Lay Summary

main points to be covered:
-the compelling question this paper tried to address,
-how does (do) the researcher(s) go about answering it
-what evidence is found
-what conclusions are made.
Homework 1: Lay Summary Instructions
In one page, write a Lay Summary for the journal article that your Lab Instructor assigned to your section. In
concise paragraphs that your aunt or uncle might understand, explain the compelling question this paper tried to
address, how does (do) the researcher(s) go about answering it, what evidence is found, and what conclusions are
made.
A lay summary is like the USA Today version of the article. You want to engage the reader using creative writing
skills, but you also must be careful that your summary neither misleads nor foster misconceptions about science.
For example, you should avoid words like, ”prove”, “correlate”, “hypothesize”, and “theorize” that are used
differently in science than in everyday language.
Summaries will be evaluated on how well they engage a lay reader, summarize the main finding(s), show language
skills, and follow the format instructions.
Assignment Details:
• Audience—Layperson such as an aunt or uncle.
• Format—Single-spaced, 10-pt font, and fits on a single page. The document should include your name, section
number, and 1-inch margins.
• Submitting the assignment— Bring a hardcopy to lab and upload a .pdf document to the specified dropbox on
D2L (unless instructed otherwise by your Lab Instructor).
• Due date—Hardcopy is due by the beginning of lab during the 2nd week of class. [Note: You will read your
summary aloud to other members of your group during lab.]
• Grading—Please look over the associated grading rubric before you write and submit this assignment.

ORIGINAL INVESTIGATION
Encephalization of Bathyergidae and comparison of brain structure
volumes between the Zambian mole-rat Fukomys anselli and the giant
mole-rat Fukomys mechowii
Dieter C.T. Kruska, Katja Steffen
Zoological Institute, Haustierkunde, Christian-Albrechts-University, Universita¨t Kiel, Olshausenstr. 40 – 60, D 24118 Kiel, Germany
Received 8 February 2008; accepted 10 April 2008
Abstract
Encephalization indices were calculated for Fukomys anselli and Fukomys mechowii by using interspecific allometric
lines of Tenrecinae (recent Eutheria with the smallest brains) and average Rodentia to compare brain sizes independent
of body size influence. These were contrasted with corresponding indices of other Bathyergidae and additionally with
other rodents. The Bathyergidae species had indices within the variation of some Cricetidae and Muridae and thus do
not differ in encephalization. F. anselli, however, had a clearly higher encephalization index than the sister species F.
mechowii. The sizes of diverse structures were measured in the brains of these two species by help of the serial section
method. No differences were found in relative composition. The lower encephalization of F. mechowii is discussed as a
special phenomenon of gigantism during phylogenetic radiation which similarly was documented for other forms.
r 2008 Deutsche Gesellschaft fu¨r Sa¨ugetierkunde. Published by Elsevier GmbH. All rights reserved.
Keywords: Fukomys anselli; Fukomys mechowii; Encephalization; Brain composition; Gigantism
Introduction
Absolute and relative size of the brain as well as
proportions of brain parts are highly diverse in
mammals (De Winter and Oxnard 2001; but see also
Finlay et al. 2001). Interspecific allometric studies of the
brain to body size relation revealed different encephalization plateaus for several orders (Baron et al. 1996;
Kruska 2005; Ro¨hrs 1966; Stephan et al. 1991). Thus,
more highly evolved species of mammalian orders have
larger brains, and in general these can be contrasted
with those less progressive and minor encephalized.
In addition, very often adaptive radiation to special
ecology and life styles within different orders is
connected with enlargement or regression of total brain
size or certain brain structures. Evidently this happened
convergently during evolution and can be shown for
different encephalization plateaus when contrasting,
e.g., semiaquatic, aquatic or arboreal species with
ground dwelling relatives (Kruska 1988, 2005).
In this connection adaptive radiation of species to a
strictly subterranean life style is of interest. This occurred
several times during phylogeny within very different
mammalian radiations. Thus, such convergence is known
from the Marsupialia (Notoryctes) and eutherian orders
as the Zalambdodonta (Chrysochloridae), Insectivora
(Talpidae) and especially within several families of the
Rodentia (Cricetidae, Geomyidae, Ctenomyidae, Octodontidae, Rhizomyidae, Spalacidae, Bathyergidae).
Adapted to fossorial activities and underground life, all
these forms share some corresponding morphological as
well as physiological characteristics.
