The challenge of being small - alternative wintering strategies in high-metabolic non-hibernating terrestrial mammals

Red-toothed shrews (Sorex spp.) are excellent model organisms to study the evolution of brain size under energetic limitations. Several Sorex spp seasonally shrink and then regrow the brain, parts of the skeleton, and brain case. While these seasonal size differences had been observed in museum records, we have documented this for the first time in repeatedly measured individuals and described the correlated changes in the size and cell architecture of the various affected brain regions. We also study the associated physiology of these shrews as they go through the stages of this phenomenon under drastically changing environmental conditions as well as the cognitive compromise this size change entails.

The smallest Mustelids (e.g.weasels, Mustela nivalis) share several similarities with shrews, including an elongated body shape, predatory behavior, and a high metabolic rate. We showed that M. nivalis and more recently the much larger and long-lived American mink (Neovison vison), and probably the European polecat (M. putorius) also exhibit Dehnel’s Phenomenon. This widened taxonomic scope is, of course, extremely relevant for the understanding of the evolution of this phenomenon and increases the promise of potential applied research using animals that exhibit Dehnel's Phenomenon. A suite of braincase size metrics on museum specimens allowed us to quantify the seasonal variation in the skulls of these mustelids after controlling for other variables such as latitude, elevation, sex, and age, which mask the pattern of the phenomenon. We are excited to have discovered seasonal skull and brain size changes in taxonomically distant species from the shrews. Here, we are collaborating with Karol Zub and Andrzej Zalewski as well as former postdoc Scott LaPoint, to address questions that may not be possible to investigate in the tiny and stress-sensitive shrews.

Currently, we are working on the underlying mechanisms and consequences of the phenomenon in a Human Frontiers funded project. We are repeatedly scanning the brains of shrews using MRI and correlating individual changes in size and structure with changes in cognitive ability. This is the subject of Cecilia Baldoni's PhD and done in collaboration with Dominik von Elverfeldt. We are also studying changes in gene expression in a suite of genes that might be associated with the change in individuals from the same population with William Thomas and Liliana Davalos. Finally, we are taking a special look at changes in the fat metabolism and how this might ultimately even be applied to human relevant research on neurodegenerative diseases with John Nieland and his team.