Dehnel's phenomenon is a seasonal reduction in body size that goes well beyond fat loss. Braincase height falls by roughly a fifth, brain mass by as much as thirty per cent, and organs including the liver lose mass through autumn and early winter, with total body mass down by about eighteen per cent. In spring the same individuals regrow, recovering much of what was lost.
The function appears to be energy economy. A shrew cannot hibernate, cannot store much fat, and must keep hunting through winter. Brain tissue is metabolically expensive, so shedding some of it lowers the daily cost of staying alive when prey is scarce. The change has been documented in individuals tracked by capture and recapture, not only by comparing different animals across seasons.
The brain shrinkage is often reported without the cognitive question attached. Whether the change carries a measurable cost in spatial memory or behaviour is not settled, and results so far are mixed. The mechanism controlling the regrowth is also not well understood, which is part of why the phenomenon attracts research on neural plasticity.