THE EARTH’S GEOMAGNETIC FIELDS OR ELECTROMAGNETIC FIELDS CANEXPLAIN BERGMANN’S, COPE’S, AND RENSCH’S RULES
Tsutomu Nishimura1*, Kaneo Mohri2,3, and Masanori Fukushima1
1
Translational Research Center, Graduate School of Medicine, Kyoto University,
Shogoin Kawara-cho 54, Sakyo-ku, Kyoto 606-8507, Japan
Phone: +81-75-751-3397; Fax: +81-75-751-3399
E-mail: t246ra@kuhp.kyoto-u.ac.jp
2
Aichi Micro Intelligent Corporation, wanowari 1, Arao-machi, Tokai, Aichi 476-8666 Japan
3
Nagoya Industrial Sciences Research Institute, 1-13 Yotsuya-dori, Chikusa-ku, Nagoya, Aichi
464-0819, Japan(Received April 23, 2008; Accepted July 9, 2008)(Abstract)
A number of ecological and evolutionary patternsor ‘rules’ dealing with body size have been proposedover the years, the most prominent being Bergmann’srule, Cope’s rule, and Rensch’s rule. The mechanisms
underlying these patterns remain enigmatic. We
focused on the relationship between magnetic field(MF) exposure and animal body size because
Bergmann’s rule holds that organisms tend to belarger at higher latitudes, where the geomagnetic field
is more than twofold stronger than at lower latitudes.
We
researchedtherelationshipbetweenelectromagnetic field (EMF) exposure and change inanimal body weight using data in the literature. We
conducted a meta-regression analysis to examine theimpact of EMF exposure on animal weight ascompared with the weight of unexposed controls.
Meta-regression showed that EMF exposure had astatistically significant positive association withrelative weight in males but not in females. The
increase in body weight would explain Rensch’s rule.
The increase in the relative weights of males would
explain Bergmann’s and Cope’s rules. Over
successive generations, animals would gradually gaina considerable amount of body size if environmental
MF and/or EMF become stronger over the course oftime, which explains Cope’s rule.
(Keywords)
Bergmann’s rule, Cope’s rule, Rensch’s rule, SSD,
magnetic field, MF, EMF, ELF, evolution, body size,
body weight
1. Introduction
A number of ecological and evolutionary patternsor ‘rules’ dealing with body size have been proposedover the years, the most prominent being Bergmann’srule (the tendency towards size increase withincreasing latitude) [1-15], Cope’s rule (the tendency
towards size increase within phyletic lineages)[16-25], and Rensch’s rule (which states that in many
animal groups, when the male is larger than thefemale, sexual size dimorphism (SSD; the ratio ofmale to female size) increases with body size, but ingroups in which the male is smaller than the female,
SSD decreases with body size) [26-30]. The
mechanisms underlying these patterns remain
enigmatic.
In the present study, we focused on the relationshipbetween magnetic field (MF) exposure and animal
body size because according to Bergmann’s rule

