Zoologischer Anzeiger 251 (2012) 331–334
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Zoologischer Anzeiger
journal homepage: www.elsevier.de/jcz
Observations on variation in skull size of three mammals in Israel during the 20th
century
Yoram Yom-Tov ∗ , Shlomith Yom-Tov
Department of Zoology, Tel Aviv University, Tel Aviv 69978, Israel
a r t i c l e
i n f o
Article history:
Received 27 May 2011
Received in revised form
28 November 2011
Accepted 22 December 2011
Corresponding editor: C. Lueter.
Keywords:
Red fox
Golden jackal
Cape hare
Temporal change
Body size
Israel
a b s t r a c t
Among mammals, food availability, especially during the growth period, is a key predictor in determining
final body size, and improved nutrition may lead to an increase in their body size. In Israel during the
last century food availability for animals commensal with humans increased greatly, due to a 16-fold
increase in the human population and the accompanying changes, such as a 135-fold increase in the area
of irrigated agriculture and the availability of large quantities of organic garbage.
Using museum material, we studied temporal changes in skull size of a sample of 89 red foxes (Vulpes
vulpes Linnaeus, 1758), 108 golden jackals (Canis aureus Linnaeus, 1758) and 117 Cape hares (Lepus
capensis Linnaeus, 1758) collected during the 20th century. Four measurements (condylobasal length,
zygomatic breadth, the length of the upper cheek teeth row and the length of the mandible) were taken
for each skull, and principal component analysis was used to combine the measurements into principal
components.
We found that skull size of the red fox increased significantly during the 20th century, possibly due
to improved food availability from man-made resources such as agricultural produce and garbage. No
temporal trend in body size was detected for the jackal and hare. These differences are discussed.
© 2011 Elsevier GmbH. All rights reserved.
1. Introduction
Temporal and geographical variation in body size of animals
is a common phenomenon, and has been related to many factors
(reviewed by Yom-Tov and Geffen, 2011). Among these factors are
predation, ambient temperature, fluctuations in various climatic
phenomena including climate change, interspecific competition
and food availability (Gosler et al., 1995; Grant and Grant, 1995;
Yom-Tov, 2003; Yom-Tov et al., 2003; Ozgul et al., 2009). Observed
reduction in body size of many species was generally attributed to
global climate change (Gardner et al., 2011; Sheridan and Bickford,
2011). On the other hand, an increase in body or skull size was
attributed to increased food availability, either by human activity
or higher primary productivity in northern latitudes (Yom-Tov and
Geffen, 2011). Recently, McNab (2010) argued that the tendency
of mammals to vary in size depends on the abundance, availability
and size of resources, and termed this pattern the “resource rule”.
Among mammals, food availability, especially during the growth
period, is a key predictor in determining final body size. Quantity
and quality of nutrition during this period affects growth rates and
final body size, and these effects on skeletal size carry over into
∗ Corresponding author. Tel.: +972 3 6409058; fax: +972 3 6409403.
E-mail address: [email protected] (Y. Yom-Tov).
0044-5231/$ – see front matter © 2011 Elsevier GmbH. All rights reserved.
doi:10.1016/j.jcz.2011.12.003
adulthood (Read and Gaskin, 1990; Ulijaszek et al., 1998; Ohlsson
and Smith, 2001; Searcy et al., 2004; Ho et al., 2010). Food availability is influenced by both biotic and abiotic factors and fluctuates
accordingly in time and space, in turn affecting body size. In Israel,
man-made food resources, such as from garbage and agriculture,
have increased greatly during the last 60 years.
During the course of the 20th century many changes took place
in the area that today encompasses Israel, the Palestinian Authority and the Golan Heights. In this area, comprising ca. 28,000 km2
(hereafter the study area), the human population grew from ca.
