What If You Had a Shark's Sense of Smell?

Can sharks smell blood from a mile away? We take a shark's sense of smell and install it in a human body. You would not smell more than you do now. You would smell far less, and the hammerhead shark helps show why. The case for the super nose looks strong. A scalloped hammerhead has around a hundred twenty-five folded plates in each nostril, more than twice as many as a sandbar shark. And the famous numbers seem to agree: a drop of blood from a mile away, one part per million, two thirds of the brain built for smell. But when researchers measured it, the super nose shrank. In twenty ten, Tricia Meredith and Stephen Kajiura found that sharks and their relatives detect key food smells in the same range as ordinary bony fish, and that a bigger nose did not mean a more sensitive one. That same year, Jayne Gardiner and Jelle Atema fitted smooth dogfish with a tube to each nostril. When one side got the smell between a tenth of a second and half a second earlier, the sharks turned toward it, even when the other side was a hundred times stronger. In two thousand seven, they showed that sharks also need the lateral line, the flow sensors along the body, to follow a scent trail. Then the install. In twenty twenty-three, Sigrun Korsching's lab read eleven genomes from sharks and their relatives, including the great white shark and the whale shark, and found only about ten to fifty smell receptors. Humans have around four hundred. In the catshark, the receptor family behind most of the smells you know was not switched on in the nose at all. Meanwhile, John McGann traced the myth of the weak human nose to Paul Broca, and a Berkeley team showed that blindfolded people can follow a chocolate trail, and do better with two nostrils than one. Four things this video corrects. No study has shown that a shark can detect one drop of blood from a mile away. One part per million of an Olympic pool is about two and a half liters of blood, not a drop, so the popular picture is off by about fifty thousand times. The olfactory bulb is about seven percent of brain weight in a scalloped hammerhead and about three percent in a sandbar shark, and no measurement we found supports two thirds. And the recent claim that humans smell rain two hundred fifty thousand times better than sharks smell blood compares a volume of blood with the weight of one molecule, so it does not hold. The honest limits: the sensitivity figures come from electrodes on animals held still in a lab, the timing study used eight sharks of one species, and the receptor trade-off is still a hypothesis. Every figure in this video comes from a published measurement, not from a blog post. The pool and rain comparisons are our own math from those numbers. Sources are listed below so you can check them yourself. SOURCES Meredith & Kajiura, 2010 — Olfactory morphology and physiology of elasmobranchs (Journal of Experimental Biology 213:3449) Kajiura, Forni & Summers, 2005 — Olfactory morphology of carcharhinid and sphyrnid sharks: does the cephalofoil confer a sensory advantage? (Journal of Morphology 264:253) Northcutt, 1977 — Elasmobranch central nervous system organization and its possible evolutionary significance (American Zoologist 17:411), via Kajiura et al. 2005 Tricas, Kajiura & Summers, 2009 — Response of the hammerhead shark olfactory epithelium to amino acid stimuli (Journal of Comparative Physiology A 195:947) Gardiner & Atema, 2010 — The function of bilateral odor arrival time differences in olfactory orientation of sharks (Current Biology 20:1187) Gardiner & Atema, 2007 — Sharks need the lateral line to locate odor sources: rheotaxis and eddy chemotaxis (Journal of Experimental Biology 210:1925) Syed, Sharma, Policarpo, Ferrando, Casane & Korsching, 2023 — Ancient and nonuniform loss of olfactory receptor expression renders the shark nose a de facto vomeronasal organ (Molecular Biology and Evolution 40:msad076) Cox, 2008 — Hydrodynamic aspects of fish olfaction (Journal of the Royal Society Interface 5:575) Rygg et al., 2013 — A computational study of the hydrodynamics in the nasal region of a hammerhead shark (Sphyrna tudes): implications for olfaction (PLOS One 8:e59783) McGann, 2017 — Poor human olfaction is a 19th-century myth (Science 356:eaam7263) Porter et al., 2007 — Mechanisms of scent-tracking in humans (Nature Neuroscience 10:27) Geosmin odor threshold — Watson et al. 2008 and Antonopoulou et al. 2014, as cited in Water Practice & Technology 18:159 (2023) Florida Atlantic University, 2022 — Study explores shark nose shape, size and link to sensitivity of smell Boston University, BU Today — What a Shark's Nose Knows Next episode: the dog. Everyone says its nose is up to a hundred thousand times better than yours. For some smells, it is not better at all. Subscribe if you would rather have the number than the myth. #shark #sharks #scienceexplained