But more importantly, Schanks noted that it was was confounded by using the third cycle, which was a treatment cycle, as a baseline cycle for determining the change in cycle length resulting from ovarian cycle secretions. It was as flawed as the other seven studies.

The search continues

We have no evidence that human pheromones exist. But that is not the same as having evidence that they do not. The search continues, and some are hopeful that by returning to good scientific principles and by using more rigorous techniques we can avoid some of the mistakes of the past.

And what are we to make of the claim that Rava and Rav Elazar could detect pheromones? Well, we should make of it exactly what the Talmud itself makes of it. Either their conclusions were the result of God’s direct revelation to “those who fear Him” (סוד ה' ליראיו), or, and this is equally possible, they were just a lucky guess. Take your pick.

“Science is not subject to statutes of limitation or prohibitions against double jeopardy. Theories, methods, and data are forever open to critical review. Science only progresses when hypotheses and theories are given the most severe tests possible. Indeed, even when a theory passes a severe test, errors may be subsequently found in data and methods supporting that theory. This implies that neither theories, data, nor methods can be accepted with absolute certainty. Scientists are fallible, and even the peer review process is no guarantee against error. The fact that errors may occur at all levels of scientific inquiry appears to lead to the skeptical view that all of science is on an equally uncertain footing. However, by repeatedly scrutinizing theories, data, and methods to weed out errors, we can have growing confidence in those that survive. This is a never ending process, but the more we critically scrutinize previous results, the more confident we can be in those theories, data, and methods in which we fail to find errors. Perhaps future studies will find indisputable evidence of pheromones that modulate menstrual cycles, but the studies to date have not. ”

— Schank, J. Do Human Pheremones Exist? Human Nature 2006: 17 (4). 448-470.