Sex matters in the race to combat climate change

This blog post is provided by Rachael Y. Dudaniec, Alexander R. Carey, Erik I. Sve­­nsson, Bengt Hansson, Chuan Ji Yong and Lesley T. Lancaster and tells the #StoryBehindThePaper for the paper ‘Latitudinal clines in sexual selection, sexual size dimorphism, and sex‐specific genetic dispersal during a poleward range expansion’, which was recently published in the Journal of Animal Ecology.

As climate change is facilitating many species to expand their ranges into novel areas, one may wonder if males and females are responding in the same way, and if these massive disruptions are creating new interactions, or possibly, altercations, between the sexes as they battle their way through foreign territories. It is possible that sex-differences in behaviour, appearance or physiology along a range expansion gradient may affect how successful individuals are at adapting to novel environmental conditions. This can lead to variation in how different sexes become distributed across space, and how they adapt to their environments. This phenomenon of ‘spatial sorting’ of individuals that possess favourable traits can therefore affect the rates of range expansion, but little is known about how sex differences in these traits impacts such expansions.

In insects, males and females of the same species may differ in body shape, size or colour, or levels of environmental plasticity in traits that impact dispersal and reproduction. Here, we investigate how sex differences in morphology, behaviour and genomic population differentiation may impact the climate-mediated poleward range expansion of the common bluetail damselfly (Ischnura elegans) in northern Europe. We sampled 65 sites along a 583 km gradient including the expanding I. elegans range margin in Sweden, and quantified gradients in local abundance, sex ratio and sex-specific shifts in body size and mating status. Mating status was measured via the ‘copulation status’ of the damselfly, which was either solitary and unpaired, or paired, with males and females captured in the ‘mating wheel’ formation, which forms a distinctive love heart shape (see picture).  

The characteristic love heart shaped ‘mating wheel’ formation of Ischnura elegans. Photo by Rachael Dudaniec

In many insects, including I. elegans, females are conspicuously larger than males, however towards the cooler, northern range limit, male damselflies had a larger body size than in the range core, a trait that is advantageous for dispersal capacity. This increase in size was not reflected in females, resulting in relaxation of sexual size dimorphism at the expanding range edge. The reasons for the lack of change in female body size is not known, but may indicate a lack of selection for a larger, dispersal-enhancing female body size due to existing behavioural or morphological constraints.

There was also a male-biased sex ratio towards the range limit. Despite this, these larger, northern males were less likely to be found in copula with females compared to their smaller counterparts. This observation was striking given that the reverse pattern occurs in the south of the I. elegans range, where larger males are under sexual selection. These findings suggest that sexual selection is not an important driver of this increased size, but rather, effects of the climate-driven range expansion on natural selection, shaping its male passengers for a speedy journey at the expense of having less success with the opposite sex.

Complementing this field data with genetic information for 426 individuals from 25 sites, we could further see how the sexes differed in gene flow and migration rates. These data indicated that males were more likely to migrate north, and for longer distances, than females. Sex-specific differences in dispersal were also revealed by investigating how genetically similar individuals of the same sex were to each other at different spatial scales. This test showed us that females are more ‘sedentary’ as they stay more genetically similar to each other at shorter distances than do males.

When investigating how landscape and climatic features can limit or facilitate sex-specific genetic connectivity along the expansion gradient, we found that annual temperature had a strong linear effect on genetic dispersal. This temperature effect was consistent across both sexes, and in all cases an effect of landcover features on genetic dispersal (such as vegetation cover and roads) was entirely absent. Within the cooler temperatures towards the northern range edge, it was harder for gene flow to occur, representing another challenge for larger males moving longer distances as they reach their new, colder, northern frontiers.

Altogether, we present a story of male-biased, longer distance dispersal during range expansion combined with limited mating success of larger, northern males. These northern males also encountered lower temperatures that are more difficult to disperse through and provide high resistance to gene flow. These findings have emergent consequences for the evolution of the sexes and sex differences in population structure in I. elegans during range expansion. While males are more likely to colonise novel areas ahead of females in I. elegans, which is so far, exhibiting a successful range expansion in Sweden, adapting to a rapidly changing climate is not without disruption to sex-specific interactions, which may ultimately play a key role in the success of future population persistence.

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