  The first part of the paper gives a new proof of self-duality for Selmer
groups: if A is an abelian variety over a number field K, and F/K is a Galois
extension with Galois group G, then the Q_pG-representation naturally
associated to the p-infinity Selmer group of A/F is self-dual. The second part
describes a method for obtaining information about parities of Selmer ranks
from the local Tamagawa numbers of A in intermediate extensions of F/K.
