A new study suggests wild squash and megafauna had a long mutually beneficial relationship during the Pleistocene. Wild squash evolved bitter poisons known as cucurbitains in their flesh that discouraged seed consumption by rodents. However, large mammals have fewer bitter taste receptors and can consume large quantities of cucurbitains without ill effect. Most squash seeds could survive passage through the gut tract of a megaherbivore and were spread throughout the environment in fertile piles of dung. The squash plants thrived in open sunny environments created by megafauna foraging and trampling. Mammoths, mastodons, and giant ground sloths killed trees by uprooting them or by stripping off the bark. This opened woodland canopies where squash plants were exposed to direct sunlight. Trampling and wallowing also killed grass, resulting in bare soil environments where squash plants could germinate with less competition. In exchange for providing food, wild squash enjoyed a wide and continuous geographic distribution during the Pleistocene along megafauna game trails, around water sources, or wherever megaherbivores congregated.
Wild squash seeds extracted from ancient mastodon dung.
After the megafauna became extinct the range of wild squash was fragmented into relic habitats. Some species likely became extinct. Seeds found in fossil mastodon dung in Florida don’t exactly match those of any known species. However, humans began cultivating squash about the same time the megafauna died out (circa 10,000 BP). Initially, humans used the squash rinds as containers, but the seeds could be eaten as well, if the toxic pulp was washed off the edible kernels. Humans selected for mutated squash that had sweet rather than bitter flesh. Successful cultivation of improved varieties didn’t occur until wild squash became less common in the natural environment because when improved varieties backcross with wild varieties, a bitter hybrid is produced. This suggests good varieties of squash were not cultivated until Pleistocene megaherbivores were completely gone or rare. Without mastodons wild squash lost their distributors, and cultivated squash could grow with less chance of backcrossing.
The scientists who conducted this study looked at the genomes of 91 squash specimens consisting of 12 species and including 42 domesticated specimens, 30 wild specimens, and 19 specimens found at ancient archaeological sites. They determined squash was domesticated independently in several different regions in Mexico and southeastern North America. The genetic evidence suggests the Okeechobee gourd (C. okeechobeensis), today restricted to 2 regions of Florida, is essentially the same species a C. lundelliana, a wild squash found in Mexico. The Okeechobee gourd likely had a more continuous distribution during the Pleistocene.
They also looked at the genomes of 46 species of mammals to analyze the bitter taste receptors of each. Smaller mammals and humans have many more bitter taste receptors than elephants or rhinos. Smaller mammals are more vulnerable to toxins and need to avoid them. Large mammals have greater tolerance to toxins because of their physiology and size.
Kistler, L., et. al.
“Gourds and Squash (Cucurbita sp.) Adapted to Megafaunal Extinction and Ecological Anachronism through Domestication”
PNAS November 2015