Chillies fiery mystery solved

 作者:危径     |      日期:2019-03-06 08:20:05
By Greg Miller The mystery of why chilli peppers are hot has been solved – the answer is for the birds. US scientists have found evidence that the chilli’s bite may have evolved to repel animals who are ineffective at dispersing its seeds and encourage the more effective seed spreaders, like birds. They report their findings in the journal Nature. Joshua Tewksbury and Gary Nabhan videotaped native chilli plants in southern Arizona. After watching 146 hours of tape, they concluded that the peppers were eaten exclusively by birds. Packrats and cactus mice, the two most common mammals in the area, appeared to have no appetite for the spicy fruits. These dining preferences appear to be related to the peppers’ sting. Tewksbury and Nabhan captured packrats, cactus mice and curved-billed thrashers – the daring bird caught on camera eating 72 per cent of the chillies – and offered them a choice of piquant chillies, non-piquant chillies, or non-piquant hackberry fruit. As expected, the rodents avoided the piquant chillies entirely. The thrashers ate all three offerings with equal gusto. Previous studies have shown that birds are unable to sense capsaicin. In a follow-up experiment, the researchers found that chilli seeds processed by the thrasher digestive system germinated just as well as control seeds. In contrast, non-piquant chilli seeds passed by the rodents failed to germinate. Moreover, seeds eaten by thrashers tended to end up under hackberry bushes, the birds’ favourite roosts, providing a suitably shady place for the chillies to grow. Nabhan believes the findings explain why chilli plants make capsaicin, the compound that gives them their heat. It deters animals whose guts destroy chilli seeds, leaving more for creatures that disperse the seeds in favourable locations. The chilli’s strategy represents a remarkable departure in plant evolution, Nabhan says. Whereas most plants in the nightshade family (which includes chillies, as well as important food crops like potatoes and tomatoes) produce alkaloid compounds that deter a wide range of predators, chillies are only plants known to have a directed deterrence strategy. “There was something that had worked well for hundreds of thousands of years,