Viva Origino 36 (2008) 44 - 50
ⓒ 2008 by SSOEL Japan

organisms tend to be larger at higher latitudes, where
the geomagnetic field (typically around 50 µT; range
20-90 µT)[31] is more than twofold stronger than atlower latitudes. We researched the relationshipbetween extremely low frequency electromagnetic
field (ELF-EMF) exposure and change in bodyweight using data in the literature. Although humans
have been exposed to ELF-EMF in their daily lives
from electrical appliances and power lines for about acentury, ELF-EMF are also generated bygeomagnetic storms [32], volcanic activity [33],earthquakes [33], and Schumann resonance [34].
Thus, ELF-EMF may have an effect on evolutionary
processes among animals. Although static magnetic
fields (SMF) and ELF-EMF differ in terms offrequency, SMF and ELF-EMF reportedly have
similar effects on growth in plants and blood pressurein animals [35-38]. Furthermore, the clinical andhygienic effects of exposure to alternating and direct
current MFs (of tens of mT) on human beings havebeen found to be very similar [39]. As describedabove, geomagnetic fields are always in a state offlux, the fluctuations are larger at higher latitudes
where the geomagnetic field is stronger, and animals
are always moving, so we believe that SMF and
ELF-EMF are likely to have similar effects on animal
body size.
We hypothesize that a stronger MF and/or
electromagnetic field (EMF) causes animals to
become larger during evolution. In addition, we
propose that Bergmann’s rule, Rensch’s rule, andCope’s rule are all underpinned by one common
factor, MF and/or EMF exposure.
2. Materials and Methods
2. 1. Search strategy and inclusion criteria
For our meta-analysis, we focused initially on
high-quality animal studies conducted by the
National Toxicology Program (NTP), which is madeup of four charter agencies of the US Department of
Health and Human Services. The NTP has conductedstudies to assess the risks, especially with respect tocarcinogenicity, of long-term exposure to ELF-EMF.
We identified trials conducted according to themethods of the NTP study by performing a search of
the MEDLINE database (1990–April 2007). The
search terms used were ‘carcinogenicity’ and‘magnetic field’, and the search was restricted topapers in English that described animal studies. The
titles and abstracts of the articles identified using thisprocess were scanned to exclude any trials that were

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Viva Origino 36 (2008) 44 - 50
clearly irrelevant. The full text of the remaining
articles was read to determine whether they contained
information on the topic of interest. The reference
lists of the selected articles were reviewed for
additional pertinent articles. In our analyses, we
included only studies that were conducted by the
NTP or according to the methods of the NTP study,
which ran for 2 years, and in which animals exposed
to ELF-EMF and the sham-exposed control animals(more than ~50 in each group) were dealt with in thesame way, in the same room and at the same time.
We did not assess the quality of the methods used in
the primary studies; because the studies wereconducted by the NTP or according to the methods of
the NTP study, we presumed that these studies wereof high quality. Two of the authors extracted the data
independently. The following data were collected foreach article: publication data (first author’s last name,year of publication, country in which the study was
performed); study design; number of animals; animal
characteristics (sex, age); interventions (magnetic
flux density, duration of exposure, exposure
hours/day); and weights of animals and number ofsurvivors at each assessment time point. Differencesin data extraction were resolved by consensus afterreferring back to the original article. Publication bias
was not assessed because long-term studies
conducted by the NTP or according to the methods of
the NTP study tend to be published whether the
results are positive or negative.
2. 2. Statistical analysis
We conducted a meta-regression analysis [40] to
examine the impact of ELF-EMF exposure on
relative weight (percentage of control weight). We
defined the relative weight as the weight of animals
exposed to ELF-EMF divided by the weight ofcontrol animals at the same assessment time point in
the same study. We defined ELF-EMF exposure asmagnetic flux density × duration of the study (weeks)× exposure hours per day. We logarithmicallytransformed all ELF-EMF exposure values to achievea more symmetric distribution of values. The natural
logarithm of the relative weight was the response(dependent) variable, and ELF-EMF exposure was
the explanatory (potential effect modifier) variable.

