Lemniscomys barbarus or L. zebra. Most zoos, incl. Hellabrunn, label theirs as L. barbarus, but I suspect this is based on "what everybody does" rather than real knowledge. The two are very similar and there has been quite some taxonomic confusion between them (to further complicate matters, it appears L. zebra as currently defined is a species complex). The faint stripes along the sides exclude Rhabdomys.
