June 16, 194996 gms.
September 26, 1949230 gms.
September 27, 1949230 gms.
October 18, 1949260 gms.
October 27, 1949250 gms.
October 29, 1949220 gms.
November 8, 1949235 gms.
November 15, 1949245 gms.
November 24, 1949240 gms.
November 26, 1949240 gms.
November 30, 1949240 gms.
December 20, 1949260 gms.
February 18, 1950230 gms.
April 5, 1950290 gms.
April 7, 1950300 gms.
October 7, 1950320 gms.
November 29, 1950345 gms.
March 23, 1951340 gms.

Another example, showing winter cessation of growth in a male at even smaller size is shown below. This was in the winter of 1950-1951.

November 9145 gms.
November 28175 gms.
November 29165 gms.
January 10180 gms.
January 11175 gms.
March 1225 gms.
March 23200 gms.

Longevity

The longest span of records for an individual woodrat recorded was 991 days in a female, already adult when she was first caught on November 18, 1948. Other relatively long spans of records were: 827 days in a male, adult when first caught on March 16, 1952; 754 days in a female, also adult when first captured; 649 days in a male first captured as a juvenile; 465 days in a male, adult when first captured; 409 days in a male, juvenile when first captured; 399 days in a female, juvenile when first captured; 395 days in a female, adult when first captured; 390 days in a female, adult when first captured; 366 days in a male, adult when first captured. Of these eleven individuals (six females and five males) whose records cover more than a year, eight were already adult when first caught. These eleven rats represent only 4.3 per cent of the total number captured. Our study was made at a time when populations were shrinking and disappearing, and obviously individual spans would have been longer if we had been working with a stable population. In most instances the spans of our records represent only small parts of the life spans of the individuals involved. Nevertheless, our records emphasize the potentially greater longevity of the woodrat as contrasted with the various smaller rodents living in the same area. Of several thousand individuals of the genera Mus, Zapus, Reithrodontomys, Peromyscus, Sigmodon, and especially Microtus, none is known to have survived so long as two years, and only a few individuals are known to have survived so long as one year after being marked.

Summary

Plant succession resulting from land use practices created habitat conditions especially favorable for woodrats in the late nineteen forties in northeastern Kansas, and particularly on the University of Kansas Natural History Reservation. With protection from prairie fires, woody vegetation had encroached onto areas that were formerly grassland, and, later, fencing against livestock permitted dense thickets of undergrowth to develop. In this region the woodrat usually lives in a forest habitat, and requires for its house sites places that are especially well sheltered, as in matted thickets of undergrowth, root tangles exposed along eroded gully banks, hollow stumps or tree trunks, bases of thorny trees with multiple trunks for support, thick tops of fallen trees, or, especially, rock outcrops with deep crevices.

At the time of their maximum population density in or about 1947, woodrats probably averaged several per acre on the woodland parts of the Reservation. In the autumn of 1948, 17 were caught on the ten-acre tract of woodland that was live-trapped most intensively. By then, however, the population had already undergone drastic reduction, as shown by the fact that there were many unoccupied and disintegrating houses throughout the woodland. While the time and manner of mortality was not definitely determined, circumstantial evidence suggests that the downward trend began in early March, 1948, when record low temperatures and unusually heavy snowfall coincided with the time when parturition normally occurs. The rigorous weather conditions then may have been injurious, not only to the newborn litters but to the females comprising the breeding stock. Nevertheless, the population remained moderately high through 1948, but by early spring of 1949 more than three-fourths of the adults and subadults present in late autumn had been eliminated. Again, unusually severe winter weather seemed to be the underlying cause, as in January precipitation was the heaviest on record in 81 years, with penetrating sleet storms, persistent ice glaze, and occasional brief thawing followed by sudden drops to extremely low temperature.

After the drastic reduction in the winter of 1948-49, the population did not recover. Although no further sudden reductions due to extremes of weather were noted, the trend seemed to be one of gradual, progressive decline throughout the following period of years. Deterioration of the habitat, as the developing forest shaded out undergrowth, and inroads of certain predators may have been important in preventing recovery of the population. Many kinds of predatory mammals, hawks, owls, and snakes probably take woodrats occasionally, but the spotted skunk, long-tailed weasel, horned owl, timber rattlesnake and pilot black snake are considered to be by far the most important predators because of their habits and prey preferences. Few actual records of predation on woodrats were obtained because of their scarcity during most of the period covered by our study.