ARTICLE IN PRESS
www.elsevier.de/mambio
1616-5047/$ – see front matter r 2008 Deutsche Gesellschaft fu¨r Sa¨ugetierkunde. Published by Elsevier GmbH. All rights reserved.
doi:10.1016/j.mambio.2008.04.002 Mamm. biol. 74 (2009) 298–307
Corresponding author. Tel.: +49 431 880 4513;
fax: +49 431 880 1389.
E-mail address: dkruska@zoologie.uni-kiel.de (D.C.T. Kruska).
Concerning brain size and brain composition it could
be assumed that this form of a rather protected life
underground would be connected with smaller brains,
reduced sense organs and smaller sensory brain parts,
which would result in lower encephalization compared
with relatives adapted to non-fossorial life styles.
However, this was not the case in Chrysochloridae and
Talpidae since independent of body size these forms
clearly had larger brains than the non-fossorial Tenrecinae, which have the smallest brains of all recent
Eutheria (Stephan et al. 1991). Likewise brains of the
blind Spalax also were larger compared with those of
laboratory rats within the order Rodentia (Frahm et al.
1997).
Mole-rats of the family Bathyergidae are another
strictly fossorial group. They are endemic to Africa
south of the Sahara and evolved within the hystricognath rodents in a special and separated radiation from
early miocene ancestors. No sister-group relationship of
these forms to any other rodent radiation could be
established (Thenius 1969; Honeycutt et al. 1991). The
taxonomy and systematics of the Bathyergidae are still
under question but in general two subfamilies are
recognized today. The Bathyerginae includes two species
of Bathyergus which are slightly larger in body size, dig
with their enlarged forefeet and claws and live solitary.
The other subfamily, the Georychinae, comprises
Georychus capensis, Heliophobius argenteocinereus, Heterocephalus glaber and an uncertain number of
Cryptomys species (Scharff et al. 2001). Recently a new
genus Fukomys was recognized valid for some former
Cryptomys species although both genera are not clearly
separated from each other by morphological traits or
morphometric differences. The new genus was characterized by allozyme, nuclear and mitochondrial DNA
markers and high karyotypic diversity with diploid
numbers from 48 to 70 versus the rather stable 2n ¼ 54
for Cryptomys (Ingram et al. 2004). Altogether 14
species of Fukomys are described until now (Kock
et al. 2006) and about five of Cryptomys. However,
except for F. mechowii the other Georychinae are
smaller sized and all use enlarged chissel-like incisors
for fossorial activities. Heterocephalus and some
Fukomys species are the only mammals that are
known for their eusocial organization in family clans
(Bennett and Jarvis 1988; Burda 1990; Burda and
Kawalika 1993; Jarvis 1981, 2001; Lacey and Sherman
1981).
Since there only is little information on brains of the
Bathyergidae (Pirlot 1990) the aim of this study is firstly
to get insight in the general encephalization of some
bathyergid species compared to Tenrecinae and nonfossorial rodents. Secondly the relatively small Fukomys
anselli will be compared with the larger sister species
Fukomys mechowii concerning relative proportioning of
the brain.
Material and methods
Ten (five males, five females) F. anselli and 11 (five males, six
females) F. mechowii were obtained alive from Prof. H. Burda,
University of Duisburg-Essen. The F. anselli were in the first or
second generation bred under human care. The founder
individuals of this breeding colony originated from the region
of Lusaka, Zambia. They were of the 2n ¼ 68 karyotype
population which led to the description of this new species
(Burda et al. 1999). The F. mechowii individuals were caught in
the wild from the population near Ndola, Zambia and kept
under human care for several months. The individuals were
adult and between 1.5 and 5 years old.
All the animals were sacrificed under deep anaesthesia.
Total body weights were recorded immediately after death and
the brains were dissected from the skulls and freshly weighed.
Visceral organs were additionally dissected, weighed and
stored in formalin (10%). Net cadaver weights were calculated
as total body weight minus viscera weight (Table 1).