We used a weighted regression model so that more
precise trials had a greater influence on the result of
the analysis. To correspond to a meta-regression
analysis, studies were weighted using the number ofsurvivors. A P-value of 0.05 indicated statisticalsignificance. Statistical analyses were performed
using SAS ver. 8 (SAS Institute Inc., Cary, NC,
USA).
3. Results
Our search of the MEDLINE database initiallyyielded nine articles; however, some of these did not
specifically address the topic of our analysis and were
excluded, which left six potentially relevant articles.
We read the full text of these articles and checked thereference lists for other relevant articles. We
identified four trials that were conducted by the NTP
or according to the methods of the NTP [41-45]. We
obtained detailed data on weight changes for the four
trials from a report [46] and book [47]. The generalcharacteristics of the trials are described in Table 1. A
total of 1288 rats and 1576 mice (50% female) wereinvolved in these controlled trials: 992 rats and 1184
mice in the ELF-EMF exposure groups, and 296 ratsand 392 mice in the control groups. The details of thestudies are as follows.
The study of Yasui et al. [41] was acarcinogenicity test in rats using a 50 Hz sinusoidal
ELF-EMF. In that study, male and female F344 rats,
48 per exposure group, were sham exposed (shamcontrol) or exposed to 500 µT (Group 1) or 5000 µT(Group 2) ELF-EMF for 2 years. Animals were
exposed from 5 to 109 weeks of age. The average
exposure time was 22.6 h/day. No significant
increases in the incidence of leukemia were observed.
Similarly, incidences of brain and intracranial tumors
did not increase in the exposed groups. The
incidences of both benign and malignant neoplasms
did not differ significantly between the exposed andsham exposed groups, with one exception: fibroma of
the subcutis occurred slightly more commonly in
exposed male rats than in sham exposed male rats.
However, this difference was considered to be notstatistically significant when evaluated with respect
to historical control data from the laboratory of Yasui
et al.

Table 1. Details of the studies included in our meta-analysis and the populations studied- 45 -

Viva Origino 36 (2008) 44 - 50
The study of Boorman et al. [43] was a 2-year
whole-body exposure study that was conducted toevaluate the chronic toxicity and possible
oncogenicity of a 60 Hz (power frequency)
ELF-EMF in rats. Groups of 100 male and 100
female F344/N rats were exposed continuously to apure, linearly polarized, transient-free 60 Hz
ELF-EMF at flux densities of 0 µT (sham control), 2µT (Group 1), 200 µT (Group 2), or 1000 µT (Group3). An additional group of 100 male and 100 female
F344/N rats received intermittent (1 h on/1 h off)
exposure to a 1000 µT ELF-EMF (Group 4).
Mortality patterns, and the total incidence andnumber of malignant and benign tumors in all groups
exposed to ELF-EMF were similar to those found in
sex-matched sham controls.
The study of McCormick et al. [44] was a 2-year
whole-body exposure study that was conducted toevaluate the chronic toxicity and possible
oncogenicity of a 60 Hz (power frequency)
ELF-EMF in mice. Groups of 100 male and 100
female B6C3F1 mice were exposed to a pure, linearlypolarized, transient-free 60 Hz ELF-EMF at flux
densities of 0 µT (sham control), 2 µT (Group 1), 200µT (Group 2), or 1000 µT (Group 3). An additionalgroup of 100 male and 100 female B6C3F1 micereceived intermittent (1 h on/1 h off) exposure to a1000 µT ELF-EMF (Group 4). A small but
statistically significant increase in mortality wasobserved in male mice exposed continuously to the1000 µT ELF-EMF, but mortality patterns in all othergroups of mice exposed to ELF-EMF werecomparable to those found in sex-matched shamcontrols. Body weight gains and the total incidence
and number of malignant and benign tumors weresimilar in all groups.
The study of Mizuki et al. [45] was a whole-body
exposure study that was conducted to evaluate theeffect of a 50 Hz (power frequency) ELF-EMF on
cancer rate in mice. Groups of 96 male and 96 female
AKR mice were exposed to 50 Hz ELF-EMF at flux
densities of 0 µT (sham control), 500 µT (Group 1:

