To disperse their spores, truffles rely on animals eating other animals

Truffles, which come in many guises, not just those regarded as delicacies by humans, are the fruiting bodies of fungi—the underground equivalents of mushrooms. Unlike mushrooms, however, they cannot shed their spores into the air, to be scattered by the wind. Instead, they rely on being eaten and the spores they contain (pictured) then being deposited elsewhere once they have passed through the gut of the diner.

But that, by itself, would not disperse them far. Most truffle-eaters are small mammals, mainly rodents and shrews, with commensurately small home ranges. Spores these animals pass in their faeces will not have travelled any great distance from the fungal mycelium that spawned them, and will thus end up competing with their parent rather than spreading parental genes elsewhere.

Noting this anomaly, Ryan Stephens at East Tennessee State University and Michael Joyce at the University of Minnesota wondered whether small mammals are thus mere pawns in this tale of fungal reproduction. The real work, they suspected, as they explain in a paper in Ecology Letters, is done by others.

They knew from studies elsewhere that plant seeds in the guts of small animals subsequently eaten by predators often survive their journey through this second gut. Since predators have larger ranges than small mammals, this helps spread those seeds around. They wondered if something similar was happening with truffles.

They therefore trapped small mammals—mostly mice, voles, squirrels and shrews—at 40 sites in the wildernesses of eastern Minnesota and southern Wisconsin, and searched their guts for spores. They also collected predator faeces, gathering 267 samples deposited by wolves, coyotes, foxes, bobcats and members of the weasel family called fishers.

Analysis of these two sets of samples showed the same fungal spores were present in both. Though the richness and total load of spores in predators’ scats was typically lower than that in the guts of voles and squirrels, it was similar to that in shrews and mice. And, though Dr Stephens and Dr Joyce could not rule out the possibility of predators foraging directly for truffles, there is little evidence from other studies that this happens.

Predators, therefore, seemed as well placed as small mammals to carry spores around. The question remained—how far can they carry them? A search of the scientific literature showed that food takes about nine hours to traverse a fisher’s digestive track and 17 hours for that to happen in a wolf. Some experiments with radio collars then showed that a fisher would travel, on average, 2km in this amount of time, and a wolf 3.5km.

All this suggests truffle-spreading involves not one, but two meals: that of the animal which eats the truffle, and that of the animal which eats the animal that eats the truffle. Bear this gruesome fact in mind next time the waiter is grating some bianco d’Alba onto your tagliatelle. ■