Virtually all mammals wisely choose to avoid eating chilli peppers and other foods that taste ‘hot’. New research shows that Chinese tree shrews have evolved to eat large quantities of chillies in their diet by tuning down their taste buds to the chemical that makes these foods spicy.
Yalan Han of the Kunming Institute of Zoology in China, and their colleagues found that Chinese tree shrews (Tupaia belangeri chinensis) in the lab were happy to feed on chilli peppers, unlike lab mice which stop eating food if it’s too spicy. To see whether this unusual behaviour was due to reduced numbers of pain receptors on the tongue, or reduced sensitivity of those receptors, the team looked at gene expression patterns and found that while mice and tree shrews had similar numbers of pain receptors, the tree shrews’ receptors were less responsive to capsaicin – the chemical that gives chillies their spice.
Tree shrews are thought to share a relatively recent common ancestor with primates, just 85 million years ago.
Spiciness is down to chemicals called capsaicinoids, which include capsaicin in chilli peppers and related chemicals in peppery plants like Piper boehmeriaefolium, which is naturally abundant in Chinese tree shrew habitat. These compounds evolved to prevent herbivores eating the leaves and stems of plants that produce them – chemical warfare against would-be attackers. Capsaicinoids stimulate the TRPV1 receptor which is found on the surface of pain-sensitive cells, particularly those on the tongue. TRPV1 receptors are designed to detect dangerously hot food and produce a pain response to protect the animal from burning itself, but evolution has exploited a quirk in their structure meaning they are also triggered by dietary compounds like capsaicin.
Looking to the genome, the team were able to track the reduced sensitivity of the tree shrews’ TRPV1 receptor to a mutation that made a single amino acid change – just one link in the protein chain – at the site that capsaicin binds, making the receptor less likely to bind capcaisin molecules that hit it. This simple change makes the shrews taste less of the spiciness of their food.
The ability to feed on Piper boehmeriaefolium and other spicy plants that are common in the tree shrews’ natural environment may have driven evolution of this simple but effective way to increase tolerance to spicy foods.
The moral of the story is this – never get into a chilli-eating contest with a tree shrew.