650,000 inhabitants between 1900 and 1903 (Rupin, 1920) to about
1.62 million in 1942 (Baki, 1966). Following establishment of the
State of Israel in 1948, population size increased exponentially,
reaching ca. ten million in 2010 in the study area (of which ca.
7.5 million in Israel proper; Statistical Abstracts of Israel, 2010),
i.e. a 16-fold increase. This population increase led to an increased
use of land for human needs – construction of buildings, roads etc.
Many changes also took place in agricultural practices. The area of
worked land increased by ca. 50% to ca. 3000 km2 (Avitsur, 1977;
Statistical Abstracts of Israel, 2009), the irrigated area increased
by ca. 135-fold, from 15 km2 at the beginning of the 20th century (Avitsur, 1977) to about 2000 km2 approximately 100 years
later (Statistical Abstracts of Israel, 2009). Primary production, particularly in a relatively arid country such as Israel, is determined
primarily by rainfall, which is a good predictor of body size for
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Y. Yom-Tov, S. Yom-Tov / Zoologischer Anzeiger 251 (2012) 331–334
several species of Israeli mammals (Yom-Tov and Geffen, 2006). The
fact that green vegetation and running water in agricultural crops
became available throughout the year extended the period of plentiful food into the otherwise normally dry summer. The increased
and reliable agricultural production thus benefited some species
commensal with humans (Yom-Tov and Mendelssohn, 1988, 1999).
Growth of the human population, accompanied by a rise in living standards, has also led to a significant increase in the amount
of solid waste produced (more than one million tons in 2000;
Statistical Abstracts of Israel, 2007). Many small settlements and
military bases created their own (illegal) garbage dumps, where
the waste remained largely untreated. Garbage in Israel currently
includes about 41% organic material, and until recently this percentage was as high as 75%, providing a readily available source of
food for birds and mammals (Oskrovsky et al., 2009). Thus, food
availability for mammals that are either commensal with humans
or feed on agricultural produce has increased greatly since 1948.
Yom-Tov and Geffen (2011) contend that animal body size acts
as a barometer: if food is plentiful during the animal’s period of
growth, its body size will increase in parallel. Hence, museum collections that hold zoological specimens can serve as a tool by which
to draw conclusions regarding past food conditions. The development of natural history collections in Europe and the USA since the
19th century created a market for mammalian specimens from various parts of the world, including Palestine. In Palestine, collecting
during the 19th and early 20th century was carried out by travelers (e.g. Canon H. B. Tritram), European missionaries (e.g. the priest
E. Schmitz) and locals (e.g. I. Aharoni), who either sold or donated
their specimens to European collections. Many of these specimens
are currently housed in the Berlin Museum of Natural History and
in the British Museum of Natural History, London. After 1948 collection continued, and following the establishment of zoological
collections in Israel the specimens have been preserved mainly in
the Zoological Museums of Tel Aviv University and the Hebrew University, Jerusalem. The availability of skulls collected more than 80
years ago, when the study area was under-developed, provides an
opportunity to compare old and recent samples.
The red fox (Vulpes vulpes), golden jackal (Canis aureus) and
Cape hare (Lepus capensis) are medium-size mammals that are
widely spread in the Palearctic region (Corbet, 1978). In Israel they
occurred throughout the country, and are particularly common
near agricultural settlements where they feed on agricultural products (Mendelssohn and Yom-Tov, 1999). In much of their range,
both the red fox and the jackal are commensal with man, feeding on
agricultural products, poultry, road kills and garbage (Assa, 1990;
Macdonald and Barrett, 1993; Mendelssohn and Yom-Tov, 1999;
Lanszki and Heltai, 2002; Jensen and Sequeira, 1978). In Europe the
red fox inhabits cities, where it feeds mainly on garbage (Macdonald
and Barrett, 1993).
In the present study we used specimens from the above museums to examine the hypothesis that, during the last century, body
size of the red fox, golden jackal and Cape hare in Israel has
increased due to increased food availability resulting from agricultural stability and a constant supply of garbage. The reason for
selecting these three species was due to the relatively large samples
available from both before and after the period of great expansion
of the human population in Israel.
