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Grunting for worms: Seismic vibrations cause Diplocardia earthworms to emerge from the soil

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Harvesting earthworms by a practice called 'worm grunting' is a widespread and profitable business in the southeastern USA. Although a variety of techniques are used, most involve rhythmically scraping a wooden stake driven into the ground, with a flat metal object. A common assumption is that vibrations cause the worms to surface, but this phenomenon has not been studied experimentally. We demonstrate that Diplocardia earthworms emerge from the soil within minutes following the onset of grunting. Broadband low frequency (below 500 Hz) pulsed vibrations were present in the soil throughout the area where worms were harvested, and the number of worms emerging decreased as the seismic signal decayed over distance. The findings are discussed in relation to two hypotheses: that worms are escaping vibrations caused by digging foragers and that worms are surfacing in response to vibrations caused by falling rain.
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Biol. Lett. (2009) 5, 16–19
doi:10.1098/rsbl.2008.0456
Published online 14 October 2008
Animal behaviour
Grunting for worms:
seismic vibrations cause
Diplocardia earthworms to
emerge from the soil
O. Mitra
1
, M. A. Callaham Jr
2
, M. L. Smith
1
and J. E. Yack
1,
*
1
Department of Biology, Carleton University, Ottawa,
Ontario, Canada K1S 5B6
2
Forestry Sciences Laboratory, USDA Forest Service,
Athens, GA 30602, USA
*Author for correspondence (jyack@ccs.carleton.ca).
Harvesting earthworms by a practice called
‘worm grunting’ is a widespread and profitable
business in the southeastern USA. Although a
variety of techniques are used, most involve
rhythmically scraping a wooden stake driven
into the ground, with a flat metal object. A
common assumption is that vibrations cause
the worms to surface, but this phenomenon has
not been studied experimentally. We demon-
strate that Diplocardia earthworms emerge
from the soil within minutes following the onset
of grunting. Broadband low frequency (below
500 Hz) pulsed vibrations were present in the
soil throughout the area where worms were
harvested, and the number of worms emerging
decreased as the seismic signal decayed over
distance. The findings are discussed in relation
to two hypotheses: that worms are escaping
vibrations caused by digging foragers and that
worms are surfacing in response to vibrations
caused by falling rain.
Keywords: earthworm; vibration; escape; rain; moles
1. INTRODUCTION
In the southeastern USA, worm ‘grunting’ is com-
monly practiced to collect earthworms for fish bait. A
wooden stake is driven into the ground and typically
scraped with a long metal object until worms come to
the soil surface. Worm grunting is economically and
ecologically important in localized regions. Appreci-
able income can be generated by collecting worms,
but this can result in negative effects on earthworm
populations and their associated benefits to plant
nutrition (Hendrix et al. 1994;Callaham & Hendrix
1998). Hendrix et al. (1994) reported significant
reductions in earthworm biomass due to bait harvest
in Florida, while noting that during some seasons
thousands of earthworms per hectare per day can be
harvested via grunting. Similar practices are used to
collect earthworms elsewhere, including England,
where worms are ‘charmed’ out of the ground by
‘twanging’ the handle of a garden fork, the tongs
of which are inserted into the soil ( Edwards &
Bohlen 1996).
Despite their prevalence, worm grunting and
similar techniques have not been examined experi-
mentally. Vibrations have not been recorded, and the
relationship between vibrations and worm emergence
has not been documented. We record the vibrations
being transmitted through the soil using a typical
worm grunting technique, and test the hypothesis
that earthworms surface in response to a vibration
stimulus. The hypotheses explaining the adaptive
significance of earthworms surfacing in response to
vibrations are discussed.
2. MATERIAL AND METHODS
Experiments were conducted in the pine (Pinus palustris and Pinus
elliottii )-dominated forests of Apalachicola National Forest, Liberty
County, Florida (compartments 100 and 110) between 08.00 and
10.00, 14–15 April 2008. Both sites had recently undergone
prescribed burning and had very little emergent vegetation or
detritus obscuring the ground surface, making it easier to find
worms. For this reason, burned areas are attractive to bait harvest-
ers, and are preferentially used shortly after prescribed fires have
been conducted (Hendrix et al. 1994).
Grunting vibrations were generated by one of the authors
(M.A.C.), who had previously used this technique to survey
earthworms ( Hendrix et al. 1994). Grunting was performed by first
hammering a wooden stake into the soil to a depth of approximately
30 cm. The top end of the stake was then rubbed with a long
(100!7 cm) flat metal object with a smooth surface (figure 1a;
supplementary video in the electronic supplementary material).
Specific locations within each site were evaluated for the presence
or absence of earthworms, and if earthworms were observed to
surface we moved a short distance away (approx. 20 m) to perform
full trials. Visual inspection of the trial area prior to grunting
indicated that no worms were visible on the soil surface.
