. Bulletin. Science; Natural history; Natural history. j i_ 0 1000 2000 3000 4000 MEAN ALTITUDE 3 7 Z < UJ S 6 c r2 = + X 50 52 54 56 58 60 MEAN TEMPERATURE 62 64 LO 3 7 Z < UJ S 6 D. r2 = Y = + X 150 200 250 300 350 MEAN FREEZE-FREE PERIOD Figure 2. Correlations of rainfall (inches); B. Mean length of the freeze-free period (days) itter size with climatic and physical factors. A. Mean annual altitude (feet); C. Mean temperature (Fahrenheit); D. Mean Differences in litter sizes of the California ground squirrel could be responses to different densitie
. Bulletin. Science; Natural history; Natural history. j i_ 0 1000 2000 3000 4000 MEAN ALTITUDE 3 7 Z < UJ S 6 c r2 = + X 50 52 54 56 58 60 MEAN TEMPERATURE 62 64 LO 3 7 Z < UJ S 6 D. r2 = Y = + X 150 200 250 300 350 MEAN FREEZE-FREE PERIOD Figure 2. Correlations of rainfall (inches); B. Mean length of the freeze-free period (days) itter size with climatic and physical factors. A. Mean annual altitude (feet); C. Mean temperature (Fahrenheit); D. Mean Differences in litter sizes of the California ground squirrel could be responses to different densities of breeding populations caused by other inimical factors related to latitude. Tomich (1962) believed that excessive rainfall was the most important adverse factor affecting popula- tions of California ground squirrels. However, rainfall at the northern localities for which mean litter sizes were available averaged about three times that at the southern localities and the corresponding correlation (Table 2. Fig. 2A) indicated litter sizes were smaller where rainfall was heaviest. A low level of correlation was also found when altitude was used as the independent vari- able (Table 2. Fig. 2B). The correlation between litter size and mean temperature at the various collection sites was not significant (Table 2. Fig. 2C). Lord's (1960) hypothesis could be used to explain inverse relationships between latitude and litter size in hibernating species of mam- mals. This would require that the length of periods of dormancy be directly correlated with latitude, and that associated differences in rates of mortality be of sufficient magnitude to affect the observed differences in litter size. However, the California ground squirrel both hibernates and estivates (Linsdale. 1946). In southern Cali- fornia. Linsdale (1946) observed active ground squirrels in all months of the year, but found that adults spent as long as eight months and juveniles as long as 5% months in their burrows. Pleas
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