linearly polarized horizontal), or 500 µT (Group 2:
ellipsoidal). No significant difference was detected
between the exposed and sham exposed groups foreach kind of tumor. Hematologically, there was nodifference between the exposed and sham-exposed
animals that were euthanized.
To investigate the impact of ELF-EMF exposure
on relative weight, we performed a meta-regression
analysis. The regression included 294 data points
from four trials for both males and females. For
males, we obtained an estimate that was statisticallysignificantly different from zero for the regression
coefficient of the relative weight on log ELF-EMF
exposure (coefficient 0.23, standard error [SE] 0.067,
p = 0.0007, intercept 97.4) (Fig. 1). The regression
coefficient is the estimated increase in the relative
weight per unit increase in the covariate. Thus, therelative weight is estimated to increase by 0.23 perunit increase in log ELF-EMF exposure. The
estimated relative weight of the covariate can bederived from the regression equation: relative weight= 97.4 + 0.23 ×log ELF-EMF exposure. Fig. 1 shows
the relative weight estimates according to log
ELF-EMF exposure and shows that ELF-EMF
exposure is positively associated with weight.
In females, the regression included 294 data points
from four trials. For females, we obtained an estimatethat was statistically significantly different from zerofor the regression coefficient of the relative weight on
log ELF-EMF exposure (coefficient 0.06, standard
error [SE] 0.05, p = 0.25, intercept 99.5) (Fig. 2). The
relative weight is estimated to increase by 0.06 perunit increase in log ELF-EMF exposure. The
estimated relative weight of the covariate can bederived from the regression equation: relative weight= 99.5 + 0.06 × ELF-EMF exposure. Fig. 2 shows therelative weight estimates according to log ELF-EMF
exposure and shows that ELF-EMF exposure was not
associated with weight in females.
4. Discussion
We reviewed the results of four studies in which

Figure 1. Meta-regression of change in relative weight on log ELF-EMF exposure formales. The circles on the graph represent the experimental groups in the studies
included; the size of the circles indicates the weighting according to the number ofsurvivors.

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Viva Origino 36 (2008) 44 - 50

Figure 2. Meta-regression of change in relative weight on log ELF-EMF exposure forfemales. The circles on the graph represent the experimental groups in the studies
included; the size of the circles indicates the weighting according to the number ofsurvivors.

animals exposed to ELF-EMF and animals that weresham exposed were compared over a period of 2
years. Meta-regression revealed that ELF-EMF
exposure had a statistically significant positive
association with weight in males (p = 0.0007), but not
in females (p = 0.25). It seems clear that these bodyweight gains in males were caused by the effects of
ELF-EMF, because food consumption did not differ
between the ELF-EMF and sham exposure groups inthe studies of Yasui’s and Mizuki’s groups [47],although the other studies did not present data on
food consumption.
Regarding the mechanism of weight gain, it isinteresting that mild increases in plasma thyroid
hormones (e.g., thyroxine) [48] and prolactin [49]have been found in pregnant lactating dairy cows thatwere exposed to ELF-EMF. Thyroxine is known toplay key roles in growth, metabolism, reproduction,and somatic differentiation in developing and adultanimals. Also in pregnant lactating dairy cows,
exposure to the electric component of these fields
alone (10 kV/m) did not affect either prolactin or
thyroxine [50]. It is possible that MFs exert an effecton body size via plasma thyroxine and/or prolactin.
In fact, injections of prolactin and thyroid hormones
have been found to promote weight gain in malereindeer [51].
It is also possible that EMFs exert an effect onskeleton size via alteration of the proliferation andactivity of bone cells. In support of this hypothesis,
pulsed EMF stimulation has been used clinically formore than twenty-five years for the treatment ofpatients with delayed fracture healing and non-unions[52-57]. Furthermore, a substantial number of in vitro
studies have shown that EMFs have positive effects
on the proliferation and activity of bone cells [58-61].
Our results correspond with previous findings that
ELF-EMF causes an increase in the body weight ofmice and cattle [62-65]. Electrical fields alone did not
produce any change in the body weight of pregnant
lactating cows relative to unexposed controls [50],whereas MF did [66]. These results suggest that MF
impact on the weight gains of animals.
Our finding that ELF-EMF exposure had astatistically significant positive association withweight in males with the evidence for sex differences
in a variety of effects of various types of magnetic
fields [67-72]. This result would explain Rensch’srule, which states that when the male is larger than
the female, SSD increases with body size, but when
the female is larger than the male, SSD decreases
with body size. That is, the mean body weights ofanimals that are sensitive to MF would increasedepending on magnetic flux density and frequency. In
many mammals and birds, the male is larger than thefemale, which may be caused by a difference insensitivity to MF.
Our finding that there was an increase in relative
weight in males would explain Bergmann’s andCope’s rules. The mechanism underlying Bergmann’srule has remained a mystery to date, but at higher
latitudes the geomagnetic field is more than twice asstrong as at lower latitudes. Our hypothesis is that,these changes in geomagnetic fields might cause
organisms to grow in size. This would explain
Bergmann’s rule (the tendency towards size increase
with increasing latitude). These animals would gain aconsiderable amount of body size over generations, if
their surrounding environmental MF and/or EMF
become stronger. This would explain Cope’s rule (thetendency for body size to increase over evolutionarytime). We believe that the ELF-EMF generated bygeomagnetic storms, volcanic activity, earthquakes,
or Schumann resonance have influenced animal bodysize during the course of evolution. In addition, thevulnerability to extinction of animals that adhere to
Cope’s rule would be explained by loss of the MF
and/or EMF that supports animal body size.
Recent studies suggest that Bergmann’s rule holds
not only for endothermic vertebrates [3,4,73], butalso for some ectothermic vertebrates; specifically,
Bergmann’s rule applies to most turtles [7] and- 47 -