Seismic vibrations were recorded using two set-ups: linear
geophone arrays were used to examine the amplitude decay over
distance, while equidistant arrays were used to examine the
frequency composition of the vibrations. Linear arrays consisted of
four identical vertical geophones ( DT20DX 4.5 Hz, Dynamic
Tech, Salt Lake City, Kolkata, India) buried at a depth of 10 cm,
and placed at intervals of either 1.8 or 3 m from the wooden stake.
Geophone signals were amplified (M-10MX, Edirol/Roland,
Los Angeles, CA) and recorded onto data recorders ( PMD671,
Marantz, Kanagawa, Japan). The equidistant arrays consisted of
four sensor types: three geophones including a DT20DX 4.5 Hz, a
GS-20DM 28 Hz (Oyo Geospace, Houston, TX), a GS-100
100 Hz and a microphone (ATM10a, Audio Technica, Tokyo,
Japan). Each was positioned either 0.9 or 3.6 m from the stake and
buried 10 cm in the soil. All geophones were amplified and
recorded as described above, while the microphone was connected
directly to the data recorder. Data files were transferred to a laptop
computer and analysed using Raven Bioacoustics Research Pro-
gram (Cornell Laboratory of Ornithology, Ithaca, NY). Our
objectives were to record the relative amplitudes and general
frequency composition of vibrations. To do this, we used geophones
that measure only the vertical component and velocity of vibrations.
Each trial consisted of a harvesting and recording phase. The
harvesting phase preceded the recording phase, although all record-
ing equipment was placed in position before the harvesting phase
began. Five individuals searched for surfacing earthworms during
the harvesting phase. The location of each worm was flagged, and
the specimen was collected to prevent recounts. When worms
ceased to surface, the recording phase began. A map was made,
indicating the positions and distances between worm flags, sensors
and the stake. All experiments were videotaped using a camcorder
(DCR-TRV19, Sony, Tokyo, Japan).
3. RESULTS
Worm grunting generated a train of distinct seismic
vibrations that proved to be successful in extracting
Diplocardia worms (figure 1b) from the soil surround-
ing the stake. Individual strokes were 760.8G99.8 ms
in duration (nZ25), repeated at a rate of one stroke
every 1.2G0.1 s (nZ15). Each stroke consisted of an
initial discreet noise (produced by the metal contact-
ing the stake) followed by a broadband ‘grunt’
Electronic supplementary material is available at http://dx.doi.org/
10.1098/rsbl.2008.0456 or via http://journals.royalsociety.org.
Received 13 August 2008
Accepted 24 September 2008 16 This journal is q2008 The Royal Society
(caused by stroking the metal against the stake;
figure 1c). Seismic signals were broadband, with all
sensors indicating thatmostoftheenergywas
concentrated below 500 Hz with a dominant fre-
quency of 97.3G11.7 Hz (nZ15).
Soil-borne vibrations were strong enough to be felt
when standing several metres away from the stake.
Vibrations were still detected by our farthest geo-
phone placed 12 m away from the stake, but the
relative amplitude decayed markedly beyond a few
metres (figure 2a,b). Spectral changes were also
apparent as the distance from the stake increased.
Vibrations below 500 Hz were still prominent at our
furthest recording distance (12 m), and frequencies
above 1.5 kHz were typically attenuated beyond 7 m
from the stake.
Worms began to emerge from their burrows within
54–131 (nZ5) s following the onset of vibrations.
Following emergence, they crawled across the soil
surface and remained on the surface after vibrations
ceased (at least until the specimens were collected).
Surfaced worms did not show directional preferences,
since they were observed moving both away from and
towards the stake. The size of the worms that
emerged ranged between 7 and 30 cm in length.
The largest number of worms surfacing in a single
trial was 41, during 22 min of grunting in an area
approximately 18 m in diameter centred on the stake
(figure 2a). Data from the spatial distribution of
surfaced worms from all trials show that the number
of worms surfacing decreases as distance from the
stake increases (figure 2c).
4. DISCUSSION
Our results demonstrate that the Diplocardia earth-
worms respond to the seismic vibrations generated by
the worm grunting technique. Worms emerge from
their burrows within a few minutes following the
onset of vibrations, and the number of worms surfa-
cing is positively correlated with the signal strength.
Although the practice of grunting for harvesting fish
bait is well known in the southern USA, there are few
documented accounts of worms responding to
vibrations. A few authors have briefly commented
(a)(b)
(c)
relative
amplitude
frequency (kHz)
0
2.5
2.0
1.5
1.0
0.5
246
time (s)
81012
Figure 1. Worm grunting in the Apalachicola National Forest. (a) One of the authors demonstrating the worm grunting
technique, whereby a wooden stake is vibrated using a flat metal object. (b) An earthworm emerging from its burrow.
(c) Oscillogram and corresponding spectrogram of seismic vibrations recorded at 1.8 m from the stake during a linear
array recording.
Grunting for worms O. Mitra et al. 17
Biol. Lett. (2009)
upon the worm grunting technique as a means of
collecting Diplocardia mississipiensis,Diplocardia flori-
dana or Pheretima diffringens (Vail 1972;Hendrix et al.
