 
Summary: The burrow system of the fossorial form of the water vole
(Arvicola terrestris scherman Shaw.) (Mammalia, Rodentia) :
an approach using graph theoretical methods and simulation models
by J.P. AIROLDI ' et D. DE WERRA2
'Centre deformation du Brevet dfEnseignementSecondairelBiologie,
Universite' de Berne, Gertrud Wokerstrasse 5, CH  3012 Berne, Suisse.
Dkpartement de mathirnatiques, Ecole polytechnique fe'de'rale,
MA  Ecublens, CH  1015 Lausanne, Suisse.
Summary.  41 burrow systems of the fossorial form of the water vole (Arvicola terrestris
L.), a small rodent inhabiting meadows, pastures and orchards in central Europe, have been ana
lysed using methods of graph theory. Four main types exist : a) linear structures (trees), b) trees
with a few cycles, c) mixture of linear and cyclic structures, d) essentially cyclic structures. The
intersections with 3 branches clearly predominate (97%). The lengths of the edges (segments joi
ning a deadend and an intersection or two intersections) follow an exponential distribution with
a mean and a standard deviation being usually very close. The nest is generally closer to a cen
tral point than would be expected by chance. The connections between any two intersections or
an intersection and a deadend can be summarised in the form of an adjacency matrix ; a method
of reducing its size by eliminating the trees connected to the cyclic part was used when compu
ting the shortest paths, in order to save computing time and space.
Two simulation models are briefly described. The first one is rather simple and produces
