NO. 38658 LIZARDS—-RAND AND HUMPHREY 15 
specific competition between most lizard species and, consequently, no 
selection pressure for mechanisms that would reduce the competition. 
If this is true, how can one account for the observed specialization 
in both climatic and structural niche? The answer is, of course, 
intraspecific competition in the Darwinian sense. When any species 
can exploit the available resources (in the broadest sense) more 
effectively by specializing in part of them, the specialization itself will 
be selected for unless this selection pressure is countered by selection 
in other directions. Specialization, by definition, involves loss of eco- 
logical amplitude, and selection against specialization is largely 
generated by temporal fluctuations in environmental conditions that 
generate a selection pressure for ecological amplitude (Pianka, 1967). 
In stable habitats—and the inside of the tropical rain forest is one 
of the most stable—the selection against specialization should be at 
its lowest. 
It may be that low counterselection against specialization, rather 
than any unusually strong selection pressure for it, has produced some 
of the extreme specializations we see in tropical forest forms. 
We have not the data to choose between these two hypotheses, 
which, in fact, are not mutually exclusive. It seems most likely that 
lizards as a block do not occupy a monolithic trophic niche but share 
a niche in a complex way with many other animals. It also seems likely 
that the low population levels in the forest lizards result in mini- 
mizing the importance of selective forces that arise from interspecific 
competition between many lizard species. 
Conclusions 
(1) The Belém area contains several different habitats and no 
species of lizard studied ranges uniformly through the area. (2) The 
forest is richest in number of species and the evidence suggests that 
this is at least in part because of its greater structural complexities. 
(3) Two types of thermal relationships with the environment are 
demonstrated by the lizard fauna: the heliothermic in both open and 
forest habitats and the nonheliothermic inside the forest. The latter 
show about as much homeothermism as the former because of the 
relatively low ambient temperature variation inside the forest. (4) 
The distribution of three species of macroteiid suggests that they re- 
place one another ecologically. A fourth macroteiid that is syntopic 
with at least two of these is much larger. Two Anolis species that are 
syntopic and occupy very similar structural niches are different in 
size. (5) These replacements are probably important, but the im- 
pression of the distributions is one of irregular overlaps and gaps 
between niches of species. (6) The apparently unoccupied niches are 
