By shrinking their brains in winter, common shrews save energy during a period of food scarcity. Photo: ImageSelect
Shrews save energy in winter by allowing their brain cells to ‘dry out’.
He brain of the common shrew (Sorex araneus) shrinks by nearly 10 per cent in winter because brain cells lose water, not because they die. This is the finding of new research using advanced MRI techniques (Current Biology, 1 September). It is one of the few examples of a mammal that can shrink its brain so drastically – and so completely reversibly.
Water distribution
For their study, the researchers caught wild common shrews in southern Germany, both in summer and in winter. The animals were examined using diffusion microstructure imaging, an MRI technique that tracks water molecules in brain tissue and thereby reveals the distribution of water inside and outside cells. In addition, the biologists counted neurons and glial cells in various brain regions.
The analyses showed that intracellular water decreases markedly in winter, whilst more fluid accumulates outside the cells. The cells shrink but remain intact. The number of neurons and glial cells stays the same, whilst in the olfactory bulb their numbers actually increase in winter.
The shrinkage process
The discovery makes clear that the shrinkage process – known as Dehnel’s phenomenon – is an active adaptation to winter. Common shrews have an extremely high metabolic rate and can barely store fat. Because they do not hibernate or enter torpor, they continue to forage actively throughout winter. By shrinking their brains, they save energy during a period of food scarcity. Not all brain regions responded in the same way, however.
The pons, medulla and hippocampus showed the greatest volume reduction, whilst the midbrain actually became slightly larger. The neocortex and cerebellum also shrank, but differed from other brain regions in their water distribution.
These regions may partly stabilise their structure in order to preserve functions such as perception and motor control. The researchers also found evidence that the water-channel protein aquaporin-4 is less expressed in the hippocampus and cortex during winter. This could facilitate the shift of water out of the cells. Apoptosis genes were also found to be suppressed, allowing cells to be preserved.
Brain atrophy
The study draws a comparison between this process and brain atrophy in humans, where disruptions in water balance and aquaporin-4 lead to irreversible loss of function. In common shrews, by contrast, it is a controlled and reversible adaptation, which the researchers suggest could provide insight into the repair mechanisms that fail in neurodegenerative diseases.
Whereas brain shrinkage in humans is disastrous, in common shrews it proves to be an ingenious strategy for surviving winter. In spring, the brain and skull grow back, allowing the animals to enter the summer season with a fully restored brain.