2 hares were collected in 1864) and the rest after 1941. The specimens selected for this study were adults with data on locality and
year of collection. Skulls of young specimens (open sutures between
the bones of the skulls or whose permanent teeth had not fully
erupted) were not measured. For each specimen we noted from
the museum catalogue its sex (if available), body mass (when available) and fitted data on latitude, longitude, and mean annual rainfall
for the locality of collection. Most specimens collected after 1932
were sexed and had data on body mass and body length, but only
4 foxes, 10 jackals and 9 hares collected before 1932 were sexed
and almost none had data on body mass and length. Using digital calipers, four measurements were taken from each skull to an
accuracy of 0.1 mm: condylobasal length (CBL), zygomatic breadth
(ZB), the length of the upper cheek teeth row (from the front of the
canine to the back of the last molar in the fox and jackal and the
molars in the hare; UT) and the length of the mandible (M). Some
of the specimens had data on body mass that we used to examine
the relationship between body mass and skull size.
Due to large morphological variability expressed in a strong
Bergmannian cline in body size, the systematic position of the
hares in Israel was a matter of controversy. Some authors (Wilson
and Reeder, 1983) recognized two species, Lepus europeus in the
Mediterranean region of the country and L. capensis in the southern
desert region, while others designated all Israeli hares to L. capensis,
with cline (Mendelssohn and Yom-Tov, 1999; Suchentrunk et al.,
2000). In this study we accept the latter opinion and regard all
Israeli hares as a single species.
2.1. Statistics
All skull measurements were log-transformed for normality. We
used principal components analysis (PCA) to combine the information of the four skull parameters (CBL, ZB, UT and M) into principal
components. In order to account for the possible effect of rainfall
on body size (Yom-Tov and Geffen, 2006), PC1 was corrected for
annual rainfall. Precipitation data were kindly provided by Ronen
Kadmon (Department of Evolution Systematic and Ecology, The
Hebrew University, Jerusalem, Israel). These data originate from
the Israel Meteorological Service and provide various climate data
for nearly every locality in Israel. For each specimen, we assigned
data for mean annual rainfall of the locality of its collection, and the
residual value of PC1 was used in a linear regression to examine the
effect of year of collection on body size. All tests were carried out
using JMP (ver.9.0.0, SAS Inc).
Most specimens collected before 1932 were not sexed. To
account for the possibility that the two sexes were presented differently in the periods before and after 1932, we ran discriminant
function analysis on all specimens of the fox and the jackal whose
sexes differ in size. No such analysis was carried for the hare
because it is not sexually dimorphic (Mendelssohn and Yom-Tov,
1999). Specimens whose sex was predicted at a level above 80%
were allocated to their predicted sex. A two-way analysis of variance (ANOVA) was run on residual PC1 to determine differences in
sex ratio between the periods and between sexes on size.
3. Results
2. Materials and methods
Skulls of 89 red foxes, 108 jackals and 117 hares were measured
at the Zoological Museum of Tel Aviv University, Berlin Museum
of Natural History, and Natural History Museum, London. Of the
above specimens, 19 foxes (21.3% of the sample), 19 jackals (17.4%)
and 25 (21.4%) hares were collected between 1909 and 1932 (and
For all three species, PCA clumped the four skull measurements
into four variables. Eigenvalues of the first principal component
(PC1) for the fox, jackal and hare were 2.5728, 2.8249 and 3.3790,
respectively, and the proportion of variance explained by this factor was 64.3%, 71.1% and 84.5%, respectively. For all three species
Eigenvalues for the other principal components were smaller than
1 and were not used in further analyses.
