These data were obtained as part of a field study of social behavior of the prairie vole, Microtus ochrogaster, in alfalfa fields at the University of Illinois Biological Research Area, Phillips Tract. Those fields were also used in a 25-year research study on Microtus ochrogaster populations and Microtus pennsylvanicus (see Getz, L.L. 2024. Monitoring of Microtus ochrogaster and Microtus pennsylvanicus populations in three different habitats in east-central Illinois, 1972 to 1997. ver 1. Environmental Data Initiative. https://doi.org/10.6073/pasta/2dae8f08578ce31e7817c92a0f6acc87).
All nests of Microtus ochrogaster in the study areas were located and monitored twice weekly (3.5-day interval) over a 63-month period in alfalfa habitat, with very few skips of scheduled trapping (mostly during vacation periods, meeting attendance or inclement weather). Data were collected in two fields, in the first from October 1980 - July 1984 and in the second field from June 1983 to May 1987. In addition to alfalfa, ragweed (Ambrosia sp.), goldenrod (Solidago sp.), wild parsnip (Pastinaca sativa) and bluegrass (Poa pratensis) were present in the fields.
All nests used by adult females were monitored. Females were identified on the basis of data from 1) monthly 3-day live trapping sessions of the entire study area, with traps positioned in a 10m grid pattern, a 3-day trapping session each month in which grid stations from known nests were trapped at a distance >15m, and 3) twice weekly monitoring of nests. The nests were identified by use of ultraviolet reflective powder. Four or five multiple-capture live-traps were placed near the burrow openings to each underground nest or in runways leading to a surface nest. Traps were set for two 28 hour periods each week for as long as the nest was occupied. Cracked corn was used as bait. The traps were set shortly after sunrise on Monday and checked at 2-4-h intervals until midnight, and twice on Tuesday morning, for a total of about seven checks. This trapping schedule was repeated Thursday morning through Friday morning. The twice weekly regime of monitoring nests was maintained throughout the study except during the week the entire grid was trapped, when nests were monitored only Monday through Tuesday.
Note that normally during each month, there are two dates that are only one day apart. The second date is the beginning date of the grid trapping that was conducted each month. In actuality, the mid point of this session is about 3.5 days from the former and 4 days until the next behavior trapping session. So, simply using the date as a 3.5 interval between "sessions" gives the correct interval.
Voles were individually marked by toe-clipping and weighed at first capture. At each capture location, animal number, sex, and reproductive condition were monitored. Weights of juveniles (<+20g) were used to estimate age at first capture (by reference to weights of animals of known age in a laboratory colony). Animals >+30 g were considered to be adult and capable of reproductive activation.