Rosa Rubicondior: Unintelligent Design - The Extreme Lengths To Which a Shrew Goes To Survive

Wednesday, 16 April 2025

Unintelligent Design - The Extreme Lengths To Which a Shrew Goes To Survive

Masked Shrew, Sorex cinereus

Photo © P. Myers,
Mammal Images Library of the American Society of Mammalogists.

New UNCG Research Shows Southern Shrews Shrink in Winter | College of Arts & Sciences

The already peculiar beliefs that creationists must entertain about their supposed creator deity have become even stranger with the discovery that North America's masked shrew actually shrinks as part of its winter survival strategy.

Imagine you're designing a small mammal intended to survive in an environment where food becomes scarce, and temperatures fall too low for typical warm-blooded activity during winter. You've already solved this issue effectively with hibernation — a successful strategy employed by numerous animals, even large ones like bears. Yet, apparently deeming hibernation alone insufficiently complex for the masked shrew, your design involves the creature further reducing its energy needs by physically shrinking, despite the fact that its small size was already a significant part of the survival challenge.

Nevertheless, this is precisely the type of scenario a creationist must accept if they reject evolution as an explanation. Evolution, as a blind, pragmatic process working without foresight, readily generates such unlikely and counterintuitive adaptations through trial and error.

Yet prominent creationist advocates continue to assert that their supposed intelligent designer deliberately creates these seemingly nonsensical adaptations—ironically claiming these designs are beyond the capability of natural evolutionary processes.

This example of Dehnel's phenomenon is the subject of a recent research paper in The American Naturalist by researchers based at the University of North Caroline Greensboro (UNC Greensboro). It is described in a UNC Greensboro press release:

What exactly is Dehnel’s phenomenon~ Are there any other examples of it in addition to the masked shrew? Dehnel’s phenomenon is a biological adaptation characterised by a cyclic seasonal reduction and subsequent regrowth of body mass and specific organs in certain small mammals, particularly observed in shrews and weasels. Named after Polish zoologist August Dehnel, who first described the phenomenon in 1949, it is primarily considered an energy-saving strategy in response to harsh winter conditions when food resources are limited.

Typical characteristics of Dehnel’s phenomenon include:

Significant seasonal shrinkage of overall body size (body mass, bone density, and even brain size).
Reduction in internal organs, particularly the brain, liver, kidneys, and spleen.
Reversal of this shrinkage during the warmer months, regaining previous body and organ size.

Examples Beyond the Masked Shrew (Sorex cinereus):
While the masked shrew is a classic example, Dehnel’s phenomenon has been documented in several other species:

Common Shrew (Sorex araneus)

Frequently studied example across Europe.
Shows notable seasonal changes in skull size and brain mass, shrinking up to 15-20% in winter and regrowing in spring.

Eurasian Least Shrew (Sorex minutus)

Exhibits similar seasonal shrinkage and regrowth patterns in brain size and overall body mass.

American Least Shrew (Cryptotis parva)

Also demonstrates a seasonal reduction in body mass, organ size, and energy requirements.

Stoats and Weasels (Mustela spp.)

Some mustelids, notably the stoat (Mustela erminea), display seasonal skeletal and organ size fluctuations akin to Dehnel’s phenomenon.
Stoats can undergo significant seasonal reductions in skull dimensions, brain size, and other internal organs, reversing these changes in warmer months.

Adaptive Significance:

The primary adaptive advantage of Dehnel’s phenomenon is reducing the metabolic rate and energy demands during winter, enhancing survival when food is scarce. This physiological flexibility allows these mammals to conserve energy in extreme climates, especially crucial given their typically small size and high metabolic rates.

Overall, Dehnel’s phenomenon is a fascinating example of physiological plasticity, demonstrating evolution's ability to drive complex, reversible adaptations in response to environmental challenges.

New UNCG Research Shows Southern Shrews Shrink in Winter
Newly published research from UNC Greensboro biology professor Dr. Bryan McLean and colleagues shows that the masked shrew, a small, mole-like mammal found in the Appalachian Mountains, shrinks its body and braincase to conserve energy during winter months.
The study, published in the May 2025 issue of The American Naturalist, found that the masked shrew (Sorex cinereus) reduces its body mass by 13 percent in the colder months; the creature then grows larger in spring when conditions improve. In addition to a shrinking body, the team also found seasonal changes in the height of the creature’s braincase (the portion of the skull that houses and protects the brain) and the length of the femur.
Shrinking the body and its parts is in fact a clever survival strategy, and it’s one that’s important for us to understand as mammals face a constantly changing planet.