Y. Yom-Tov, S. Yom-Tov / Zoologischer Anzeiger 251 (2012) 331–334
Table 1
Results of two-way ANOVA on the first principal component (PC1) calculated from
four skull parameters and corrected for mean annual rainfall, on foxes and jackals
whose sex was known. Periods are before and after 1932. For the fox: F3,66 = 13.1783,
R2 = 0.3746, P < 0.0001; for the jackal: F3,86 = 13.5238, R2 = 0.3205, P < 0.0001.
4
3
df
F
P
df
F
P
1
1
1
3.34
1.85
0.27
0.0014
0.0683
0.7849
1
1
1
0.4283
0.2493
−0.60
0.0002
0.2493
0.5531
Table 2
Results of linear regressions relating year of collection to residual PC1 for the red
fox, golden jackal and cape hare.
Parameter
Vulpes vulpes
Canis aureus
Lepus capensis
N
F
R2
P
Intercept
Slope
77
8.4887
0.1017
0.0047
−31.0006
0.0158
105
0.9731
0.0093
0.3262
−7.6364
0.0039
114
0.5057
0.0045
0.4785
−6.1189
0.0031
For all three species PC1 was significantly related to (log
transformed) body mass (for the fox: F1,34 = 7.7091, R2 = 0.1848,
P = 0.0089: for the jackal: F1,74 = 35.9711, R2 = 0.3271, P < 0.0001; for
the hare: F1,67 = 103.8337, R2 = 0.6078, P < 0.0001). In all 3 species
PC1 was also highly and significantly correlated with each of its
components (in all cases P < 0.0001).
To account for the possibility that the two sexes were presented
differently in the periods before and after 1932, we ran a two-way
analysis of variance (ANOVA) on residual PC1, on foxes and jackals
whose sex was known. Residual PC1 of the hare was not affected
by sex (t31,42 = 0.5865, P = 0.5605; see also Mendelssohn and YomTov, 1999). For the foxes and jackals, residual PC1 was significantly
related to sex (males were larger), but was nearly significant for the
period of collection (P = 0.0683), and there was no the interaction
between these two factors (Table 1). The last result indicates that
there was a no significant difference in sex ratio between the two
periods and all data were combined for further analyses.
The effect of year of collection on residual PC1 was examined by
a linear regression. For the fox, the relationship between residual
PC1 and year of collection was significant (Table 2, Fig. 1). Cubic
spline indicates that red fox skull size increased over the early
period of the 20th century (Fig. 1). For the jackal and hare, the
relationship between year and PC1 was not significant (Table 2).
4. Discussion
We found that skull size of the red fox (but not of that of the
jackal and the hare) increased significantly during the 20th century, but only 10% of the variation in skull size was explained by
the year factor. It appears that the increase started at the 1930s
and stabilized at the 1980s. Temporal increase in body size of the
red fox was also reported in Denmark (Yom-Tov et al., 2003) and
Spain (Gortázar et al., 2000; Yom-Tov et al., 2007), and the authors
attributed this trend to increased food availability. We suggest
that the trend observed in this study for the red fox was also due
increased food availability during the 20th century.
Previous studies on material from the second half of the 20th
century did not detect changes in skull size of the red fox (Yom-Tov,
2003; Meiri et al., 2009) and the jackal (Yom-Tov, 2003). Regarding the fox, the difference between our present results and those
of Yom-Tov (2003) and Meiri et al. (2009) are probably due to the
difference in the periods of study: whereas Yom-Tov’s (2003) and
Meiri et al. (2009) samples were from the second half of the 20th
Residual PC1
Sex
Period
Sex × Period
Year & PC1
5
Jackal (n = 88)
Fox (n = 54)
333
2
1
0
-1
-2
-3
1900
1920
1940
1960
1980
2000
Year
Fig. 1. The relationship between skull size (PC1, corrected for mean annual rainfall)
of the red fox and year of collection (for linear regression: F1,76 = 8.4887, R2 = 0.1017,
P = 0.0047; For the cubic spline: F1,76 = 2.0652, R2 = 0.1063, P = 0.0409).
