In the middle of Florida’s Apalachicola National Forest,
a bizarre, almost magical scene is unraveling.
Sliding a metal strip over a wooden stake,
a master summoner is sending deep croaking noises reverberating through the area.
And, as if in a trance,
hundreds of earthworms begin emerging from the soil.
Worm grunting
This is worm grunting, also called worm charming or fiddling.
It’s a tradition that’s been practiced for more than a century,
but its inner workings were a mystery until only recently.
Worms collectively undertaking an underground exodus
seems especially unbelievable when you consider how vulnerable this makes them.
Hypothesis
So why is surfacing worth the risk?
Over the years, people have proposed a number of imaginative hypotheses.
One was that worms were somehow charmed by the noise,
like the rats from the medieval Pied Piper legend.
Okay, sounds fun, but how would the worms actually become bewitched?
Another hypothesis was that worm grunting tickled their bodies,
so they emerged to end the aggravation.
Whimsical!
But worm grunting vibrates the ground’s surface.
If worms were evading the vibrations, wouldn’t they burrow deeper instead?
Perhaps the most popular hypothesis was that worm grunting mimicked falling rain
and the worms fled to avoid drowning.
In 2008, biologist Kenneth Catania tested this hypothesis,
setting up three arenas filled with soil
and 300 individuals of the large species of earthworm
found in the Florida Panhandle.
After an hour of rain, water had pooled at the surface,
but only two earthworms emerged.
The rest remained buried and healthy.
So, unlike those containers, this hypothesis just didn’t hold water.
Catania decided to explore another route of inquiry.
In 1881, Charles Darwin published his final work,
a bestseller that rivaled his most well-known books at the time:
“The Formation of Vegetable Mould, Through the Action of Worms,
with Observations on their Habits.”
Yes, it was literally called that—
and it was the culmination of 40 years of earthworm investigations.
Within it, Darwin noted that worms sometimes left their burrows
when the ground trembled and mentioned an interesting hypothesis:
maybe they flee because they believe they’re being pursued by moles.
Catania got to work testing this hypothesis himself.
He found that Eastern moles had astounding tracking abilities,
could eat their weight in worms every day,
and were abundant in the Florida Panhandle.
When Catania released a single mole into worm- and soil-filled arenas,
about 30% of the worms crawled to the surface in the first hour—
a markedly different result from the control and rain trials.
And when he recorded the vibrations produced
by worm grunters and moles digging,
their frequencies overlapped substantially.
This was it.
Over hundreds of thousands of years,
these earthworms evolved a behavior that helped them escape a top predator.
Aboveground, they were immune to the moles,
which usually stayed subterranean.
But then humans came along.
And, funnily enough, we aren’t even the only ones
that take advantage of this behavior.
Herring gulls and wood turtles also sometimes drum their feet on the earth
to summon worms.
So then why does this behavior persist?
Why
Scientists think it’s beneficial for a prey species
to maintain its adaptations against a more frequent predator,
even if it makes it more vulnerable to a rarer one.
Many insects, for example, use flight to avoid predation.
But painted redstarts take advantage of this:
they boldly flash their colorful tail and wing feathers to elicit this response,
then catch the insects as they try to fly away.
It seems the prey species’ response remains
simply because it’s beneficial most of the time.
For over a century, humans in the southern US, the UK, and elsewhere
have been unknowingly exploiting the worm’s escape response.
The current world record for “most worms charmed”
was set by a 10-year-old British girl in 2009.
Wiggling a fork in the ground and hitting it with a stick,
she made 567 worms surface in just 30 minutes.
Charming, really.
