On September 12, 1941, the writer accompanied Park Naturalist George C. Ruhle to the Lava Beds National Monument which was at the time associated for administrative purposes with Crater Lake National Park. Fifty live-traps were set out in the vicinity of Tule Lake and 32 adult Peromyscus maniculatus were taken. These were measured and skilled and the pelages added to the study skin collection at Crater Lake National Park. A statistical study of the measurements is being recorded here for the reference of students of the distribution and systematics of small mammals.

Methods: All measurements were taken of recently anaesthetized specimens removed from the live traps. The mouse is stretched to a uniform tension on a measuring board and the dimensions of the total length, tail length and foot length to the end of the longest claw are recorded with pins. The dimensions are then taken with a vernier caliper. The ear is measured, with the same instrument, from notch to tip. Body length is obtained by subtracting the tail length from the total length. The tail percentage is then obtained by dividing the tail length x 100 by the body length. It allows a better comparison of mice of different size than actual tail length. Tests have shown that this method of measuring recently anaesthetized mice can be done with an average error not greater than one-tenth of a millimeter.

Body Length: This standard measurement varies with age. All specimens included were 85 mm or more in length which is taken as the lower limit of adult body length. This allows comparison with series from other places with a similar lower limit but does not, of course, entirely eliminate differences in average size due to age differences in populations. The arrays are tabulated below:

In Peromyscus, as in man, female mice have a better life expectation than males. Female adults in the Lava Beds are older and therefore bigger than males. In this series they are also more numerous.

These comparisons are made with a sample of 72 mice from a sage brush association in Silver Lake, Oregon, a sample of 45 mice from Crater Lake South Rim and a sample of 27 mice from a yellow pine and deer brush association at the South Entrance to Crater Lake National Park. Lava Beds mice do not differ statistically in body length from any of these other samples.

The tail percentage is usually bound to be smaller in larger mice. These females averaging larger than the males have relatively shorter tails.

It has been found repeatedly that the relative tail length in Peromyscus varies directly with the amount of precipitation and therefore of vegetation covering the ground. Tails are longer where ground cover is more abundant. The Lava Beds are the most arid of the habitats in which series have been taken. They differ in relative tail length slightly from the South Entrance series and considerably from the Crater Lake Rim series the differences being 5.9 +/- .95 per cent. This difference is six times its standard error and therefore statistically significant.

Females have slightly longer ears presumably because they are larger mice.

Lava Beds mice do not differ from the other samples in this character.

Foot Length: Besides age differences there is a constant sex difference in foot length in Peromyscus the females having the smaller feet. The sexes are presented separately but not combined figure is given because of the sex difference.

It may be seen that the usual sex differences holds for the females with greater body length have the smaller average foot length.

The Lava Beds series does not differ in foot length in either sex from either of the Crater Lake series. There are small but statistically significant differences in the foot length of both sexes between the Lava Beds and Silver Lake series. It is possible this is due to the differences in the sandy soil cover at Silver Lake and the pumiceous or relatively unweathered rock fragment cover in the other regions.

Summary

1. Deer mice were abundant in September 1941 in the Lava Beds National Monument.
2. The 32 adults taken there resemble series of Peromyscus maniculatus gambelii taken at other areas in California and Oregon.
3. Comparisons of the Lava Beds series with mice taken at Silver Lake, Oregon, and at the Rim and the South Entrance of Crater Lake National Park showed no statistical difference in body length or ear length.
4. The Lava Beds series differed statistically in relative tail length from the Crater Lake Rim series but not from the Silver Lake or South Entrance series.
5. The gradient in relative tail length varies with the vegetative cover of the regions from which the series were taken: Rim, South Entrance, Silver Lake, Lava Beds.
6. The Lava Bed series exhibited a small but statistically significant difference in foot length in both sexes from the Silver Lake series but not from either Crater Lake series.
7. The softer soil cover at Silver Lake may be a factor in the production of this difference in foot length between the Lava Beds and Silver Lake mice.