To get an impression on the encephalization of Bathyergids
in general, data on brain and body weights for some species
were used from the literature and geometric means of the
individual values of both Fukomys species (Table 2). The
interspecific allometric line for Tenrecinae according to
Bauchot and Stephan (1966) served as one reference line and
the interspecific allometric line for average Rodentia as
another. The latter has previously been calculated using brain
body size data of 64 species from 20 families of Protrogomorpha, Myomorpha, Glirimorpha, Sciuromorpha, Caviomorpha, and Hystricomorpha (Kruska 1980, 1988; for slope and
position of several mammalian interspecific allometric lines see
ARTICLE IN PRESS
Table 1. Individual brain and body size data of the two
Fukomys species F. anselli and F. mechowii
No. Species Sex TBW NCW BW
18142 Fukomys anselli Female 58 30 1.18
18181 Fukomys anselli Female 80 44 1.23
18406 Fukomys anselli Female 54 31 1.22
18410 Fukomys anselli Female 66 38 1.21
18411 Fukomys anselli Female 50 29 1.11
18164 Fukomys anselli Male 83 46 1.35
18180 Fukomys anselli Male 100 53 1.33
18407 Fukomys anselli Male 50 29 1.21
18408 Fukomys anselli Male 80 46 1.24
18409 Fukomys anselli Male 81 46 1.21
— Fukomys mechowii Female 270 — 2.11
18162 Fukomys mechowii Female 255 142 2.21
18975 Fukomys mechowii Female 166 104 2.21
18976 Fukomys mechowii Female 225 129 2.45
18987 Fukomys mechowii Female 194 106 2.32
18988 Fukomys mechowii Female 177 102 2.25
18137 Fukomys mechowii Male 309 189 2.83
18977 Fukomys mechowii Male 256 147 2.40
19111 Fukomys mechowii Male 442 255 2.09
19112 Fukomys mechowii Male 352 187 2.36
19113 Fukomys mechowii Male 330 179 1.98
TBW ¼ total body weight (in g); NCW ¼ net cadaver weight (in g);
BW ¼ brain weight (in g).
D.C.T. Kruska, K. Steffen / Mamm. biol. 74 (2009) 298–307 299
also Kruska 2005). Encephalization indices for the Bathyergidae species were then estimated in relation to the
interspecific allometric line for Tenrecinae (EI te). Calculations
were also done in relation to the line for average Rodentia (EI
ro) (Table 2). The indices characterize the encephalization level
of a species above the plateau of Tenrecinae (EI te ¼ 100) or
above as well as below that of average Rodentia (EI ro ¼ 100).
Average Rodentia have body size independently about 2.5
times larger brains than Tenrecinae (EI te ¼ 244). The
encepalization indices of Bathyergidae are then compared
with those of some other rodents.
The brains of the two Fukomys species were fixed in AGF
fluid (being a mixture of 80 ml alcohol 80%, 10 ml glacial
acetic acid and 10 ml formalin 40%), after 3 days stored in
80% alcohol and later photographed from dorsal, lateral and
ventral view. They were then embedded in paraffin. Six (3f,
3m) brains of F. anselli and the same number of F. mechowii
were used for serial sectioning. Accordingly they were cut
totally at 10 mm or at 20 mm in the frontal plane. About 250
equidistant sections per brain were mounted on slices and
Nissl-stained with cresyl violett. The 80–90 of these, again
equidistant and covering the whole brain, were photographed
and projected on photographic paper at a known enlargement.
On these the structures listed in Tables 3–6 were at first
delineated in the same way as shown for rats in Kruska
(1975a, b), then cut out and finally weighed. By use of the
paper weight, enlargement of photographs, section thickness
and the distance between sections the serial section volume for
each structure was determined (Stephan 1960; Kruska and
Stephan 1973). Due to tissue shrinkage during fixation and
histological processing the brain volume resulting from the
sum of the major brain regions of the serial sections is
considerably smaller than the fresh brain volume (fresh brain
weight divided by 1.036 ¼ specific gravity of brain substance).
The extent of shrinkage is different for each brain. Here, the
brains of F. anselli shrank by 44.1%, 50.0%, 44.8%, 45.2%,
47.1%, 48.1% and those of F. mechowii by 46.1%, 33.7%,
36.8%, 37.0%, 38.0% and 37.3%. Consequently a conversion
factor was calculated for each brain (being volume of fresh
brain/sum of serial section volumes) and the serial section
volumes of all structures were converted into fresh tissue
values. These are documented in Tables 3 and 4.
ARTICLE IN PRESS
Table 2. Brain and body weight data of several species of
Bathyergidae and calculated encephalization indices in relation
to the allometric line for Tenrecinae (EI te) and for average
Rodentia (EI ro)
Species TBW BW EI te EI ro Source
Fukomys anselli 68.3 1.23 200 82 This study
Fukomys mechowii 265.0 2.28 158 65 This study
Cryptomys hottentotus 130.2 1.36 148 60 Pirlot (1990)
Georychus capensis 132.3 1.79 192 79 Pirlot (1990)
Bathyergus janetta 268.0 2.06 142 58 Pirlot (1990)
Heterocephalus glaber 33.3 0.43 111 45 Pirlot (1990)
The values for F. anselli and F. mechowii are geometrical means from
Table 1. TBW ¼ total body weight in g; BW ¼ brain weight in g.
