Abstract

Within the large order of bats, sexual size dimorphism measured by forearm length and body mass is often female-biased. Several studies have explained this through the effects on load carrying during pregnancy, intrasexual competition, as well as the fecundity and thermoregulation advantages of increased female body size. We hypothesized that wing shape should differ along with size and be under variable selection pressure in a species where there are large differences in flight behaviour. We tested whether load carrying, sex differential migration, or reproductive advantages of large females affect size and wing shape dimorphism in the common noctule (Nyctalus noctula), in which females are typically larger than males and only females migrate long distances each year. We tested for univariate and multivariate size and shape dimorphism using data sets derived from wing photos and biometric data collected during pre-migratory spring captures in Switzerland. Females had forearms that are on average 1% longer than males and are 1% heavier than males after emerging from hibernation, but we found no sex differences in other size, shape, or other functional characters in any wing parameters during this pre-migratory period. Female-biased size dimorphism without wing shape differences indicates that reproductive advantages of big mothers are most likely responsible for sexual dimorphism in this species, not load compensation or shape differences favouring aerodynamic efficiency during pregnancy or migration. Despite large behavioural and ecological sex differences, morphology associated with a specialized feeding niche may limit potential dimorphism in narrow-winged bats such as common noctules and the dramatic differences in migratory behaviour may then be accomplished through plasticity in wing kinematics.

Highlights

  • Among the large order of bats, sexual size dimorphism (SSD) is often female biased or absent [1,2,3,4] despite social structures that are typically linked to sexual selection and male-biased SSD in other groups of mammals, including harems and leks [5, 6]

  • We found sex differences in forearm length and body mass between female (N = 357) and male (N = 88) common noctules at the time of capture

  • Forearm lengths measured from photos were 0.34 mm shorter than forearm lengths taken externally with calipers during captures (F1,132 = 47.63, P < 0.001), likely as it was difficult to locate the olecranon process on the photos that serves as the proximal landmark of the elbow

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Summary

Introduction

Among the large order of bats, sexual size dimorphism (SSD) is often female biased or absent [1,2,3,4] despite social structures that are typically linked to sexual selection and male-biased SSD in other groups of mammals, including harems and leks [5, 6]. Big mothers may be advantageous to species that migrate over long distances, as they time reproduction to maximum food availability so that both the females and their offspring can migrate back to over-wintering sites. This hypothesis is supported by studies that find longer forearms in females of most tested species of vespertilionids, there are no other relationships to load carrying after adjusting for size [2]. The relationship between femalebiased size differences, selection for big mothers, and other morphological sex dimorphism in load carrying and shape is, still unresolved

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