Shaken, not stirred: a serendipitous study of ants and earthquakes
1 Department of Biology, University of Nevada at Las Vegas, NV 89154-4004,
USA
2 School of Animal, Plant and Environmental Sciences, University of the
Witwatersrand, PO Wits 2050, South Africa
* Author for correspondence (e-mail: jrlighton{at}aol.com)
Accepted 6 June 2005
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Summary |
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Key words: ant behavior, earthquake prediction, Messor pergande
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Introduction |
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Ants are attractive, practical candidates for quantitative behavioral and
physiological observations. Traffic kinetics on foraging trails can be
characterized by video analysis, and it is possible, with sensitive
flow-through respirometry, to measure a foraging trail's aerobic catabolism
(Lighton and Duncan, 2002).
Traffic kinetics and catabolism, when analyzed in combination, can yield a
comprehensive record of a colony's foraging and energy allocation strategies
(Lighton and Duncan, 2002
).
Various earthquake-precursor-related effects have been proposed that may
affect insect physiology or behavior (see
Ikeya et al., 1998
). If
anecdotal reports linking earthquakes and their precursors to changes in ant
colony activity even to the extent of colony evacuation (Ulomov and
Malashev, 1971) are correct, then recording and analyzing ant behavior
may be an effective means of earthquake prediction, and would yield
interesting information on colony-level responses to external,
density-independent stimuli.
A testable null hypothesis in this regard would be that ant foraging
behavior (and any other readily quantifiable aspects of ant behavior or
physiology) does not change immediately prior to or during a major earthquake.
Testing such a heuristic hypothesis is not, however, a project for which one
might realistically expect to be funded. Earthquakes cannot be predicted at
present, so testing the utility of ant behavior as a predictive mechanism is
challenging because obtaining detailed ant foraging data by chance prior to
and during a large earthquake is unlikely. Yet such an event has happened. We
report here a serendipitous field study of ant foraging (undertaken in the
course of a field validation of laboratory foraging energetic data;
Lighton and Duncan, 2002) that
took place in the Mojave Desert, California, when the magnitude 7.4 Landers
earthquake struck.
The large S-wave displacements made it difficult for the researchers to
stand upright or walk during the event, but `E' markers were placed in the
data acquisition record during the main temblor and during the sporadic
aftershocks that occurred during the rest of that recording. We refer below to
the Landers earthquake day as E-day. Thus, serendipitously
(Merton and Barber, 2003), we
had recorded an ant colony's response to a significant earthquake, the most
powerful to strike in the USA in four decades
(Sieh et al., 1993
), and its
associated aftershocks. Recordings on 3 subsequent, earthquake-free days
served as controls.
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Materials and methods |
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Data acquisition
We measured kinematic and catabolic parameters in a 15 cm section of the
foraging trail approximately 50 cm from the nest entrance. The trail passed
over a Masonite board, painted flat white, under a transparent respirometer
chamber on which an orthogonal S-VHS video camcorder was trained. The chamber
length (150 mm) served as the dimensional calibration. An identical chamber,
through which no ant trail passed, served as the baseline for the respirometry
system. Calibration markers established the dimensional scale at ant level.
Kinematic data (number of ants in the chamber, length of each ant, speed and
direction of each ant) were obtained by analysis of the video recordings,
using a program written by J.R.B.L. The video recordings were synchronized
with a prototype Sable Systems field portable TR-2 respirometry and data
acquisition system
(www.sablesystems.com).
Catabolic and kinematic data were synchronized by noting the acquired number
of samples of the catabolic/temperature recording on the sound track of the
video recorder, which was turned on for ca. 30 s at intervals of
510 min. A flow rate of 1 l m1 was maintained through
both chambers by means of mass flow control valves. Simultaneously,
temperature was measured using a copperconstantan thermocouple at `ant
height', 3 mm above ground level, and also recorded by the data acquisition
system. The measurement of aerobic catabolism in insects via
CO2 production is well-validated and details have been published
elsewhere (Lighton, 1991; for
more details, see especially Lighton and
Duncan, 2002
).
Using this system, we obtained data on traffic patterns (number and speed of ants entering and leaving the colony), mass distributions (from body length data via a separately obtained body length/body mass allometric regression), ant temperature and aerobic catabolism (via carbon dioxide production) of the foraging trail, all as functions of time. Because temperature continuously increased within each of the 4 days reported here, it served as a convenient abscissa for data analysis (we could alternatively have used time of day, but temperature is of more direct biological significance to the ants). Because E-day was characterized by not only by the Landers quake itself but by several aftershocks, any and all of which could have affected ant activity (Ulomov and Malashev, 1971), our analysis treats E-day as a discrete earthquake-affected entity for purposes of comparison with the 3 subsequent control days, which were seismologically uneventful.