1994;Edwards & Bohlen 1996). It has also been
reported that mechanical disturbances such as dig-
ging, power motors and even walking can cause
worms to emerge ( Darwin 1881;Kaufmann 1989;
Edwards & Bohlen 1996). Interestingly, some animals
have been noted to employ a similar technique to
capture worms. Wood turtles (Clemmys insculpta)
stomp their front feet at a rate of about once
per second (similar to the grunting rate), which
induces earthworms to surface (Kaufmann 1986,
1989). Similarly, different birds have been noted to
‘paddle’ the Earth with their feet, or peck hard on a
rock to force earthworms to surface (Darwin 1881;
Tinbergen 1960;Edwards & Bohlen 1996). Although
in these reports it is implied that seismic vibrations
cause the worms to surface, the vibrations were not
recorded. Other reports of vibration-mediated beha-
vioural responses mainly describe rapid escape
reflexes, which do not resemble the locomotory
response observed during our trials. Darwin (1881)
observed that earthworms rapidly retreated into their
burrows when presented with a vibration made
by playing notes on a piano, upon which a worm
within a container was placed. Herz et al. (1967)
similarly describe the unconditioned response of
Lumbricus terrestris to a ‘mild vibratory stimulation’ as
‘an initial sharp contraction which appears to habitu-
ate upon repeated presentations’, but also demon-
strate that when the stimulus is ‘more intense’ the
worm extends the anterior portion of its body.
Perhaps this latter response more closely reflects the
response to worm grunting.
Why do earthworms surface in the presence of
seismic vibrations? One hypothesis is that the
vibrations resemble those caused by rain, and the
worms emerge from the soil to avoid drowning
(Kaufmann 1986), low oxygen levels ( Minnich 1977)
or to enhance dispersal (Butt & Nuutinen 2005).
Preliminary recordings of light rainfall show that most
energy falls below 500 Hz (O. Mitra 2008, unpub-
lished data), corresponding to the same frequency
range as grunting. A second hypothesis, first proposed
by Darwin (1881) in response to indirect reports of
worms surfacing in response to vibrations, and then
by Tinbergen (1960) in relation to gull paddling, is
that earthworms are responding to vibrations caused
by the burrowing of predatory moles. Vibrations
generated by digging moles have not been formally
described as far as we know. However, other fossorial
mammals produce vibrations (for orientation or
communication purposes) that fall within the fre-
quency range of worm grunting stimuli (Mason &
Narins 2001). It should be noted that evidence for
mole tunnelling was observed at our trial sites in
Florida. Since other vertebrates and invertebrates use
seismic cues to detect predators ( Mason & Narins
2001;Cocroft & Rodrı
´guez 2005), the hypothesis
seems worthy of further testing.
We conclude that the Diplocardia earthworms
can be induced to surface in response to seismic
vibrations. Based on the available evidence, it is
proposed that these vibrations are mimicking those
of rain or predatory fossorial mammals, but these
hypotheses remain to be formally tested by fully
characterizing the natural vibrations and conducting
playback studies. Additional studies on Diplocardia
should adopt a neuroethological approach to eluci-
date the behavioural and physiological responses to
vibrations the worms would experience in their
natural environment. It should also be established
whether this behaviour is observed in other earth-
worm species. The species reported to respond to
worm grunting in Florida are almost exclusively
Diplocardia mississippiensis or D. floridana (Vail 1972;
Hendrix et al. 1994), while it has been noted that
30
20
10
024
distance from stake (m)
68
102
(b)(a)
N
(c)
10
relative amplitudeno. of worms
1
1.8 m
7.3 m
Figure 2. Distribution of recordings and worm emergence sites. (a) A map indicating the locations of surfaced worms
(circles) during a single instance of grunting, geophones in a linear array (squares) and the wooden stake (star). The worm
grunter is facing the linear array. Scale bar, 1 m. (b) Relative amplitude decay of vibrations recorded from a linear geophone
array ( yZ42.998 e
K0.3066x
,R
2
Z0.9406). The inset shows the waveforms of four grunts recorded at 1.83 and 7.32 m.
(c) The number of worms surfacing as a function of the distance from the stake. Data are plotted using sliding window
comparisons (using 100 cm windows with 50 cm increments).
18 O. Mitra et al. Grunting for worms
Biol. Lett. (2009)
L. terrestris does not respond similarly to such
vibrations (Reynolds 1977). There are an estimated
7000 species of earthworms worldwide (Hendrix
et al. 2008), but surprisingly little is known about the
behaviour and life-history traits of any one species.
Additional questions regarding how biotic (e.g. tun-
nelling habits, local predators, seasonal and diurnal
rhythms) and abiotic (e.g. soil moisture, temperature,
time of day) factors influence the sensitivity to
vibrations in different species should be addressed in
future studies.
We thank the personnel at Apalachicola National Forest for
permission to use the study area. Financial support was
provided by the Natural Sciences and Engineering Research
Council of Canada, the Canadian Foundation for Inno-
vation and the Ontario Innovation Trust (to J.E.Y.).
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