The smallest terrestrial mammal, the Etruscan shrew (Suncus etruscus), is about as big as a person’s thumb and no heavier than a couple of paper clips. 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.
“This phenomenon of an animal that is not a hibernator still implementing these energy saving strategies is just stunning,” says Christine Schwartz, a neuroscientist who studies hibernation at the University of Wisconsin La Crosse and was not involved in the work.
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.
Adult neurogenesis is something that’s very rare and happens in very low numbers. And if someone shows that there are massive changes in neurons, increases in neuron numbers in the cortex in a mammal, that’s . . . another stellar finding.—Dina Dechmann, Max Planck Institute of Animal Behavior
When Saikat Ray was a graduate student in Michael Brecht's lab at the Bernstein Center for Computational Neuroscience in Berlin, he was curious to see if Dehnel’s phenomenon also exists in white-toothed shrews, the subfamily that includes the Etruscan shrew. They already had a colony of Etruscan shrews in the lab, says Ray, who is now a postdoc in Nachum Ulanovsky’s lab at the Weizmann Institute in Israel, because the animals’ tiny brains are a helpful model system for studying more of the brain at once than are the brains of larger mammals, such as mice or rats.
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. One layer of the somatosensory cortex decreased in width by 28 percent in winter and increased by 29 percent the following summer. Neuron numbers also increased by 42 percent in this level of the cortex from winter to summer.
The most surprising part of this finding, according to Ray, is that “the brain shrinkage effect [is] happening in the lab, but it’s also very specific in that it seems to happen only in particular regions and is not a general effect everywhere.”
These physical changes also translated to changes in brain function. In the somatosensory cortex, the researchers classified three groups of neurons: activated, suppressed, and unaffected by whisker touch. Neurons that were suppressed had a more than two-fold higher activity in spring and summer than in fall and winter. This change in activity could help the animals conserve energy, the authors hypothesize in the paper.
The researchers “put them in an MRI and . . . thus were able to show a change in the volume of [one layer of] this brain region, the neocortex, which is super important across an animal’s lifetime. It’s beautiful. And they show not only that the cortex changes in size and thickness, but also which parts of the cortex are affected and at which stage in the animal’s life,” says Dina Dechmann, a behavioral ecologist at the Max Planck Institute of Animal Behavior in Germany who did not participate in the work.
The authors didn’t find changes in other parts of the brain or a winter decrease followed by a summer increase in the animals’ overall mass, “so it’s not affecting other things that usually are affected by Dehnel’s phenomenon,” Dechmann tells The Scientist.
Another area for future research, according to Ray, is how the recovery in neuron numbers happens in the spring and summer. “It seems like neurons are coming back, but exactly how that’s happening . . . is pretty open right now,” he says. “It’s not clear if they’re just new neurons being born or if there are other kinds of cells, which are getting converted into neurons.”
“Adult neurogenesis is something that’s very rare and happens in very low numbers,” says Dechmann. “And this is—in mammals, as far as we know—restricted to the hippocampus [and the olfactory bulb]. And if someone shows that there are massive changes in neurons, increases in neuron numbers in the cortex in a mammal, that’s . . . another stellar finding. Learning how animals make new cells in the brain has so many potential” applications.
“We see brain shrinkage. We see loss of cells, and those are often linked to really detrimental events,” agrees Schwartz. “But the fact that these animals go through that, but then they just bounce back and they regain that loss is something that makes this so interesting and so important to study.”
S. Ray et al., “Seasonal plasticity in the adult somatosensory cortex,” PNAS, doi:10.1073/pnas.1922888117, 2020.
Correction (December 4): The story has been updated to clarify the lab in which the work began. The Scientist regrets the error.