Tests of earthquake effects
All of our null hypotheses generated for the purpose of assessing the
effect of the Landers earthquake on our Messor pergandei colony are
necessarily a posteriori. We had no rational reason to formulate them
in advance of the earthquake.
Our primary test for an earthquake effect involved traffic direction ratio (TDR). We define TDR as the ratio of inbound ants to total traffic (inbound plus outbound), with 0.5 denoting an equal number of inbound and outbound ants. We analyzed TDR as a function of ambient temperature on E-day and the 3 subsequent days. Because earthquakes and/or their precursors have allegedly caused disturbances of ant colony activity up to and including colony evacuation (e.g. Ulomov and Malashev, 1971), we considered it plausible that the Landers quake would have changed the ratio of inbound to outbound foragers if, indeed, the colony reacted to the Landers earthquake or its precursors. Thus, our TDR a posteriori null hypothesis was that TDR would, as a function of temperature and thus time of day, not differ between E-day and the 3 control days.
We next examined `motivational state', which we define as the component of
ant running speed not explained by temperature, which is the primary
determinant of ant running speed (Rissing,
1982). Motivation can therefore be quantified by examining the
residuals of the linear regression between traffic speed and temperature at
ant height. Any stimulus that elicits escape or panic behavior will drive
measured running speeds up or down from the running speed/temperature line.
This corresponds to a subjective impression of the ants being more or less
active `than usual'. Thus, our motivational state a posteriori null
hypothesis was that ant running speed, as a function of temperature, would not
differ between E-day and the 3 control days.
Next, we looked for earthquake-related changes in body size polymorphism on
the trail. Adult workers of Messor pergandei vary in body mass from
about 1 to 10 mg, and tend to differ in behavior (e.g. picking up heavier or
lighter loads), depending on their mass
(Davidson, 1978). If seismic
activity affects the mass distribution of workers leaving the nest, perhaps by
causing a reallocation of work tasks within the nest, then the mean mass of
ants on the trail (as determined from body length by video analysis and
lengthmass regression analysis) will change. Thus, our body size
polymorphism a posteriori null hypothesis was that body size
polymorphism, i.e. minimum, maximum and mean worker body mass as a function of
temperature and thus time of day, would not differ between E-day and the 3
control days.
Finally, we examined the aerobic catabolism of the trail on a mass-specific
basis. Trail aerobic catabolism is largely a function of temperature and
running speed, with running speed itself also being a function of temperature
(Rissing, 1982). If
earthquake-related effects modulated aerobic catabolism at a given temperature
and activity rate, this would lead to anomalous catabolic data on E-day. Thus,
our catabolic a posteriori null hypothesis was that trail aerobic
catabolism, as a function of temperature and thus time of day, would not
differ between E-day and the 3 control days.
Statistics
Data analysis was performed using Sable Systems Datacan software, and with
statistical programs written by J.R.B.L. and validated against SYSTAT version
4 (Wilkerson, 1988). Means are
accompanied by standard deviations (S.D.) and
N, and were compared using Student's t-test or by analysis
of variance (ANOVA). Regressions were calculated by the method of least
squares and were compared using analysis of covariance (ANCOVA).
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Results |
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Body size polymorphism
For each measurement interval on each day we determined the minimum,
maximum and mean masses of the ants traveling through the respirometry chamber
towards and away from the nest. E-day did not differ significantly from the
three control days in the values of any of these parameters (ANOVA:
P>0.2). Our body size polymorphism a posteriori null
hypothesis is not falsified.
Trail aerobic catabolism
Trail aerobic catabolism increased exponentially with ant-height
temperature in Messor pergandei, approximately doubling every
10°C, with temperature explaining 70% of the catabolism variance
(Fig. 3). E-day did not differ
from the control days in this relationship (P>0.15). Our trail
aerobic catabolism a posteriori null hypothesis is not falsified.
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Discussion |
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Other investigators have studied certain forms of vertebrate behavior prior
to earthquakes statistically (Schaal,
1988) and found no significant predictive effects. Our findings
are in accordance with theirs. Fascinating though the behavior and physiology
of ants may be, we conclude that at least in the species and conditions we
examined, they cannot be employed as reliable predictors or even sensors of
earthquakes.
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Acknowledgments |
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References |
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