The Scientist magazine investigates what’s going on inside the skulls of the tiniest terrestrial mammals. Etruscan shrews, it turns out, cope with winter’s demands by getting rid of some of their brain mass, which grows back for summertime:
To have enough energy to survive, it must eat eight or more times its body weight daily and therefore doesn’t hibernate. Instead, according to a study published November 30 in PNAS, in winter, these shrews lose 28 percent of the volume from their somatosensory cortex, which likely helps them conserve energy.
Scientists have shown before that red-toothed shrews, which belong to a group separate from the Etruscan shrew, are born and grow to their full body size in a single summer. Then in autumn, they start to shrink all over—in their spine length, skull, brain, bones, organs such as the liver, and body weight—reaching their smallest size in the winter. Somewhere around February, they start to grow again and reach a second size peak as they sexually mature in the spring. Then they reproduce just once, and, shortly after, die. This cycle is known as Dehnel’s phenomenon.
Starting in summer, the researchers repeatedly conducted MRI scans of the brains of 10 shrews each season for a year. They found that brain volume decreased in the winter, despite keeping the animals under a constant 12-hour light-dark cycle, at a consistent temperature, and with unlimited access to food. When they limited food in the summer in different shrews, they saw a decrease in brain thickness. This indicated that the phenomenon is subject to both internal cues related to their age or to the passing of time and external influences, such as the availability of food.
Using another group of animals, Ray’s team pinpointed the shrinkage in the brain to the somatosensory cortex, the area that receives sensory input from the animals’ whiskers, which they use in hunting.
You can read Ray’s research here, at PNAS.