Table 3. Absolute volumes (in mm3
) of diverse brain structures in 6 (3 females, 3 males) Fukomys anselli individuals
Fukomys anselli 18411 18142 18181 18409 18180 18164
Pure brain tissue 1027.260 1116.848 1160.705 1129.175 1246.159 1279.176
Medulla oblongata 117.236 126.244 128.375 143.119 152.528 132.775
Cerebellum 177.231 169.333 193.753 204.897 213.982 225.052
Mesencephalon 60.321 73.122 60.374 47.654 66.950 77.806
Diencephalon 86.394 97.947 100.529 100.199 110.440 109.612
Telencephalon 586.078 650.202 677.674 633.306 702.259 733.931
Neocortex 272.478 303.577 332.056 302.412 322.284 366.566
Corpus striatum 66.871 75.610 74.620 67.220 76.393 77.571
Allocortex 246.729 271.015 270.998 263.674 303.582 289.794
Neocortex (grey matter) 246.371 275.544 297.551 271.209 289.860 328.745
Neocortex (white matter) 26.107 28.033 34.505 31.203 32.424 37.821
Olfactory allocortex 131.148 144.054 144.801 146.078 161.529 147.998
Bulbus olfactorius 43.359 45.842 47.218 47.614 54.851 29.506
Regio retrobulbaris 3.600 5.358 6.739 6.234 6.603 6.906
Tuberculum olfactorium 10.182 13.093 12.106 11.597 11.546 13.860
Regio praepiriformis 43.618 43.464 46.048 45.207 53.597 53.844
Nucleus amygdala 19.621 22.526 20.864 22.250 22.317 29.271
Basal nuclei 10.768 13.771 11.826 13.176 12.615 14.611
Non-olfactory allocortex 115.581 126.961 126.197 117.596 142.053 141.796
Septum 16.960 16.111 16.062 16.923 19.291 19.263
Hippocampus 60.612 65.009 70.947 64.538 77.316 76.819
Schizocortex 38.009 45.841 39.188 36.135 45.446 45.714
300 D.C.T. Kruska, K. Steffen / Mamm. biol. 74 (2009) 298–307
In order to compare the composition of the brains of both
species and to get an idea about individual variability, all the
diverse structures were calculated as percentages of pure brain
tissue (Tables 4 and 5). This pure brain tissue value resulted
from calculations of fresh brain volume minus volumes of
ventricles, nerves, hypophysis, epiphysis and parts of the spinal
cord which had remained with the brains. Additionally mean
percentage values were calculated for any given structure of
both species and then compared with corresponding values of
brains from wild caught Norwegian rats Rattus norvegicus
(Kruska 1975a, b; Kruska and Schott 1977).
Results and discussion
Individual data on brain and body size of Fukomys
anselli and F. mechowii are summarized in Table 1. In a
double log plot of these data the values for F. anselli are
adjusted to an intraspecific allometric line with a slope
around a ¼ 0.20 (not shown) as is typical for other
mammals (Kruska 1980, 1988, 2005) but this was not
the case for the data of F. mechowii.
Concerning the encephalization degree of Bathyergidae limited data for brain and body sizes of other species
were found in literature (Pirlot 1990) only of single
individuals per species. These were compared with the
geometrical means of the data of the two Fukomys
species (Table 2). As can be seen in Fig. 1 the values of
the Bathyergidae species are placed between the interspecific allometric lines of the Tenrecinae and average
Rodentia. This means, after adjusting for body size the
species have larger brains compared with Tenrecinae,
but they have smaller brains compared with rodents on
average. This is also documented by the encephalization
indices (Table 2). These are greater than 100 in case of
EI te and smaller than 100 for EI ro. However, the
Bathyergidae species have reached different encephalization levels, e.g., Fukomys anselli and Georychus
capensis have the largest brains followed by Fukomys
mechowii, Cryptomys hottentotus and Bathyergus janetta. Very clearly H. glaber has the smallest brain which is
only slightly larger than that of Tenrecinae. Although
for most of the Bathyergidae these values are rather
uncertain because of limited data, the values for the two
Fukomys species are valid.
