Mushroom mind

Mushrooms and other kinds of fungi are often associated with
witchcraft and are the subjects of longstanding superstitions.
Witches dance inside fairy rings of mushrooms according to
German folklore, while a French fable warns that anyone foolish
enough to step inside these ‘sorcerer’s rings’ will be cursed by
enormous toads with bulging eyes. These impressions come
from the poisonous and psychoactive peculiarities of some
species, as well as the overnight appearance of toadstool
ring-formations.
Given the magical reputation of the fungi, claiming that they might
be conscious is dangerous territory for a credentialled scientist.
But in recent years, a body of remarkable experiments have shown
(https://bit.ly/34owgb3) that fungi operate as individuals, engage
in decision-making, are capable of learning, and possess short-term
memory. These findings highlight the spectacular sensitivity of
such ‘simple’ organisms, and situate the human version of the mind
within a spectrum of consciousness that might well span the entire
natural world.
Before we explore the evidence for fungal intelligence, we need to
consider the slippery vocabulary of cognitive science. Consciousness
implies awareness, evidence of which might be expressed in an
organism’s responsiveness or sensitivity to its surroundings. There
is an implicit hierarchy here, with consciousness present in a
smaller subset of species, while sensitivity applies to every living
thing. Until recently, most philosophers and scientists awarded
consciousness to big-brained animals and excluded other forms of
life from this honour. The problem with this favouritism, as the
cognitive psychologist Arthur Reber has pointed
(https://bit.ly/3mZDw3p) out, is that it’s impossible to identify a
threshold level of awareness or responsiveness that separates
conscious animals from the unconscious. We can escape this dilemma,
however, once we allow ourselves to identify different versions of
consciousness across a continuum of species, from apes to amoebas.
That’s not to imply that all organisms possess rich emotional lives
and are capable of thinking, although fungi do appear to express the
biological rudiments of these faculties.
It turns out that this question doesn’t have a simple answer.
Mushrooms are the reproductive organs produced by fungi that spend
most of their lives below ground in the form of microscopic filaments
called hyphae. These hyphae, in turn, branch to form colonies called
mycelia. Mycelia spread out in three dimensions within soil and leaf
litter, absorbing water and feeding on roots, wood, and the bodies of
dead insects and other animals. Each of the hyphae in a mycelium is
a tube filled with pressurised fluid, and extends at its tip. The
materials that power this elongation are conveyed in little packages
called vesicles, whose motion is guided along an interior system of
rails by proteins that operate as motors. The speed and direction of
hyphal extension, as well as the positions of branch formation, are
determined by patterns of vesicle delivery. This growth mechanism
responds, second by second, to changes in temperature, water
availability, and other opportunities and hardships imposed by the
surrounding environment.
Hyphae can detect ridges on surfaces, grow around obstacles, and
deploy a patch-and-repair system if they’re damaged. These actions
draw upon an array of protein sensors and signalling pathways that
link the external physical or chemical inputs to cellular response.
The electrical activity of the cell is also sensitive to changes in
the environment. Oscillations in the voltage across the hyphal
membrane have been likened to nerve impulses in animals, but their
function in fungi is poorly understood. Hyphae react to confinement
too, altering their growth rate, becoming narrower and branching
less frequently. The fungus adapts to the texture of the soil and
the anatomy of plant and animal tissues as it pushes ahead and
forages for food. It’s not thinking in the sense that a brained
animal thinks – but the fundamental mechanisms that allow a hypha
to process information are the same as those at work in our bodies.
We tend to associate consciousness and intelligence with the
appearance of wilfulness or intentionality – that is, decision
making that results in a particular behavioural outcome. Whether
or not humans have free will, we take actions that seem wilful:
she finished her coffee, whereas her friend left her cup half full.
Fungi express simpler versions of individualistic behaviour all
the time. Patterns of branch formation are a good example of their
seemingly idiosyncratic nature. Every young fungal colony assumes
a unique shape, because the precise timing and positions of branch
emergence from a hypha vary. This variation isn’t due to genetic
differences, since identical clones from a single parent fungus
still create colonies with unique shapes. Although the overall
form is highly predictable, its detailed geometry is usually
irreproducible. Each mycelium is like a snowflake, with a shape
that arises at one place and time in the Universe.
Fungi also show evidence of learning and memory. Working with
fungi isolated from grassland soil, German mycologists
(https://bit.ly/3FWSFtV) measured the effect of temperature changes
on the growth of mycelia. When heated up quickly for a few hours
 the mycelia stopped growing. When the temperature was reduced
again, they bounced back from the episode by forming a series of
smaller colonies from different spots across the original mycelium.
Meanwhile, a different set of mycelia were exposed to a mild
temperature stress before the application of a more severe
temperature shock. Colonies that had been ‘primed’ in this way
resumed normal growth very swiftly after the severe stress, and
continued their smooth expansion, rather than recovering here and
there in the form of smaller colonies. This outcome suggests that
they developed some defensive mechanisms that enabled them to
brush off the more severe stress. The fungi retained this
biochemical memory for up to 24 hours after the mild temperature
shock, but forgot soon afterwards, and succumbed to additional
heat stress as if they had learned nothing.
