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

Scott Lapoint,Dina K N Dechmann,Martin Wikelski,Lara Keicher,Karol Zub

doi:10.1098/rsos.160947

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.

Highlights

Seasonal fluctuations in resources are a common but, from an evolutionary viewpoint, difficult problem for animals to solve
We present evidence of two rare phenomena: an absolute growth overshoot of juvenile braincase depth (BD) and a seasonally driven BD change in the adult skull
El Niño events triggered body length decreases in adult marine iguanas (Amblyrhyncus cristatus) that later re-grew, but are an example of a reactive bidirectional size change [67]

Summary

Introduction

Seasonal fluctuations in resources are a common but, from an evolutionary viewpoint, difficult problem for animals to solve. Species respond to such extrinsic challenges with behavioural adaptations, such as food caching [1], or physiologically, for example by suppressing bone resorption during hibernation [2]. Biologists have long been intrigued by body size (e.g. skull length), quantifying its variation and identifying its drivers, yielding descriptions of ecological patterns and ‘rules’ (e.g. Bergmann’s or Allen’s rules) These patterns probably arise in response to several variables combined (i.e. resource availability), not solitary factors (e.g. latitude or precipitation) (sensu [9])

Methods

Results

Conclusion