century only, the present sample represents the entire century. This
indicates that the observed change is not a result of climate change
or temperature increase, as most climate changes occurred during the second half of the 20th century. On the other hand, food
availability at the beginning of the century would thus appear to
have been much poorer than later in the century. The increase of
food availability is due to the dramatic increase in human population and the concomitant increased in agricultural produce and
garbage. Human population is now 16-fold larger than a hundred
years ago and its standard of living increased at least 10-folds (YomTov and Mendelssohn, 1988). The area of worked land increased by
ca. 50% and the irrigated area increased by ca. 135-fold (Avitsur,
1977; Statistical Abstracts of Israel, 2009). The present availability
of green vegetation and running throughout the year extended the
period of plentiful food into the otherwise normally dry summer.
In addition, garbage in Israel currently includes about 41% organic
material, and until recently this percentage was as high as 75%,
providing a readily available source of food for birds and mammals
(Oskrovsky et al., 2009). Thus, food availability for mammals that
are either commensal with humans or feed on agricultural produce
has increased greatly since 1948.
In contrast to the fox, no significant trend in skull size was found
for the other two species studied here. This is surprising, as the
golden jackal is as commensal as the red fox (Mendelssohn and
Yom-Tov, 1999; Lanszki and Heltai, 2002). We suggest that the difference may be due to different diet composition of the two species.
The fox is smaller than the jackal, feeds on smaller food items, and it
is more omnivorous (Mendelssohn and Yom-Tov, 1999). It is possible that the foxes are better adapted to take advantage of the extra
food availability, mainly from agriculture that became available and
rodent pests that consume it. However, there is no quantitative
information on diet composition of the two species, and until such
information will be available this suggestion remains hypothetic.
Yom-Tov (2003) found body length (but not skull size) of the
golden jackal in Israel increased significantly during the second
half of the 20th century, and this result may look contradictory
to our present finding of no change in skull size. However, body
parameters such as body mass, may change on seasonal and daily
basis (Baldwin and Kendeigh, 1938; King, 1972; Blem, 1990), and
is affected by many other predictors, including reproductive state,
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Y. Yom-Tov, S. Yom-Tov / Zoologischer Anzeiger 251 (2012) 331–334
time of the day (Lehikoinen, 1987), and the size of last meal. Even
linear traits, such as body length, may also vary during the year
in relation to body conditions. On the other hand, among most
mammals skull size is determined during the period of growth and
does not change afterwards (an exception being the Dehnel Effect;
Dehnel, 1949). Thus, the small but significant increase in the jackal’s
body length reported by Yom-Tov (2003) does not contradict our
present finding. Also, the fact that only 18.5% and 32.7% of the variation in skull size of the red fox and the golden jackal, respectively,
is explained by body mass demonstrates that skull size and body
mass are, at least to some extent, products of different selection
pressures.
The fact that hares did not increase in body size is less surprising.
Although many hares regularly feed on agricultural produce, we
have no direct information on as to where individual hares forage
for food, and more than a third of the hares in our sample came
from the desert areas, where there has been little change in human
density during the study period.
Acknowledgements
We thank Tsila Shariv and Arie Landsman of the Zoological
Museum at Tel Aviv University for their continuous help over the
years. Dr. Richard Sabin and Roberto Portela Miguez of the Mammal Department at the Natural History Museum, London and Dr
Frieder Mayer of the Mammal Department at the Berlin Natural
History Museum and their staff provided very valuable help during
our visits to their collections. We are grateful to Eli Geffen for statistical advice, to Ronen Kadmon for providing precipitation data
and to Naomi Paz for editing this article. We are grateful to two
anonymous reviewers for their useful comments. A SYNTHESYS
grant (GB-TAF-1060, made available by the European Community
Research Infrastructure Action under the FP6 Structuring the European Research Area Programme) to YYT enabled us to carry out this
study.
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