The single-celled fungus present as baker’s and brewer’s yeast,
Saccharomyces cerevisiae, has also shown (https://bit.ly/3HSUbhp)
the capacity for cellular memory. Yeasts primed by salt exposure
become better at responding to other kinds of chemical stress that
follow. Memorisation appears to be present in other microorganisms,
but filamentous fungi are particularly interesting because mycelia
can spread over large areas and transmit information over long
distances through their hyphae. That’s very different from learning
and memory within populations of dispersed cells. Decomposer fungi
that rot wood manifest this information transfer as they fan out
beneath the leaf litter, searching for dead and damaged trees,
fallen branches and other sources of food. When one part of the
mycelium finds some woody debris, nutrients extracted are
distributed across the whole colony, which focuses its growth away
from barren to fertile locations on the forest floor. The mycelium
is operating as more than a simple sum of its individual hyphae;
it’s like an integrated multicellular organism.
By responding to the need to create the shortest connections between
its food stations, the slime mould achieved the same economies
as human architects
When researchers (https://go.nature.com/3sWKAl9) followed
the transfer of nutrients in the lab, further remarkable discoveries
were in store. In a tray of soil, hyphae were observed to make
contact with a block of beechwood. They grew over its surface
and penetrated the solid structure, secreting enzymes that broke
down the polymers in the wood and released sugars that fuelled
their metabolism. Once the fungus exhausted the energy in the
woodblock, it grew out in all directions, foraging once again. Here
is where the mindfulness of the fungus becomes clear. When a
mycelium located a second block of beechwood and was then placed
in a fresh tray, it would emerge from the same side of the block
that had allowed it to hit pay-dirt the first time. It remembered
that growing from a particular face of the woodblock had resulted
in a food reward before, and so sought to repeat its prior success.
The fungus in these experiments showed spatial recognition,
memory and intelligence. It’s a conscious organism.
Simple forms of learning and memory have been studied in slime
moulds for many years. Slime moulds are not fungi, but relatives
of that microscopic celebrity of the biology classroom, the amoeba.
They form glistening yellow colonies called plasmodia that ooze
over rotting wood and eat bacteria. These gooey monsters can cover
the entire stump of a tree when there’s enough moisture. Smaller
versions can be nurtured with oat flakes in the lab, where the
flattened plasmodia move from flake to flake, keeping the whole
organism alive with a network of veins that pulse with fluid. In a
2010 experiment (https://bit.ly/3JJiPTe), when its plasmodium was
surrounded by oat flakes arranged in the same pattern as the cities
circling the Japanese capital, it created a pattern that was strikingly
similar to the layout of the railway system around Tokyo. By
responding to the need to create the shortest connections between
its food stations, the slime mould achieved the same economies
as human architects.
The behavioural complexity of fungi (https://bit.ly/3pVZ3Mv)
increases when they interact with living trees and shrubs rather
than dead wood. Some of these relationships are destructive while
others are mutually supportive. Pathogenic fungi can be very cunning
in how they feed on plants and evade their defences. Mycorrhizal
fungi are more cooperative, penetrating tree roots and establishing
tight physical connections through which they pass water and
dissolved minerals to the trees, in return for food produced by
photosynthesis. The mycelium of the mycorrhizal fungus operates
as an accessory root system for the tree, spreading over a larger
territory with its filamentous hyphae than the plant can cover with
its own rootlets.
This complex symbiosis relies on continuous chemical communication
between the fungus and the plant that affects the development
of both partners. Mycorrhizas support the productivity of the entire
ecosystem, spurring some fungal enthusiasts to reimagine forests
as superorganisms connected through a ‘wood-wide web’ of fungi.
This is an intriguing idea, but allusions to the internet create
problems. For starters, it does a disservice to the fungi: unlike
the internet, fungi generate their own information through active
interactions with their vegetal partners. Secondly, the computer
metaphor has been adopted by those who attribute supernatural
properties to the fungi, which is one of the reasons why fungal
behaviour has often been (wrongly) relegated to the fringes of
‘real’ science. While an expert on fruit flies can tell a fellow
biologist that they study insect behaviour without provoking mirth,
any mention of studying mushroom cognition is almost certain to
raise eyebrows.
Fungal expressions of consciousness are certainly very simple. But
they do align with an emerging consensus that, while the human
mind might be particular in its refinements, it’s typical in its
cellular mechanisms. Experiments on fungal consciousness are
exciting for mycologists because they’ve made space for the study
of behaviour within the broader field of research on the biology
of fungi. Those who study animal behaviour do so without reference
to the molecular interactions of their muscles; likewise,
mycologists can learn a great deal about fungi simply by paying
closer attention to what they do. As crucial players in the ecology
of the planet, these fascinating organisms deserve our full attention
as genuine partners in sustaining a functional biosphere.