Dr Bryan McLean, first author
Department of Biology
University of North Carolina at Greensboro, NC, USA.
Known as Dehnel’s phenomenon, this seasonal shrinking has been observed in other mammals but most often in shrews, which are small, insect-eating animals unrelated to rodents. Dehnel’s phenomenon is an extreme example of “phenotypic plasticity” – the ability of an organism to alter its physical form in response to environmental changes.

We don’t know how common Dehnel’s phenomenon is among mammals, but we know it is rarer in nature than other energy-saving strategies mammals use, like hibernation.

Dr Bryan McLean.
McLean and his team of graduate and undergraduate students analyzed 125 masked shews that were trapped in North Carolina’s Pisgah National Forest from 2021 to 2023. The team used “pitfall traps” buried in leaf litter to capture the shrews. The animals were weighed in the field, then brought to UNCG’s Joint School of Nanoscience and Nanotechnology for microCT scans to examine various skeletal dimensions. Specimens and associated data are archived in the UNCG Mammal Collection. Researchers from Georgia Southern University were also involved in the research.

“Our population of masked shrews is the southern-most yet studied for these multiple different traits,” said McLean, “and the femur measurements we made are the first to show the magnitude of seasonal change in the long bones of the skeleton. This shows that shrews rapidly remodel much of their skeleton.” Most prior studies of the phenomenon come from Europe. To place their new results in context, the researchers also conducted a meta-analysis of 74 other studies from across the Northern Hemisphere, combining that research with their own findings to understand what factors drive Dehnel’s phenomenon. McLean and his team developed statistical models that accurately predicted the amount of body shrinkage they observed based solely on the climate at the North Carolina site.
This analysis reveals the generality of Dehnel’s phenomenon in Sorex shrews. Across many populations of shrews on three continents, the degree of body mass and braincase height shrinkage is greatest in areas with the lowest cool-season temperatures. So, fall and winter temperatures predict Dehnel’s phenomenon in these animals. Phenotypic plasticity is a key way that shrews and many other species respond to changes in temperature. By learning more about this process, we can start to understand how mammals buffer against rapidly changing climates.

Dr Bryan McLean.

Abstract
Dehnel’s phenomenon describes a seasonal and reversible winter decrease in body size, which is a trait that predicts total energy demand. However, the phenomenon remains less well studied than common energy-saving or energy-seeking strategies of mammals. Here, we explore the generality of Dehnel’s phenomenon in Sorex shrews on three continents. First, we use new field sampling to document seasonal phenotypic change in masked shrews (Sorex cinereus) in North America at the lowest latitude yet investigated for this species (35.7°). This includes the first documentation of appendicular skeleton remodification in Sorex. Summer-to-winter decreases in S. cinereus body mass, braincase height, and femur length were 13%, 11.5%, and 8.7%, respectively, with subsequent increases of each in second-year individuals. Second, we compile a comprehensive dataset of studies relevant to Dehnel’s phenomenon to test whether seasonal plasticity in Sorex globally is related to climate, demonstrating that body and braincase plasticity are functions of cold season temperatures. Meta-analytical models for both of these traits generalized by (a) applying at both inter- and intraspecific scales and (b) predicting the seasonal change newly observed for S. cinereus. Our results support body size plasticity as an environmentally responsive innovation in these very small homeothermic mammals.

Introduction
Understanding how life histories evolve and respond to environmental change relies on trait proxies reflective of how organisms apportion resources to growth, maintenance, and reproduction (Bronson and Perrigo 1987; Sibly and Brown 2007; McNab 2008; Bronson 2009). Body size is a key trait in this respect; it predicts a variety of energetically associated attributes, including metabolic rate and, consequently, total energy demand (Sibly and Brown 2007; McNab 2012). Therefore, the linear proportions of the body and its parts (rather than total mass or volume)—that is, structural body size—can be useful for measuring body size dynamics on microevolutionary timescales and in response to temperature or resource availability, including across gradients of temperature seasonality (Meiri and Dayan 2003); upon colonization of islands (Lomolino 2005); and in response to anthropogenic pressures, such as urbanization (Hantak et al. 2021). Less well documented is how structural body size might vary within individuals and populations, where it could afford an ability to fine-tune energy needs and buffer against rapid climate and environmental changes.

