| PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 125 
that, if an animal is to be diurnally active, heating is easier to accom- 
plish than cooling in an area of high insulation. 
The foregoing indicates that Kentropy« may be a recent invader of 
the forest from a macroteiid ancestor that was adapted basically to 
living in open habitats. 
Though Hentropyz is the only primarily forest heliotherm, other 
heliotherms like Z’wpinambis and Mabuya also occur there. Low eccritic 
temperatures usually are associated with low temperature tolerances. 
Because of this and because the upper threshold of tolerance is usually 
sharper and more quickly lethal than the lower, it is probably easier 
and safer for open habitat animals to disperse through the forest than 
for forest animals to disperse through open habitats. 
The role of interspecific competition is a basic problem in animal 
ecology. One of the reasons for our interest in the distribution of Belém 
lizards was that the evolution of differences in microdistribution has 
been suggested as an important strategy in reducing interspecific com- 
petition among sympatric lizard species (Rand, 1964; Milstead, 1957; 
and others). The basis for this suggestion in lizards has come usually 
from studies of sympatric congeners, but there seems no reason that 
it could not occur between sympatric relatives of different genera. 
Kcologists( e.g., MacArthur, Recher, and Cody, 1966) who attempt to 
discover environmental parameters that will predict bird species di- 
versity seem to be assuming that a phylogenetic group as large as birds 
can be treated as an ecological unit for these purposes. We can make a 
similar assumption about the Belém lizards but, as we will show, it is 
probably wrong. 
The clearest cases wherein differences in microdistribution can be 
associated with interspecific interactions are those wherein two or more 
species occur in the same area, utilize the same structural niche, and 
replace one another in different habitats or different climatic niches. 
The only case of such replacement suggested by the distributions 
plotted in figure 1 is the three smaller macrotelids. There is some over- 
lap between these, but each species reaches its maximum abundance in 
a different environment: Cnemidophorus in the bare open patches, 
Ameiva in the edges and second growth, and Hentropyzx in the forest. 
All three have similar foraging habits and could well compete if they 
occurred syntopically. 
There are suggestions that size may be playing a role in reducing 
interaction between overlapping species. This is a strategy much seen 
in birds (Schoener, 1965, and others). In general, larger lizards take 
larger prey and this tends to reduce food competition among species 
of different sizes. In the Belém lizards, the very large macroteiid 
Tupinambis nigropunctatus widely overlaps Ameiva ameiva and Ken- 
