A Field Experiment On Minimum Territory Size in the Common Shrew Sorex Araneus1

A FIELD EXPERIMENT ON MINIMUM TERRITORY SIZE IN THE COMMON SHREW SOREX ARANEUS1 by NORA CROIN MIGHIELSEN2 (Department of Population Biology, University of Leiden, P.O. Box 9516, 2300 RA Leiden, The Netherlands) ABSTRACT The existence of a minimum territory size in Sorex araneus was tentatively tested in some small-scale field experiments. Very young immatures from two successive generations were introduced to two small, shrew-cleared islands in a dune lake. Shrews were introduced in surplus as well as in small numbers and synchronously as well as in successive cohorts. In both years similar but limited numbers of the animals introduced initially estab- lished territories; other individuals disappeared altogether within 24 hours when the area seemed completely occupied, but did not do so when some space was still left. Minimum territory size in this humid dune biotope was estimated at about 90 m2. KEY WORDS: Sorex araneus, minimum territory size. INTRODUCTION The hypothesis that a territorial animal needs a minimum area just sufficient to satisfy its own requirements is supported by a number of studies (see DAViES, 1978). This implies a minimum value for territory size which, proximately or ultimately, depends on the resources available. The lower limit in the compressibility of territory will result in an upper limit to local carrying capacity and hence population den- sity. Evidence of constancy in the initial population density of each new generation as well as a minimum territory size were found in the Common Shrew Sorex araneus during a descriptive field study in Dutch coastal dunes (CROIN MICHIELSEN, 1966). Common Shrews are iteroparous 'annuals' having only one reproductive season in the spring and summer following the calendar year of their birth. The first-litter and usually the second-litter young emerge from the nest synchronously, in early June and June/July, re- spectively. Newly weaned shrews, being socially dominant over the parent generation (MORALEVA, 1989), almost immediately try to settle down solitarily in a restricted area that is kept as a territory during the entire period of sexual immaturity. This strict territoriality breaks