In this respect it is of interest to know some
encephalization indices of other rodents with fossorial
but additionally non-fossorial activities. These were
calculated as EI ro from geometrical means of a greater
number of literature data and are as follows:
Cricetidae: Cricetus cricetus (n ¼ 57 Frahm 1973;
Adam 1973 unpubl.) EI ro ¼ 59; Mesocricetus auratus
ARTICLE IN PRESS
Table 4. Absolute volumes (in mm3
) of diverse brain structures in 6 (3 females, 3 males) Fukomys mechowii individuals
Fukomys mechowii 18162 18987 18976 19113 19112 18977
Pure brain tissue 2062.711 2134.879 2287.866 1843.912 2170.674 2249.974
Medulla oblongata 254.809 235.223 218.970 190.588 211.816 256.126
Cerebellum 383.397 345.007 341.344 374.681 380.318 373.804
Mesencephalon 102.467 91.793 119.419 102.179 99.066 106.012
Diencephalon 155.824 170.201 192.432 130.657 168.228 182.148
Telencephalon 1166.214 1292.655 1415.701 1045.807 1311.246 1331.884
Neocortex 584.429 648.692 696.370 527.950 657.719 696.155
Corpus striatum 127.337 137.229 165.825 98.893 138.804 143.977
Allocortex 454.448 506.734 553.506 418.964 514.723 491.752
Neocortex (grey matter) 508.503 557.820 617.031 472.088 591.445 616.241
Neocortex (white matter) 75.926 90.872 79.339 55.862 66.274 79.914
Olfactory allocortex 246.519 265.371 302.362 240.112 278.296 262.735
Bulbus olfactorius 69.727 78.961 86.625 79.568 86.280 77.958
Regio retrobulbaris 6.827 9.272 8.938 8.684 8.663 7.687
Tuberculum olfactorium 25.774 28.244 31.212 21.281 27.774 23.805
Regio praepiriformis 86.724 84.241 104.019 77.769 90.473 84.094
Nucleus amygdala 36.431 40.462 48.193 34.425 40.425 39.788
Basal nuclei 21.036 24.191 23.375 18.385 24.681 29.403
Non-olfactory allocortex 207.929 241.363 251.144 178.852 236.427 229.017
Septum 31.138 31.559 30.938 21.672 29.082 28.323
Hippocampus 112.357 128.695 142.656 106.873 131.310 121.994
Schizocortex 64.434 81.109 77.550 50.307 76.035 78.700
D.C.T. Kruska, K. Steffen / Mamm. biol. 74 (2009) 298–307 301
(n ¼ 61 Frahm 1973) EI ro ¼ 61; Phodopus sungorus
(n ¼ 60 Frahm 1973) EI ro ¼ 58; Clethrionomys glareolus (n ¼ 228 Lo¨bmann 1968; Adam 1973 unpubl.) EI
ro ¼ 82; Microtus agrestis (n ¼ 16 Adam 1973 unpubl.)
EI ro ¼ 71; Ondatra zibethica (n ¼ 27 Adam 1973
unpubl.) EI ro ¼ 71.
Muridae: Mus musculus (n ¼ 71 Rohn 1971 unpubl.)
EI ro ¼ 66; R. norvegicus (n ¼ 78 Kruska 1975a) EI
ro ¼ 62; Apodemus sylvaticus (n ¼ 267 Klemmt 1960;
Adam 1973 unpubl.) EI ro ¼ 91; Apodemus flavicollis
(n ¼ 37 Adam 1973 unpubl.) EI ro ¼ 90.
Spalacidae: Spalax ehrenbergi (n ¼ 8 Frahm et al.
1997) EI ro ¼ 99.
From this it can be concluded that Fukomys mechowii
has approximately the same encephalization level as the
three hamster species as well as mouse and rat. An
evidently higher encephalization is documented for
Fukomys anselli with an index comparable to Ctethrionomys glareolus. Very surprising, however, is the fact
that the strictly fossorial blind mole-rat Spalax ehrenbergi in a distinct radiation shows the highest encephalization of these rodents mentioned. The evident
difference between the two Fukomys sister species
remains surprising especially in the light of their
common ancestry and rather similar life styles.
However, concerning the outer appearance and except
for differences in size the brains of Fukomys anselli and
Fukomys mechowii are very much alike and remind of a
rodent brain (Fig. 2). They show relatively small and
lissencephalic hemispheres and a prominent cerebellum. In
dorsal view the contour of the hemispheres appears more
rectangular and less elongated as in some other rodents,
e.g. rats or mice. An occipital pol is not very prominent,
possibly greater parts of a visual area are lacking in these
microptic mammals. Viewed from lateral the olfactory
bulbs seem of normal size but the hemispheres are not very
high. They do not exceed the height of the cerebellum.
Tiny optic nerves and a small chiasma opticum are hardly
to recognize in ventral view of both brains.
