Dear Straight Dope: Isn't it true that birds do not have receptors for capsaicin, the protein that makes peppers hot, so birds will still eat the peppers and disperse the seeds? I don't remember where I heard this. Did I just dream it? Christopher Vittore

SDStaff Colibri replies:

Birds will happily eat the hottest of hot chili peppers, a fact so well known that some varieties are popularly known as “bird peppers.” For anyone who has ever “enjoyed” a five-alarm mondongo at a Mexican restaurant, two questions will immediately leap to mind: (a) How can such featherweight creatures consume enough incendiary material to vaporize a camel and still show no signs of discomfort? and (b) hokey smokes, Josephine, what do you suppose that will do to their tiny bungholes tomorrow morning?

The chili (or chile) peppers, genus Capsicum, comprise about 25 species, of which five are regularly cultivated. Of American origin, native peoples have grown them as a valued condiment for thousands of years. Columbus, who first brought them to Europe, compared them to the completely unrelated black pepper (Piper nigrum) of the Orient, as part of his PR campaign to demonstrate that he had in fact found the “Spice Islands.” The heat engendered by chilies proved so popular that they rapidly became an integral part of many Old World cuisines, including those of India, Southeast Asia, southern China and parts of Africa.

All wild chilies contain varying amounts of the chemical capsaicin and related compounds. (Sweet or bell peppers are cultivated varieties that have been selected for low capsaicin content.) Capsaicin is not a protein, but a nitrogen-containing lipid related to vanillin, the active principle in vanilla. The compound has a powerful irritant effect on certain mammalian pain receptors (nociceptors). The key receptor molecule, a protein on the outer surface of the cell, was identified in 1997. When capsaicin comes into contact with it, a cascade of intracellular reactions is triggered that is perceived by the brain as pain. These reactions are very similar to those produced by damaging heat, so it is no coincidence that we sense chilies as being hot. The effect can be so overpowering that sprays containing capsaicin are used to repel grizzly bears and even elephants.

The pepper sensation is not, properly speaking, a taste. There are only five kinds of taste buds (salt, sweet, sour, bitter and MSG, the last only recently identified). Capsaicin itself is tasteless and odorless. What we describe as the “taste” of chili might better be described as the “pain” of chili (and we perceive this in parts of the body that clearly have no taste buds, to wit, the sphincter). One possible explanation for the appeal of chilies is that the body manufactures painkilling endorphins, akin to morphine, to counteract the pain, and endorphins themselves are pleasurable. In other words, we eat chilies because it feels so good when we stop. The heat of chilies is traditionally expressed in Scoville units, a subjective scale devised by the pharmacist Wilbur Scoville in 1912. Jalapenos rate about 4,000 Scoville units, while the hottest habeneros score up to 400,000. One variety in southeast Asia has recently been evaluated at an incredible 850,000 Scoville units, a veritable vegetable Chernobyl.

The situation is entirely different for birds. While mammals will avoid food containing as little as 100-1000 parts per million (ppm) of capsaicin, birds will readily consume up to at least 20,000 ppm (mind, we’re talking food that’s 2% pure capsaicin here). The difference seems to be that bird receptor cells are largely insensitive to capsaicin. Certain chemical modifications can make capsaicin somewhat aversive to birds, which shows that it is the structure of the molecule that is the key. Capsaicin sensitivity is perhaps the most well known difference between bird and mammalian receptors, although birds also seem to be insensitive to many other substances that are irritating to mammals, including ammonia and naphthalene. (A contrasting case is methyl anthranilate, grape flavoring, which is aversive to birds but not to mammals.) This difference is exploited by some commercial bird seeds, which add chili powder or capsaicin to the mixture to deter feeder-raiding squirrels.

The reason chilies incorporate capsaicin in their fruits (and red/green peppers of course are fruits in a botanical sense, not vegetables) seems to be to ensure that their seeds are dispersed properly. When small birds consume the fruits of wild peppers the seeds pass through the gut undigested and, due to the birds’ flight range, are deposited in distant places where they can grow with less competition. If the fruits were consumed by larger mammals the seeds would either be digested, or deposited much closer to the parent plant. Studies have shown that the seeds of wild peppers are in fact dispersed almost exclusively by birds.

Given that capsaicin is so aversive to mammals, one might wonder if birds might not be able to protect themselves against predation by retaining the compound in their flesh or feathers. But although many insects do this sort of thing, it doesn’t seem to be common in birds. (One exception is the Pitohui — AKA the P-tuh-hooey! — of New Guinea, which contains a neurotoxin apparently picked up in its food.) When broiler chickens have been experimentally fed capsaicin, the compound was not detectable by human taste testers. I for one was disappointed to learn this, as a self-enchilada-ing pollo could have saved me a step the next time I was cooking estilo mexicano.

George Angehr is an ornithologist working for the Smithsonian Tropical Research Institute in Panama.

SDStaff Colibri, Straight Dope Science Advisory Board

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