FOOD AND SPACE - RESOURCES DEFENDED BY TERRITORIAL PAROUS FEMALE COLUMBIAN GROUND-SQUIRRELS

The importance of food and space, as resources defended by parous fema le Columbian ground squirrels (Spermophilus columbianus), was studied by manipulating one of these resources, that is increasing the quality of forage (through urine fertilization) on small plots within territo ries. Use of these fertilized plots by ground squirrels increased more than 100-fold when averaged over 2 years after manipulation. This inc rease, however, was not equal for each sex and age class: parous femal es used the fertilized plots relatively more, and nonparous females le ss, than either yearling or adult males. The number of parous females with territories overlapping the experimental plots also increased aft er fertilization, but the size of their territories declined only slig htly, by less than 10%. Parous females with access to the fertilized p lots, relative to those without such access, had greater body mass and larger litters that both weighed more at birth and gained body mass s ubsequently more rapidly. Parous females on territories with fertilize d plots showed higher levels of agonism than those an territories lack ing such plots. Most of the agonism was centered on the experimental p lots and more of it was directed at young of other females than at the ir own young. Such differential treatment of kin, however, did not ext end to their offspring of the previous year. We suggest that for parou s females of this ground squirrel, both food and space (at least that normally needed to supply sufficient forage) are important resources t o defend, and both may have played a significant role in the evolution of territoriality in females of this species.