The measured fresh tissue sizes of diverse brain parts
are summarized in Table 3 for Fukomys anselli and in
Table 4 for Fukomys mechowii. They, of course, differ in
size from brain to brain and species to species because of
differences in total brain size and individual variability.
The relative values (Tables 5 and 6) are more reliable for
a comparative approach. Also here a certain individual
variability can be seen in both species which in similar
dimensions also is known from other mammals and
comparable studies. Min.–max. and mean values of
relative structures sizes of the brains of both Fukomys
species are given in Table 7 and contrasted with comparable data of wild Norvegian rats (Kruska 1975a, b;
Kruska and Schott 1977). These three species show
corresponding results concerning brain proportioning.
ARTICLE IN PRESS
Table 5. Relative values of the brain structures in Fukomys anselli
Fukomys anselli 18411 18142 18181 18409 18180 18164
Pure brain tissue 100.00 100.00 100.00 100.00 100.00 100.00
Medulla oblongata 11.41 11.30 11.06 12.67 12.24 10.38
Cerebellum 17.25 15.16 16.69 18.25 17.17 17.59
Mesencephalon 5.87 6.55 5.20 4.22 5.37 6.08
Diencephalon 8.41 8.77 8.66 8.87 8.86 8.57
Telencephalon 57.06 58.22 58.39 56.09 56.36 57.38
Neocortex 26.52 27.18 28.61 26.78 25.87 28.66
Corpus striatum 6.52 6.77 6.43 5.96 6.13 6.07
Allocortex 24.02 24.27 23.35 23.35 24.36 22.65
Neocortex (grey matter) 23.98 24.67 25.64 24.02 23.26 25.70
Neocortex (white matter) 2.54 2.51 2.97 2.76 2.61 2.96
Olfactory allocortex 12.77 12.90 12.48 12.94 12.96 11.57
Bulbus olfactorius 4.22 4.11 4.07 4.22 4.40 2.31
Regio retrobulbaris 0.35 0.48 0.58 0.55 0.53 0.54
Tuberculum olfactorium 0.99 1.17 1.04 1.03 0.93 1.08
Regio praepiriformis 4.25 3.89 3.97 4.00 4.30 4.21
Nucleus amygdala 1.91 2.02 1.80 1.97 1.79 2.29
Basal nuclei 1.05 1.23 1.02 1.17 1.01 1.14
Non-olfactory allocortex 11.25 11.37 10.87 10.41 11.40 11.08
Septum 1.65 1.44 1.38 1.49 1.55 1.51
Hippocampus 5.90 5.82 6.11 5.72 6.20 6.00
Schizocortex 3.70 4.11 3.38 3.20 3.65 3.57
302 D.C.T. Kruska, K. Steffen / Mamm. biol. 74 (2009) 298–307
The telencephalon is the greatest part of the fundamental brain regions followed by the cerebellum, the
medulla oblongata and then the diencephalon and
mesencephalon which clearly are smaller. Within the
forebrain the neocortex with a prominent portion of
grey matter is only slightly greater than the allocortex.
The latter structure consists of olfactory and nonolfactory (limbic) parts to nearly similar extent although
in the Fukomys species the olfactory structures seem
slightly larger in relative size. When comparing the mean
ARTICLE IN PRESS
Table 6. Relative values of the brain structures in Fukomys mechowii
Fukomys mechowii 18162 18987 18976 19113 19112 18977
Pure brain tissue 100.00 100.00 100.00 100.00 100.00 100.00
Medulla oblongata 12.35 11.02 9.57 10.34 9.76 11.38
Cerebellum 18.59 16.16 14.92 20.32 17.52 16.61
Mesencephalon 4.97 4.30 5.22 5.54 4.56 4.71
Diencephalon 7.55 7.97 8.41 7.08 7.75 8.10
Telencephalon 56.54 60.55 61.88 56.72 60.41 59.20
Neocortex 28.33 30.38 30.44 28.63 30.30 30.94
Corpus striatum 6.18 6.43 7.25 5.37 6.40 6.40
Allocortex 22.03 23.74 24.19 22.72 23.71 21.86
Neocortex (grey matter) 24.65 26.13 26.97 25.60 27.25 27.39
Neocortex (white matter) 3.68 4.25 3.47 3.03 3.05 3.55
Olfactory allocortex 11.95 12.43 13.22 13.02 12.82 11.68
Bulbus olfactorius 3.38 3.70 3.79 4.31 3.97 3.46
Regio retrobulbaris 0.33 0.43 0.39 0.47 0.40 0.34
Tuberculum olfactorium 1.25 1.32 1.36 1.15 1.28 1.06
Regio praepiriformis 4.20 3.95 4.55 4.22 4.17 3.74
Nucleus amygdala 1.77 1.90 2.11 1.87 1.86 1.77
Basal nuclei 1.02 1.13 1.02 1.00 1.14 1.31
Non-olfactory allocortex 10.08 11.31 10.97 9.70 10.89 10.18
Septum 1.51 1.48 1.35 1.18 1.34 1.26
Hippocampus 5.45 6.03 6.23 5.79 6.05 5.42
Schizocortex 3.12 3.80 3.39 2.73 3.50 3.50
Average Rodentia
Tenrecinae
log
Total Body Weight (g)
Fukomys anselli
Fukomys mechowii
Cryptomys hottentotus
Bathyergus janetta
Georychus capensis
Heterocephalus glaber
a = 0.63
b = -0.9800
a = 0.63
b = -1.3680
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
1.0000 1.5000 2.0000 2.5000
Brain Weight (g)
log
3.0000
Fig. 1. Interspecific allometric lines of Tenrecinae and average Rodentia and specific data plots of Bathyergidae.
