Behavior and Neurology of Lizards 
N. Greenberg and P. D. MacLean, eds. 
NIMH, 1978. 
Why Brain Research on Lizards? 
Paul D. MacLean 
Laboratory of Brain Evolution and Behavior 
National Institute of Mental Health 
Those who are familiar with lizards real- 
ize that there are more reasons for conduct- 
ing research on these animals than there are 
investigators prepared to do the necessary 
work. Our own interest in lizards is easy to 
explain because they are distant relatives of 
the long-extinct mammal-like reptiles that 
are believed to have been the antecedents of 
mammals. Ordinarily, there is no apparent 
causal connection between a current hap- 
pening and something that took place a few 
hundred million years ago. But it is perfectly 
correct to say that this Laboratory would not 
be here, and that this Conference would not 
have been held, had there not been an his- 
torical link between mammals and reptiles. 
When beginning to plan for the present 
facility 20 years ago, a primary purpose was 
to use new behavioral approaches in investi- 
gating the functions of a basic part of the 
forebrain that reflects our reptilian ancestry. 
The reason will be explained after defining 
the cerebral mass in question. 
THE TRIUNE BRAIN 
In its evolution the primate forebrain ex- 
pands along the lines of three basic forma- 
tions that can be characterized as reptilian, 
paleomammalian, and neomammalian. The 
diagram in Figure 1 depicts the hierarchic 
organization of the three formations. Re- 
markably different in chemistry and struc- 
ture, and in an evolutionary sense eons apart, 
the three formations constitute three brains 
in one, or what may be called for short a 
triune brain (MacLean, 1970, 19736). This 
neural condition suggests that psychologi- 
cally we represent an amalgamation of three 
quite different mentalities (MacLean, 1975a, 
1977a). 
In mammals, the major counterpart of the 
reptilian forebrain is represented by the ol- 
factostriatum, corpus striatum (caudate 
nucleus and putamen), the globus pallidus, 
and satellite collections of gray matter. Since 
there is no term that applies to all of these 
structures, I will refer to them as the striatal 
or R-complex (MacLean, 1973c). 
Developments in histochemistry have been 
of great help in identifying the corresponding 
striatal structures in reptiles, birds, and 
mammals. The black areas in Figure 2 show 
how a stain for cholinesterase (originally 
described by George Koelle in 1954) vividly 
colors the R-complex in the monkey brain. 
Figure 1. In its evolution, the primate forebrain 
expands in hierarchic fashion along the lines of 
three basic patterns that may be characterized as 
reptilian, paleomammalian, and neomammalian 
(from MacLean, P.D., 1967). 
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