Dehnel’s phenomenon describes a plastic (i.e., environmentally mediated, nonheritable) reduction in structural body size in winter—a period of elevated thermoregulatory demand and potential scarcity in invertebrate food resources—followed by regrowth in spring. The phenomenon is rare compared with other energy-saving (e.g., torpor, hibernation) or energy-seeking (e.g., migration, dietary or behavioral shifts) strategies in mammals. It has primarily been documented in very small-bodied (<45 g) mammals possessing continuous homeothermy (McNab 2012) and can involve a reduction in overall body size (Dehnel 1949; Taylor et al. 2013; Lázaro et al. 2021.1; supplemental PDF), craniodental proportions (including braincase size; Pucek 1963; Taylor et al. 2013; Lázaro et al. 2021.1; supplemental PDF), and volumes of some organs, including brain and liver (Pucek 1965; Lázaro et al. 2021.1). Despite the resulting smaller body size and thus higher heat loss potential (given a higher surface-to-volume ratio), mass-specific metabolic rates remain comparable to those of larger summer individuals (Taylor et al. 2013; Schaeffer et al. 2020), thus breaking the expected inverse correlation between smaller body size and higher mass-specific metabolic rate (McNab 2008, 2012). The details of how this is accomplished are not fully known, although mitigating heat loss through increased fur density and disproportionately greater shrinking of the most energetically expensive organs may contribute (but for brain data, see Schaeffer et al. 2020). Dehnel’s phenomenon was first described from the common shrew (Sorex araneus) in Poland (Dehnel 1949), but subsequent research suggests that it is a strategy used by numerous other shrews (Pucek 1963; Lázaro et al. 2021.1) and select insectivores (Nováková et al. 2022), small carnivores (LaPoint et al. 2017), and rodents (McNab 2012; Mulvey et al. 2024) to reduce total energy demand and improve survival during energetically challenging periods.

Shrews in the genus Sorex (Eulipotyphla: Soricidae) are small (2–15 g), insectivorous, and obligatorily homeothermic mammals with intense energy budgets (McNab 2012). Seventy-five years after Dehnel’s (1949) original observations, Sorex shrews remain a model system for phenotypic plasticity research given their small size and high surface area/volume ratios, elevated metabolic rates, and occupancy of high-latitude and highly seasonal environments in North America, Europe, and Asia. Paradoxically, though, knowledge of Dehnel’s phenomenon in Sorex (which contains 89 species) is extremely geographically and taxonomically biased. Of studies we review here, 58% (43 of 74) focus on a single Eurasian species (S. araneus), and 25% of those (11 of 43) focus on this same species in a single country (dataset S2). These biases severely limit the ability to understand how a diversity of small mammals buffer against stochastic environments and, relatedly, to incorporate phenotypic plasticity into predictive models of climate change response.

In this study, we combine new empirical and meta-analytical datasets to explore the generality of Dehnel’s phenomenon in Sorex and its drivers at a global scale. First, we quantify the existence and magnitude of the phenomenon in a novel population of S. cinereus (masked shrew) in North Carolina, at the lowest latitude yet investigated for this species (35.7°). This is also the lowest latitude at which the proxies of body mass and braincase proportions (which are the most commonly measured) have been investigated in tandem, on any continent. In addition, we present measurements of the femur that provide the first insight into the magnitude of seasonal skeletal change in long bones of the appendicular skeleton. Second, we conduct a comprehensive meta-analysis of studies from across the Northern Hemisphere to identify potential climate drivers of Sorex plasticity, which have remained poorly understood to date. We gauge the generalizability of these models by asking whether predictions accurately extend to new species and sites (here, S. cinereus) as well as to other levels of biological organization (within species), providing new context for Dehnel’s phenomenon as both an exception to metabolic theory and a general adaptive strategy in shrews.

McLean, Bryan S.; Stierman, Kristin E.; Ivey, Leo R.; Weller, Amanda K.; Chapman, Olivia S.; Miller, Ava C.; Byrd, Jada S.; Garcia, Abigail Mendoza; Greiman, Stephen E.
Seasonal Body Size Plasticity and the Generality of Dehnel’s Phenomenon in Sorex Shrews.
The American Naturalist (2025); DOI: 10.1086/735018

Copyright: © 2025 The authors.
Published by The University of Chicago Journals. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

It defies credibility that an intelligent designer would intentionally create small mammals — or any organism — to inhabit environments so challenging they require extraordinary measures merely to survive. Even more implausible is the idea that such a designer would equip these creatures with the capacity to shrink and later regrow their bodies as part of these measures.

Yet perhaps most astonishing of all is that rational adults could regard this as evidence of supreme intelligence at work, rejecting the far more plausible explanation offered by an unguided, pragmatic evolutionary process, and instead preferring the absurd notion of what can only be regarded as an incompetent designer.

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