Growth overshoot and seasonal size changes in the skulls of two weasel species

Abstract

Ontogenetic changes in mammalian skulls are complex. For a very few species (i.e. some Sorex shrews), these also include seasonally driven, bidirectional size changes within individuals, presumably to reduce energy requirements during low resource availabilities. These patterns are poorly understood, but are likely most pronounced in high-metabolic species with limited means for energy conservation. We used generalized additive models to quantify the effect of location, Julian day, age and sex on the length and depth of 512 and 847 skulls of stoat (Mustela erminea) and weasel (M. nivalis) specimens collected throughout the northern hemisphere. Skull length of both species varies between sexes and geographically, with stoat skull length positively correlated with latitude. Both species demonstrate seasonal and ontogenetic patterns, including a rare, absolute growth overshoot in juvenile braincase depth. Standardized braincase depths of both species peak in their first summer, then decrease in their first winter, followed by a remarkable regrowth that peaks again during their second summer. This seasonal pattern varies in magnitude and timing between geographical regions and the sexes, matching predictions of Dehnel's phenomenon. This suggests implications for the evolution of over-wintering strategies in mammals, justifying further research on their mechanisms and value, with implications for applied osteology research.

Stable carbon isotopes in breath reveal fast incorporation rates and seasonally variable but rapid fat turnover in the common shrew (Sorex araneus)

“…Recently, Dehnel's phenomenon has also been described in weasels, which share several important life history traits with red-toothed shrews (Dechmann et al, 2017). This supports the general hypothesis that energy savings and/or a reduction in resource requirements are driving forces behind the evolution of this phenomenon, but the consequences for the animals are still poorly understood (Dechmann et al, 2017;LaPoint et al, 2017). In summary, Dehnel's phenomenon is thought to be an energy-saving strategy, allowing the shrews to reduce absolute food requirements when the prey abundance is lower in the cold season (Churchfield, 2002;Gliwicz and Taylor, 2002;Hyvarinen, 1969;McNab, 1991;Pucek, 1970;Taylor et al, 2013) and/or energetic costs through the reduction of high energy-consuming tissues, including the brain.…”

Virtual endocasts of fossil Sciuroidea: brain size reduction in the evolution of fossoriality

“…; Lapoint et al . ). These results show that, when working with variables such as brain and body masses that can change with the seasons, their associated limitations should be taken into consideration.…”

Geographic patterns in seasonal changes of body mass, skull, and brain size of common shrews

“…Also, the length of the spine decreases and regrows seasonally as a result of shrinkage of the intervertebral disks (Hyvärinen, 1969;Saure & Hyvärinen, 1965). Some other species of shrews, and, as has recently been found, some mustelids, also show seasonal reversible shrinkage and regrowth at least of their skulls and brains (Dechmann et al, 2017;LaPoint et al, 2017).…”

“…However, Dehnel's Phenomenon can differ greatly between populations. Braincase changes associated with Dehnel's Phenomenon in weasels ( Mustela erminea and Mustela nivalis ) vary greatly in intensity and timing between populations at different geographic locations (LaPoint et al., 2017). Previous studies on common shrews suggested a greater winter decrease in skull and body size in northeastern Europe compared to southwestern populations (Pucek, 1970; Spitzenberger, 2001).…”