We analyse the dynamics of near-extremal Reissner-Nordstr\"om black holes in
asymptotically four-dimensional Anti-de Sitter space (AdS4). We work in the
spherically symmetric approximation and study the thermodynamics and the
response to a probe scalar field. We find that the behaviour of the system, at
low energies and to leading order in our approximations, is well described by
the Jackiw-Teitelboim (JT) model of gravity. In fact, this behaviour can be
understood from symmetry considerations and arises due to the breaking of time
reparametrisation invariance. The JT model has been analysed in considerable
detail recently and related to the behaviour of the SYK model. Our results
indicate that features in these models which arise from symmetry considerations
alone are more general and present quite universally in near-extremal black
holes.Comment: 44 (=26+18) pages, 1 figure, 6 appendices; v2: references added; v3:
minor changes made; v4: additional references added, version accepted in JHE
