No accounting for taste? Giant sloths, ancient pumpkins, and evolutionary genetics in bitter taste receptors.

What can pumpkin evolution or the extinction of giant sloths tell us about sensory neuroscience? Read on, fellow nerds…

Though domesticated pumpkins and other gourds (think zucchinis, acorn squashes, butternut squashes), are edible (and tasty!), their wild cousins produce a toxic bitter compound, rendering them poisonous to humans, even in small amounts. Severe “toxic squash syndrome” has been reported after consumption of “just one or two bites” of squash tainted with this compound, known as cucurbitacin. Cucurbitacin production, in fact, defines the plant genus that contains all of our favorite edible squashes (the aptly named Curcurbita). 

 Gourds! Squash! Author:  Ian Patterson.

Gourds! Squash! Author: Ian Patterson.

Anyone who has ever picked a pumpkin and hauled it home might be wondering…why on earth would a plant produce fruit weighing more-than-some-dogs that no one can eat? A paper recently published in the Proceedings of the National Academy of Sciences attacks this question by examining the genetics of archaeological samples of gourds (this is some Jurassic Park level shit here, folks. Except WAY cooler because, well, it’s real. And it’s PLANTS) and the genetics of taste receptors in species ranging from bats and tarsiers to pandas and manatees (also polar bears, and alpacas, and tasmanian devils, and…).

  Restoration of Giant Ground Sloths (  M. americanum)   by   Robert Bruce Horsfall . Source: Wikipedia

Restoration of Giant Ground Sloths (M. americanum) by Robert Bruce Horsfall. Source: Wikipedia

It turns out that multiple types of plants—including paw-paws, American persimmons, osage oranges, and our favorite gourds—produce large fruits or large seeds that no modern animal seems to disperse. Current theory suggests that these plants evolved in a time when truly immense animals (think mammoths, mastodons, and giant ground sloths) roamed the American plains, munching down these fruits, seeds and all, and dispersing the intact seeds that survived the animals’ digestive tract. Because these plants have long outlived their evolutionary partners, in some ways they are out of place in time. Such species are said to have “anachronistic fruits” (it’s already my band name, don’t even try).

But why produce the bitter toxin? As someone who has given up on growing backyard vegetables, I can attest that large fruits with tough skins are not enough to deter menacing rodents. These smaller creatures might gnaw into the fruit but fail to disperse the seeds—a disaster for the plant (and for my shady, squirrel-ridden, and otherwise-forsaken garden). Scientists have reasoned that wild gourds may produce bitter compounds to deter these small seed predators while sparing the much larger mammals that can safely consume this amount of toxin. One potential complication is that the bitter taste might still deter the mastodons and giant sloths. That is…if they could taste it.

 Mastodon, the natural predator of bitter squashes and gourds. Source:  Wikipedia

Mastodon, the natural predator of bitter squashes and gourds. Source: Wikipedia

In fact, when scientists went sifting through the genomes of mammals large and small, they found that the larger an animal is, the fewer copies of the bitter taste receptor it has. Smaller animals who might be sickened by the toxin have evolved to be more sensitive to the taste, while larger animals may not be sensitive at all. Humans fall somewhere in the middle—less sensitive than mice and shrews, but more than pandas or manatees. In fact, the authors propose, after the extinction of their evolutionary partners, gourds might not have survived to modern times without the help of human domestication, which bred the bitter toxin out of our gourds and pumpkins.

Those sitting through the holiday meals at the children’s table might be wondering whether this same effect applies to them. Are smaller, younger humans, still learning to avoid poisonous foods, more sensitive to bitter tastes? Indeed, multiple lines of evidence indicate that your brussels and broccoli-snubbing young ones are, in fact, more sensitive to bitter taste than adults, even those adults with the same bitter taste receptor genotype (see here and here). So, maybe cut them a break, and pass along the domesticated pumpkin pie.