Effects of population density on static allometry between horn length and body mass in mountain ungulates

Christophe Pélabon,Morgane Tidière,Marco Festa‐Bianchet,Anne Loison,Jean‐Michel Gaillard,Jean‐Francois Lemaître,Mathieu Garel,Steeve D Côté,Carole Toïgo

doi:10.1111/oik.08726

Abstract

Little is known about the effects of environmental variation on allometric relationships of condition‐dependent traits, especially in wild populations. We estimated sex‐specific static allometry between horn length and body mass in four populations of mountain ungulates that experienced periods of contrasting density over the course of the study. These species displayed contrasting sexual dimorphism in horn size; high dimorphism in Capra ibex and Ovis canadensis and low dimorphism in Rupicapra rupicapra and Oreamnos americanus. The effects of density on static allometric slopes were weak and inconsistent while allometric intercepts were generally lower at high density, especially in males from species with high sexual dimorphism in horn length. These results confirm that static allometric slopes are more canalized than allometric intercepts against environmental variation induced by changes in population density.

Highlights

For morphological traits, allometric relationships describe the proportional increase in size of a trait for a proportional increase in size of the whole organism (Huxley 1932).These relationships are often expressed as a power function of the form y = axb, where y is trait size and x body size, and where a depends on the initial values of x and y, while b, the allometric exponent, depends on the relative rate of increase of x and y
Allometric relationships can be estimated during growth, among individuals measured at similar age or developmental stage or among populations or species
We analyzed the effect of population density on static allometry between horn length and body mass in four mountain ungulates to test whether allometric relationships were more sensitive to variation in individual condition in species with large horns, and whether allometric slopes and intercepts were sensitive to changes in environmental conditions mostly induced by variation in population density

Summary

Introduction

Static allometric slopes are less variable than allometric intercepts among populations (Egset et al 2011, Voje et al 2014) and they are more difficult to change by artificial selection (Egset et al 2012, Bolstad et al 2015, Stillwell et al 2016). It remains largely unknown, whether static allometric slopes are less sensitive than allometric intercepts to environmental variation affecting nutrition and growth. Evidence for reprogramming the relative growth of specific body parts following starvation in Drosophila (Bergland et al 2008) suggests that variation in nutrition can affect the allometric slope. On the other hand, Tobler and Nijhout (2010) showed that starvation only affected the intercept of the static allometry between wing mass and body mass in the moth Manduca sexta

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