D.C.T. Kruska, K. Steffen / Mamm. biol. 74 (2009) 298–307 303
relative values for the diverse structures of F. anselli with
F. mechowii differences obviously occur for the telencephalon, the total neocortex and its grey matter.
However, such differences could not be assured statistically and consequently as a main result both species
have brains of identical relative composition although
very clearly at different encephalization levels.
Compared with the rat brain composition on the
other hand some differences are obvious. Here the mean
relative values of cerebellum and telencephalon are
smaller and those of medulla oblongata, mesencephalon
and diencephalon larger. Within the smaller telencephalon of rats again the total neocortex and its grey matter
are larger as in mole-rats, while the allocortex, especially
its olfactory parts, are smaller. Maybe in comparison
with Rattus norvegicus the larger olfactory structures in
the brains of the two Fukomys species compensate to a
certain degree the lack of extensive visual structures in
their smaller diencephalon and neocortex.
In conclusion both Fukomys species have reached
encephalization levels comparable with those of some
Cricetidae and Muridae with above ground life styles. In
this example there is no convincing evidence for the
assumption that ecological niche adaptation during
phylogenetic radiation is dependent of or connected
with evolutionary brain size increase or decrease.
Moreover, F. anselli and F. mechowii have similar life
styles although the former consumes plant materials
exclusively whereas the latter is omnivorous (Burda and
Kawalika 1993). Therefore, the most striking result is
the similar relative composition of brains in these two
species which have clearly different encephalization
indices.
In this connection the following must be recognized.
Concerning the relationship of brain to body size the
following resulted from the fossil record of the Equidae
as well as of the Tylopoda: with the origin of new species
during phylogeny an increase of body size was not
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Fig. 2. Brains of Fukomys anselli (18181 – left) and Fukomys mechowii (18976 – right) in dorsal, lateral and ventral view.
304 D.C.T. Kruska, K. Steffen / Mamm. biol. 74 (2009) 298–307
always or necessarily connected with a brain size
increase similarly as the interspecific allometric relation
of recent forms (see Kruska 1982a, 1987). Very often
during evolutionary processes a certain ‘‘persistence’’ of
brain size is characteristic during phylogenetic body size
increase (Edinger 1960). On the other hand a brain size
increase is also documented for the radiation of Equidae
and Tylopoda which happened in an ‘‘erratic’’ way
rather independently of body size. For recent mammalian species Ro¨hrs (1958) mentioned several examples
for such ‘‘erratic’’ breakthroughs of interspecific allometries upward, i.e. closely related species with similar
life style and behaviour which have greater brains at the
same body size of sister species. Thus, progressive
encephalization during phylogeny was documented for
several clades, e.g., for carnivores (Kruska 1988, 2005;
Finarelli and Flynn 2007). As already was mentioned
during phylogenetic radiation of mammals interspecific
allometries also were broken through downward,
namely in cases where an extensive retardation of brain
size is opposed to a phylogenetic acceleration of body
size. As an example from the recent fauna the giant
forest hog Hylochoerus meinertzhageni can be mentioned
compared with the other species of Suidae and
Hippopotamus amphibius contrasted with the pigmy
hippo Choeropsis liberiensis (Kruska 1970, 1982b). Both
giant forms derived phylogenetically from smaller forms
closely resembling the other recent species of the family
in size (Thenius 1969). The brain size did not follow the
bodily gigantism in an interspecific mode. Consequently
both giant species clearly are less encephalized.
