Selection, habitat preference and the evolution of sexual size dimorphism in arvicoline voles

Submitted by editor on 8 December 2015.Get the paper!

In mammals, the male generally is the larger sex. For example, a mature male of southern elephant seal weights 3,200 Kg on average, whereas the females in his harem typically weigh five times less. However, females do still outweigh males in some species. For example, spotted hyena females are larger than their male conspecifics. It is this variation in the difference in size between the sexes, called ‘sexual size dimorphism’ (SSD) that has attracted the attention of evolutionary biologists ever since Darwin.

Although several explanations for the existence of variation in SSD have been proposed, and all of these somehow relate to how the strength of natural and sexual selection differs between males and females, distinguishing between them has proven to be far from easy. This is because we are rarely able to witness evolution of SSD in action, either because it is happening too slowly, or because SSD has already reached its adaptive optimum. Instead, we have to use the variation between species that we can observe today, i.e. the outcome of the evolutionary process, and test which properties of a species are associated with SSD. This is where phylogenetic comparative methods (PCM) come in.

PCM has a long history in evolutionary biology, and it was Charles Darwin himself who used them to elucidate the adaptive function of morphological features of organisms in The Origin of Species. However, because we now know that some species are more closely related to each other than others, and hence we cannot treat species as independent data points, we have to take into account phylogeny when we are looking for the traits co-evolving with SSD, We therefore applied a more modern version of PCM to data on SSD in arvicoline voles, a group of rodents that is widespread across the northern hemisphere for which the phylogeny is fairly well-resolved. This group is particularly well suited to disentangle the evolutionary drivers of SSD, as it contains species with both male- and female-biased SSD, as well as a high diversity in terms of mating system and social organization.

Although the mating system (polygamy, monogamy or promiscuity) has been proposed to be an important driver of SSD through its effect on sexual selection, our analyses revealed that SSD did not coevolve with mating system. Instead, we found that female size evolves in response to fecundity selection, with larger females giving birth to larger litters, while male size is shaped by sexual selection, with large males having a competitive advantage over smaller males. This suggests that size is subject to similarly strong selection in both sexes, and hence males and females evolve at similar rates. However, they do so largely independently from each other.

Most interestingly, we found that the best predictor of SSD is provided by habitat type, with species inhabiting open habitats (e.g. grasslands or steppe-like habitats) being more sexually dimorphic than species that live in closed habitats (e.g. forests). This is in line with food being more abundant and more evenly distributed in open habitats, allowing females to be less territorial and to form groups. As a consequence, males inhabiting open habitats are able to monopolize territories containing several females, resulting in intense between-male sexual selection for large size. In closed habitats on the other hand, food resources are patchily distributed and females exhibit strong territoriality, leading to an opportunistic (non-clumped) distribution, preventing males from monopolising multiple females and thereby a relaxation of intrasexual (male-male) competition.

Altogether, our study reveals the importance of habitat type in shaping variation in SSD across species, and it highlights the complexity of the evolution of SSD in general.

A graphical summary of one of the main results found in the study. Negative values refer to species in which males are larger than females. 

 

 

 

 

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