  Structured equations are a standard modeling tool in mathematical biology.
They areintegro-differential equations where the unknown depends on one or
several variables, representing the state or phenotype of individuals. A large
literature has been devoted to many aspects of these equations and in
particular to the study of measure solutions.Here we introduce a transport
distance closely related to the Monge-Kantorovich distance,which appears to be
non-expanding for several (mainly linear) examples of structured equations.
