Rapid evolution of sexual dimorphism in island populations of the Sulawesi Babbler

Stronger intraspecific competition on islands might explain this finding.

A few weeks ago, I wrote about the molecular underpinnings of sexual dimorphism in birds (see this blog post). In most species where males and females look different, the dimorphism is very obvious. Think of the extravagant tail feathers of the male peacock (genus Pavo) compared to the dull brown female. In some species, however, the differences between males and females are more subtle and easily missed by the casual observer. For example, in descriptions of the bird family Pellorneidae (the jungle babblers), you will often read that the sexes are similar. But a closer look at these inconspicuous birds might reveal cryptic sexual dimorphism. Indeed, a recent study in the journal Biotropica uncovered subtle morphological differences between males and females of the Sulawesi Babbler (Pellorneum celebense).

Clustering

Fionn Ó Marcaigh and his colleagues examined the morphology of birds captured on the southeast peninsula of Sulawesi and on the smaller islands of Kabaena, Muna, and Buton. They applied a cluster algorithm to determine if the birds could be sorted into different subgroups based on subtle morphological differences. This approach of unsupervised clustering is well suited for detecting sexual dimorphism in body size for monochromatic bird species. The analysis separated the sampled birds into two groups that corresponded to males and females (confirmed with molecular sexing). Male babblers were consistently larger than females. Convincing evidence of sexual dimorphism in the Sulawesi Babbler.

The clustering algorithm identified two main clusters that correspond to males (green) and females (red). From: Ó Marcaigh et al. (2020). Biotropica.

Competition

The researchers also noticed that “island birds appeared more strongly dimorphic than their mainland counterparts in each of our five morphological traits.” These islands became isolated within the last 20,000 years, suggesting very rapid evolution of sexual dimorphism in the Sulawesi Babbler. The selection pressures responsible for these fast changes are probably related to intraspecific competition (i.e. between individuals of the same species). On islands, species can often occur in higher densities because there is less competition with other species. However, these higher densities might result in more fierce competition between members of the same species, potentially culminating in sexual dimorphism when males and females adapt to different ecological niches. An interesting phenomenon on the islands that Alfred Russell Wallace already described as “anomalous”. Who knows what other discoveries await us on Sulawesi?

Sexual dimorphism was more pronounced on the islands compared to the mainland, as illustrated by the diverging lines in these graphs (green = males, red = females). From: Ó Marcaigh et al. (2020). Biotropica.

References

Ó Marcaigh, F., Kelly, D. J., Analuddin, K., Karya, A., Lawless, N., & Marples, N. M. (2021). Cryptic sexual dimorphism reveals differing selection pressures on continental islands. Biotropica53(1), 121-129.

Featured image: Sulawesi Babbler (Pellorneum celebense) © Trinity College Dublin

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