1965] 
Scherba — Microtus 
129 
While there are no data available on the Microtus population 
density changes at the study site, casual observation suggests that 
population levels fluctuate greatly from year to year. The voles are 
seldom seen during some summers, while they are abundantly evident 
moving on the ground during other years. 1962 Was such a year of 
apparently high vole density in Jackson Hole. 
THE BURROW SYSTEM 
Multiple openings of tunnels are apparent at the base of a large 
number of the mound nests. Nest 434, a moderately active mound, 
had eight such openings and this nest was selected for examination 
(Fig. 1). 
The tunnel entrances measured 3.0 to 3.8 cm. in diameter. Exca- 
vation indicated that two of the tunnels on opposite slopes led down- 
ward, then inward, below the soil surface level, then upward, termi- 
nating in an oval chamber 12.2 cm. in length and 4.6 cm. high. 
Other tunnels ended blindly within the mound, joined this chamber 
after a circuitous path or entered a second, larger, chamber described 
below. The floor of this first chamber, which was 16.8 cm. below 
the mound apex, consisted of fine soil and numerous particles of short 
green and dried plant stems, especially the rush J uncus which grows 
as a fringe on the mound exterior. 
The second chamber, larger and deeper than the one described 
above, was abundantly lined with soft dried grasses and was accessible 
by three tunnels which entered it internally from the North, South, 
and West. These tunnels terminated, eventually, in openings at the 
ant nest surface. The chamber measured 13.7 cm. in length, 10.7 cm. 
in heighth, and its floor was located 13.7 cm. below the level of the 
soil surface and beneath the center of the mound apex. (See Fig. 2). 
Presumably this chamber serves as the bedding site for Microtus. 
Excavation of a second, more active ant mound, nest 460, revealed 
seven tunnel entrances with tunnels extending circuitously into the 
mound for distances of several centimeters. All of these tunnels 
terminated blindly. 
In summary, Microtus activity results in a series of tunnels opening 
at or just above the level of the soil surface. These tunnels, turning 
frequently, either end blindly or lead into lined chambers or cavities 
located within active ant mounds. 
POSSIBLE RELATIONSHIPS 
The unexpected presence of a rodent living within active ant nests 
prompts the inquiry: Are the voles adapted to living within the 
mounds, either by virtue of being protected from ant bites, or because 
of avoidance of the voles by the ants? Further, are the ants serving 
