Common Noctule Bats Are Sexually Dimorphic in Migratory Behaviour and Body Size but Not Wing Shape

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.

Fig 1. Wing morphology of Nyctalus noctula.

A) Right wing of N. noctula with 13 landmarks that were used to calculate individual bone lengths. The blue line represents the forearm length, the metacarpals are shown in orange, the proximal phalanges in yellow, the medial phalanx of digit 3 in cyan, and the distal phalanges are in green. B) Right wing of Nyctalus noctula showing the calculation of areas, and linear distances.

Fig 2

Boxplots of forearm lengths (A), body mass (B), and body condition (C) of female (red) and male (blue) noctules. Females have longer forearms and higher body mass.

Fig 3. Wireframe shape distortions based on the first principal component in the geometric morphometric analysis.

No sex differences in wing shape were found.

Fig 4. Principal component 1 vs 2 of Procrustes scaled coordinates in the geometric morphometric analysis.

The first two principal components account for a cumulative 51.1% and 68.1% of the variation, respectively.