Just the same, within the group of tree-squirrels giant
species of the genus Ratufa likewise evolved from smaller
sized forms. Calculations on brain and body size data
from literature (Kruska unpubl.) resulted as follows: four
species of the smaller sized Sciurus had greater and rather
similar EI ro values, S. carolinensis 150, S. niger 155,
S. rufiventer 155, S. vulgaris 159. The value for the northAmerican semiarboricol Tamias striatus was 150, for
Tamiasciurus hudsonicus 169 and the African Funisciurus
carruthersi 162. In contrast the Asiatic giant squirrels
clearly had smaller values: Ratufa indica 93 and Ratufa
bicolor 119. Furthermore within the Petauristinae the
smaller Pteromys volans had an index of 142 whereas the
giant Petaurista petaurista only had 108.
In the light of these results we may conclude that the
giant mole-rat F. mechowii in contrast to F. anselli is just
another example of downward breakthroughs of interspecific allometries in the brain to body size relation.
Consequently the encephalization index is not always
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Table 7. Comparison of relative sizes of brain structures (pure brain tissue ¼ 100) in Fukomys anselli (n ¼ 6) and Fukomys
mechowii (n ¼ 6) with wild Rattus norvegicus (n ¼ 8, Kruska 1975a, b, Kruska and Schott 1977)
F. anselli F. mechowii R. norvegicus
Min.–max. Mean Min.–max. Mean Min.–max. Mean
Medulla oblongata 10.4–12.7 11.5 9.6–12.4 10.7 12.2–13.2 12.6
Cerebellum 15.2–18.3 17.0 14.9–20.3 17.4 13.7–16.5 14.9
Mesencephalon 4.2–6.6 5.5 4.3–5.5 4.9 6.3–7.3 6.8
Diencephalon 8.4–8.9 8.7 7.1–8.4 7.8 8.9–9.8 9.3
Telencephalon 56.1–58.4 57.3 56.5–61.9 59.2 55.1–58.3 56.4
Neocortex 25.9–28.7 27.3 28.3–30.9 29.8 29.3–33.0 31.1
Corpus striatum 6.0–6.8 6.3 5.4–7.3 6.4 4.6–5.4 5.1
Allocortex 22.7–24.4 23.7 21.9–24.2 23.0 18.9–21.2 20.2
Neocortex (grey matter) 24.6–25.7 24.6 24.7–27.4 26.3 25.0–28.8 27.1
Neocortex (white matter) 2.5–3.0 2.7 3.0–4.3 3.5 3.5–4.3 4.0
Olfactory allocortex 11.6–13.0 12.6 11.7–13.2 12.5 9.2–11.1 10.4
Bulbus olfactorius 2.3–4.4 3.9 3.4–4.3 3.8 2.1–3.6 3.1
Regio retrobulbaris 0.4–0.6 0.5 0.3–0.5 0.4 0.7–0.9 0.8
Tuberculum olfactorium 0.9–1.2 1.0 1.0–1.4 1.2 0.4–0.5 0.4
Regio praepiriformis 3.9–4.3 4.1 3.7–4.6 4.1 2.7–3.6 3.2
Nucleus amygdala 1.8–2.3 2.0 1.8–2.1 1.9 1.8–1.9 1.8
Basal nuclei 1.0–1.2 1.1 1.0–1.3 1.1 1.0–1.2 1.1
Non-olfactory allocortex 10.4–11.4 11.1 9.7–11.3 10.5 9.4–10.1 9.8
Septum 1.4–1.7 1.5 1.2–1.5 1.4 1.1–1.4 1.3
Hippocampus 5.7–6.2 6.0 5.4–6.2 5.8 5.6–6.5 6.0
Schizocortex 3.2–4.1 3.6 2.7–3.8 3.3 2.4–2.8 2.5
Min.–max. and mean values of individual variability are given.
D.C.T. Kruska, K. Steffen / Mamm. biol. 74 (2009) 298–307 305
and exclusively a measure for life style peculiarities.
Sometimes there might be some other phenomena
involved, especially ontogenetic and/or phylogenetic
events (see Kruska 2005).
Acknowledgements
We very much thank Prof. Dr. Heynik Burda,
Department of General Zoology, University of Duisburg-Essen for purchase of the animals and Mrs. Astrid
Ingwersen-Zwein for histological processing of the
brains and computer work. We also thank two
anonymous reviewers for some critical comments on a
former draft of the manuscript.
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