Figure 1. — Typical backyard scene in the row-house area of 
Project conducted many of their studies on the biology 
ically, a “row house” block consists of a solid row 
of houses facing one street with the doors opening 
directly on the sidewalk. Adjoining houses have a 
common sidewall. The width of houses varies from 
about 16 to 25 feet and there are usually 14 to 20 
units joined together. Backyards are of similar 
width and extend from 20 to 60 feet in depth. An 
alley runs through the center of the block. Adjoin- 
ing the alley on the other side are another series of 
backyards belonging to the row of houses facing a 
street on the opposite side of the block. Occasion- 
ally there is also a small group of row houses at 
either end of a block. Surrounding many of the 
yards there is a near-solid board fence approxi- 
mately 6 feet high. Through the fences there are 
occasional breaks due to broken or loose boards or 
where the rats have gnawed through or dug under. 
Fences are barriers which restrict movement 
through the block. The major portion of the rats 
live out of doors. Occasionally additional space 
becomes available to them in the basements or even 
in the house proper. Protection from the weather 
and predators is also provided by garages, outhouses 
Baltimore where the various members of the Rodent Ecology 
of the Norway rat. Photograph by John T. Emlen, Jr. 
of various types, outdoor toilets in some cases, and 
all sorts of miscellaneous junk. At the time of the 
initial studies made by the Rodent Ecology Project 
ample food was available in practically every yard 
in open garbage containers which ranged from 
pasteboard boxes to 20-gallon galvanized garbage 
cans, which however lacked adequate covers. In 
the early 1940’s, at least, before the municipal pro- 
gram of environmental control had any effect, this 
description fits to varying degrees the habitat avail- 
able to rats in substandard housing areas, as well 
as in those which were given a higher rating. 
Due to the structure of the blocks each formed 
the limits of a single population. There was little 
exchange of members between adjoining blocks. 
Even within a block there was considerable re- 
striction of ranging by rats. Studies of marking 
and release conducted by Emlen, Stokes, Davis, 
and myself have regularly shown that the popula- 
tion of a single block is composed of smaller aggre- 
gates or colonies. Each has its own restricted home 
range, but there is some overlap in the space en- 
compassed by the movements of members of ad- 
joining colonies. 
3 
