



Using the cotton rat. the Oklahoma Unit is studying the 

 relations of animal density to social behavior, physio- 

 logical condition, and regulation of numbers in wild 

 populations. View of the Oklahoma southern prairie 

 habitat type (top), runway and live trap (middle), and 

 field preparation of study materials (bottom). (Photos 

 by Robert Cox) 



Rodent pojntlations in relation to grazing use. — 

 A study of rodent populations and intensity of 

 livestock grazing has been active on Colorado 

 ranges during the past 7 years. Kesults tend to 

 dispel the belief that high rodent and rabbit pop- 

 ulations are closely associated with overgrazing. 



On a desert shrub area in western Colorado no 

 significant difference in Peromyscus numbers was 

 found between areas grazed in winter by sheep 

 and cattle and adjacent ungrazed areas. On 10,- 

 000-foot Black Mesa in western Colorado there 

 was no detectable difference in white-footed mouse 

 numbers with three intensities of cattle use on 

 sage-grass range. Similarly, there appeared no 

 correlation of mouse numbei-s with intensity of 

 cattle use on grasslands of the Manitou area at 

 8,000 feet. There were, however, twice as many 

 Peromyscus on grazed areas as within adjacent 

 ungrazed exclosures. The three intensities of 

 grazing use produced no detectable difference in 

 pocket goplier numbers on either tlie Black Mesa 

 or Manitou areas; however, there were three times 

 as many gophers per acre within vmgrazed ex- 

 closures as found on grazed ranges at Manitou. 



Meadow voles were virtually absent on grazed 

 Manitou ranges, but were es{)ecially abundant 

 within the exclosures. On Black Mesa, moi.st areas 

 with heavy cover periodically held substantial 

 vole numbers despite moderate grazing. 



Lagomorphs were studied only on the Badger 

 Wash area. Here, pellet counts, strip flushing 

 counts, and drives indicated greater numbers of 

 black-tailed jacks and cottontails on the ungrazed 

 range. 



Response of cotton rat to population density. — 

 A fact coming increasingly to attention as a result 

 of present-day study of natural regidation of wild- 

 life populations is that disease and predation are 

 of less importance as population regulators than 

 they were long thought to be. Instead, the state 

 of balance within the endocrine system, combined 

 with social behavior, has come to l>e regarded as 

 imiwrtant in natural regulation, usually even more 

 important than the food supply. 



Study of these phenomena by the Olchihoma 

 I 'nit has involved two discrete wild populations 

 of cotton rats, one dense and the other relatively 

 dilute. 



Kesults siiowed tliiil if the suiimier |)()|)iilal ion 

 was suiliciently low. tlie cotton rat, luoiv or less 

 a seasonal breeder, extended its reproductive 

 season into the late fall or winter. Or, if the 



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