Breeding on the leading edge of a northward range expansion: differences in morphology and the stress response in the arctic Gambel's white-crowned sparrow.

Jesse S Krause,Ashley Asmus,Simone L Meddle,John C Wingfield,Jonathan H Pérez,Laura Gough,Lisa N Quach,Natalie Boelman,Karen R Word,Shannan K Sweet,Michaela A Mcguigan,Helen E Chmura

doi:10.1007/s00442-015-3447-7

Abstract

Individuals at the forefront of a range shift are likely to exhibit phenotypic traits that distinguish them from the population breeding within the historic range. Recent studies have examined morphological, physiological and behavioral phenotypes of individuals at the edge of their range. Several studies have found differences in the hypothalamic–pituitary–adrenal (HPA) axis activity in response to acute restraint stress in individuals at the range limits. HPA axis activation leads to elevations in glucocorticoids that regulate physiology and behavior. Here we compare the hormonal profiles and morphometrics from Gambel’s white-crowned sparrows (Zonotrichia leucophrys gambelii) breeding at the northern limit of the population’s range to those birds breeding within the historic population range. Birds breeding at the northern limit experienced a harsher environment with colder temperatures; however, we found no differences in arthropod prey biomass between the northern limit and more southern (historic) sites. Males at the northern limit had higher body condition scores (mass corrected for body size) compared to individuals within the historic range, but no differences were found in beak and tarsus lengths, wing chord, muscle profile or fat stores. In males during the pre-parental stage, before breeding commenced, HPA axis activity was elevated in birds at the northern limit of the range, but no differences were found during the parental or molt stages. Females showed no differences in HPA axis activity during the parental stage. This study suggests that “pioneering” individuals at the limits of their breeding range exhibit physiology and morphology that are distinct from individuals within the historic range.Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-015-3447-7) contains supplementary material, which is available to authorized users.

Highlights

The geographical range of a species is determined by the ecological niche where evolution has selected the traits that maximize fitness (MacArthur 1972)
This study suggests that “pioneering” individuals at the limits of their breeding range exhibit physiology and morphology that are distinct from individuals within the historic range
Temperatures were colder at range limit than historic range during the pre-parental (t = 5.98, P < 0.001) and parental (t = 3.72, P < 0.001) stages but not during molt (t = 1.90, P < 0.06)

Summary

Introduction

The geographical range of a species is determined by the ecological niche where evolution has selected the traits that maximize fitness (MacArthur 1972). Species ranges are often highly plastic, shifting or expanding to match spatial shifts of their niche (Sexton et al 2009). As an individual moves to the range margins, the specialized traits may no longer be optimal for the environment that is encountered because habitat quality has declined. Changes in species’ geographic range are one of the most commonly reported responses to climate change. A metaanalysis of 1700 plant and animal species indicates that ranges are shifting at a rate of 6.1 km per decade towards the poles (Parmesan and Yohe 2003).

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