Viva Origino 36 (2008) 44 - 50
Table 2. Proportions of different animal groups that adhere to Bergmann’s rule and have larger males

salamanders [5]. Squamate reptiles (lizards andsnakes) are a clear exception: the converse
Bergmann’s rule applies to most species ofsquamates (i.e., they are smaller in cooler
climates)[7]. In contrast, recent studies suggest that
Cope’s rule holds for a variety of taxa, including
Cenozoic mammals [74-76] and dinosaurs [77].
Rensch’s rule describes a pervasive macroecologicalpattern that has been observed in a wide range of taxa,
including mites, water striders, lizards, snakes, turtles,
hummingbirds,
songbirds,
andprimates[27,29,30,78,79]. Given the fact that Cope’s rule
applies to dinosaurs, which were endothermic, but
not reptiles, which are generally ectothermic, it seems
that endothermy is an important factor in MF effects.
Of vertebrates, only mammals and birds areendothermic. If our hypothesis is correct, mammals
and birds are endothermic should meet following
conditions: 1) males should be larger than females; 2)
they should adhere to Bergmann’s rule; and 3) theirancestors should have adhered to Cope’s rule. In fact,mammals and birds both meet all three of these
conditions (if dinosaurs are regarded as the ancestorsof birds). In fact, we evaluated the relationshipbetween 1) and 2) by using data published by
Blanckenhorn et al. [80] (Table 2). The number ofclasses that adhere to Bergmann’s rule and that havelarger males is 18 of 23 (78.3%) mammal and birdclasses, 4 of 14 (28.6%) reptile classes, and 11 of 57(19.3%) classes of other animals. There was astatistically significant difference between the
mammals and birds combined and reptiles (p=0.004;
Fisher's exact test), and between the mammals andbirds combined and the other animals (p <0.0001;
Fisher's exact test). These results show a strong
relationship between 1) and 2) for mammals andbirds.
As outlined above, we believe that MF and/or
EMF are fundamentally connected with animal
evolution, and we have named this new field of study‘magneto-evolution’. Because MF and/or EMF may
have influenced not only animal body size but also
many other characteristics, more research is neededin this new field.
Acknowledgment
We thank Ms. Yoko Teraguchi, Ms. Harue Tada
and Dr. Satoshi Teramukai (Translational ResearchCenter, Graduate School of Medicine, Kyoto
University, Kyoto, Japan) for their constructive
comments and suggestions.
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