  We report on rate-dependent fracture energy measurements over three decades
of steady crack velocities in alginate and gelatin hydrogels. We evidence that,
irrespective of gel thermo-reversibility, thermally activated "unzipping" of
the non-covalent cross-link zones results in slow crack propagation, prevaling
against the toughening effect of viscous solvent drag during chain pull-out,
which becomes efficient above a few mm.s$^{-1}$. We extend a previous model
[Baumberger {\it et al.} Nature Materials, {\bf 5}, 552 (2006)] to account for
both mechanisms, and estimate the microscopic unzipping rates.
