Truffles, which are also known as hypogeous sequestrate fungi, don’t depend on wind for spore dispersal like most of their closely related cousins. Instead, these hidden, ectomycorrhizal globules of spore filled fungal tissue entice foraging mammals to eat them. If you have ever smelled or tasted truffles, you quickly realize their ecological function. The pungent, umami earthiness demands attention as your sinuses gather and retain the essence of the forest floor. I haven’t eaten truffles in quite some time, but it’s one of those special tastes you never forget. Once eaten by forest dwelling critters, the spores pass unperturbed through the mammalian digestive system and are dispersed far away from where they were consumed. If these droppings land near plants that they can engage in mycorrhizal mutualisms with, it still takes 7-10 years for their mycelial networks to develop. Even then, truffles only begin to develop 4-6 years after these underground networks are formed.
Although their spores can remain viable for nearly a decade, truffles are highly dependent on their mammalian counterpart. With truffles being found all over the world, different mammals have filled that role of disperser. Before reading about the hypogeous sequestrate fungi, I assumed that pigs where the main dispersers of truffle spores in Europe and Asia (the old world) because of their wide use by truffle hunters. They do indeed disperse these fungal spores, but are not the truffles target species. Although pigs are voracious eaters, they are omnivores that use truffles to supplement their diet rather than exclusively feed on them. A mammal that has a specialized diet of these fungi are the prime dispersers for this group of species. There seems to be a common syndrome for becoming the main disperser of truffles.
In North America, a major fungal specialist that disperses truffle spores is from an unlikely rodent-the Northern flying squirrel (Glaucomys sabrinus). Like other squirrels in its family, they have a keen sense of spatial memory and can remember and locate truffles by creating complex mental maps. One study found that fungal spore germination was actually enhanced when spores passed through the digestive system of Northern flying squirrels. Red spruce seedlings that where treated with feces from G. sabrinus had improved inoculation of ectomycorrhizal truffle fungi, opposed to seedlings inoculated directly with the truffle spores. These flying squirrels are vital in maintaining the heath of these northern forest communities by providing nutrient sequestering fungi to vulnerable seedlings. On a different continent, another small mammal helps support its forest ecosystem by dispersing fungal spores.
In Australia, an improbable group of mammals specializes in truffle consumption and dispersal. The marsupial Bettongia gaimardi is a type of bettong or that is highly mycophagous. Depending on the season, a bettong’s diet is made up of 40-90% fungal tissue. This study showed that fungal specialists like these disproportionally consume and transport fungal spores more efficiently than generalist mammals. With Australian soils being some of the most ancient, eroded soils on the planet, these fungal dispersers are keystone species that allow eucalyptus and acacia trees to access scant soil nutrients by inoculating their roots with symbiotic fungal spores.
After reading up on these fungi and their dispersers, a common trend is beckoning. These mammalian dispersers have an enormous range relative to their body size. An animal’s home range is usually relative to its body size, as larger animals can travel longer distances with their extensive energy stores to find larger amounts of food. Bettongia gaimardi like its marsupial cousins have evolved an efficient form of movement, using digitigrade locomotion. Like kangaroos, bettongs store nearly half of the energy required for one hop in the form of tension. The tension gathered in leg tendons is released for the next hop, which ultimately reduces its energy consumption. This is a vital adaptation for efficient movement in the arid Australian bush. An even more efficient way of moving is found in the other truffle disperser, the Northern flying squirrel. Although these creatures must climb, once high enough they effortlessly glide through the air, traversing great distances with little caloric demand. So why would selection favor truffle dispersers to be small mammals with enormous home ranges?
The patchy distribution of truffles helps explain this pattern. Truffles are located sparsely throughout habitats because their tree host is patchy, and/or the droppings from their other mutualist is excreted randomly. The mammals that have wide ranges can access these truffles patchy distribution and have formed a long-lasting mutualism that encourages forest heath throughout the world. It always amazes me to see drastically distant species carrying out analogous ecological functions in vastly different habitats. The frigid northern part of North America, and the arid eastern part of Australia both support nutrient sequestering, truffle-forming fungi that have evolved specialist relationships with small, wide ranging mammals. Nature never ceases to amaze me.
I’d like to thank Dr. Susan Nuske, an author of the Australian publication for sending me the paper that I didn’t have access to. You made this post possible!