(First posted on July 21, 2006) Some plants do not want to get eaten. They may grow in places difficult to approach, they may look unappetizing, or they may evolve vile smells. Some have a fuzzy, hairy or sticky surface, others evolve thorns. Animals need to eat those plants to survive and plants need not be eaten by animals to survive, so a co-evolutionary arms-race leads to ever more bizzare adaptations by plants to deter the animals and ever more ingenious adaptations by animals to get around the deterrents.
One of the most efficient ways for a plant to deter a herbivore is to divert one of its existing biochemical pathways to synthetise a novel chemical - something that will give the plant bad taste, induce vomiting or even pain or may be toxic enough to kill the animal.
But there are other kinds of co-evolution between plants and herbivores. Some plants need to have a part eaten - usually the seed - so they can propagate themselves. So, they evolved fruits. The seeds are enveloped in meaty, juicy, tasty packages of pure energy. Those fruits often evolve a sweet smell that can be detected from a distance. And the fruits are often advertised with bright colors - red, orange, yellow, green or purple: "Here I am! Here I am! Please eat me!"
So, the hot peppers are a real evolutionary conundrum. On one hand, they are boldly colored and sweet-smelling fruits - obvious sign of advertising to herbivores. On the other hand, once bitten into, they are far too hot and spicy to be a pleasant experience to the animal. So, what gives?
Back in 1960s, Dan Johnson had an interesting proposal he dubbed "directed deterrence" which suggested that some plants may make choices as to exactly which herbivores to attract and which to deter. Hot peppers are prime candidates for such a phenomenon. What is hot in peppers is capsaicin, a chemical that elicits a sensation of pain when it bind the vanilloid receptors in the nerve endings (usually inside the mouth) of the trigeminal nerve. As it happens, all mammals have capsaicin receptors, but it was found, relatively recently, that birds do not.
To test that hypothesis, Josh Tewksbury used two variants of hot peppers - one very hot (Capsicum annuum) and the other with a mutation that made it not hot at all (Capsicum chacoense) - and offered both as meals to rodents (packrats and cactus mice) and to birds (curve-billed thrashers).
All species ate the sweet kind about equally. When Josh offered them identically prepared meals made out of the hot stuff, the two rodents refused to eat it while the birds happily munched on it.
The study appeared in 2001 in Nature (pdf) and I saw Josh give a talk about it at that time as he was joining our department to postdoc with Dr.Nick Haddad. While my lab-buddy Chris and I gave him a lot of grief in the Q&A session on his lenient criteria of what constitutes a "hungry animal" (he needed them to be hungry for the feeding tests), still the main conclusions of the study are OK.
More importantly, it really happens in nature. Mammals avoid hot peppers out in Arizona where Josh studied them (and made videos of their behavior), but the birds gorged on peppers. When he analyzed the droppings of rodents and birds fed peppers, he saw that seeds that passed through avian intestinal tracts were fully fertile, while seeds eaten by mammals were chewed, crushed, broken or semi-digested and not fertile at all.
Additionally, the thrashers tend to spend a lot of time on fruiting shrubs of different kinds. While there, they poop. The hot pepper seeds in the droppings germinate right there and this is an ideal shady spot for them to grow.
What a great example of (co)evolutionary adaptations. Next time on this blog, the second Big Question: Why do we like to eat hot peppers?
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Umm...because they taste good? Because the pain they cause produces such a wonderful endorphin rush? Because food spoils rapidly in the tropics, so the anti-microbial properties helped keep our ancestors alive?
I'd be partial to the drug idea, actually - the whole anti-microbial thing only works after we started putting it in our food. Based on my sample size of one, I'd go with the pleasure/pain explanation.
Hmm, at least Seagulls go have working capsaicin receptors. When they eat sambal out of a restaurant trash you can see them crashing into the water to flush it away.
Or maybe that is just an urban legend that's told around here.
There are a variety of very hot peppers grown in the Pacific islands that people call 'bird peppers', precisely because birds love them.
I have heard that some people coat the peanuts in their bird feeders in chilli powder to deter squirrels from eating them. The birds don't notice, apparently.
Well, that's nifty. Thanks for re-posting that!
I'll sound all smart at my next dinner party while eating jalapeno poppers.
I feed pepper suet in my backyard because the bird seed store folks told me that the squirrels wouldn't bother it due to the pepper but that the birds wouldn't be able to taste the pepper.
Thanks for explaining how this came about!
I found out birds did not experience the "heat" of peppers long before the idea emerged in the scientific community.
I had a peach-faced lovebird that seemed to enjoy vomiting up its food and them re-eating it, so I put mashed chilli on the perch where this occurred. The bird vomited and ate the vomit along with the chilli and did not bat an eyelid.
your mom is hot