The dual Bonahon-Schläfli formula

Given a differentiable deformation of geometrically finite hyperbolic 3-manifolds (M_t)_t, the Bonahon-Schläfli formula expresses the derivative of the volume of the convex cores (CM_t)_t in terms of the variation of the geometry of its boundary, as the classical Schläfli formula does for the volume of hyperbolic polyhedra. Here we study the analogous problem for the dual volume, a notion that arises from the polarity relation between the hyperbolic space H^3 and the de Sitter space dS^3. The corresponding dual Bonahon-Schläfli formula has been originally deduced from Bonahon's work by Krasnov and Schlenker. Here, making use of the differential Schläfli formula and the properties of the dual volume, we give a (almost) self-contained proof of the dual Bonahon-Schläfli formula, without making use of Bonahon's original result.