Do gut microbiota contribute to reproductive isolation between Nightingale species?

A recent study explores this intriguing hypothesis.

The expression “gut feeling” has received an entirely different meaning. Recent studies have documented how the microorganisms in an individual’s intestines can influence its behavior (see for example this paper). These findings suggest that gut microbiota might play an underappreciated role in several biological processes. They could even contribute to speciation. Experiments with fruit flies (Drosophila), for instance, indicated that lineage-specific microbiota influenced assortative mating among the flies, potentially giving rise to a premating barrier. Alternatively, microbiota could act after mating if hybrids suffer from incompatible microbiotic combinations. Regardless of the exact mechanism, it seems feasible that gut microbiota could play a role in the origin of new species. A recent study in the journal BMC Ecology and Evolution tested this idea in two closely related bird species: the Common Nightingale (Luscinia megarhynchos) and the Thrush Nightingale (L. luscinia).

Individual Variation

Camille Sottas and her colleagues studied the gut microbiota of 18 Common Nightingales and 18 Thrush Nightingales. For both species, half of the individuals were collected in sympatric regions, while the other half originated from allopatric locations. This sampling design allowed the researchers to disentangle the different factors contributing to the diversity in gut microbiota. They nicely explain the reasoning behind potential patterns in the introduction: “A higher divergence [in gut microbiota] in sympatry would imply a stronger effect of habitat use or diet, while a higher divergence in allopatry would indicate a stronger effect of geographical region on the gut microbiota divergence.”

Sequencing the microorganism community at three sections of the intestine (duodenum, jejenum and ileum) uncovered twelve bacterial phyla and 126 genera. Analyses of the consequent diversity patterns revealed no significant differences between the species and their geographic origins. In fact, 79% of the variation in microbiota could be explained by individual differences. These results suggest that the gut microbiota composition is unlikely to contribute to reproductive isolation between these nightingale species.

Statistical analyses of different diversity measures in allopatry and sympatry revealed no significant differences in the gut microbiota of Thrush Nightingale (TN, in blue) and Common Nightingale (CN, in red). From Sottas et al. (2021).

Negative Results

The answer to the title of this blog post is thus a resounding no. Gut microbiota do not play a role in speciation (at least when it comes to these nightingales). You might be wondering why I decided to dedicate a blog post to a negative result. This decision aligns with the goal of this blog – and my personal mission – to generate attention for less “sexy” topics. Most newspapers and popular science websites tend to focus on scientific research that captures the imagination of the general public and generates clicks. A negative result is thus not that interesting for these attention mongers. From a scientific perspective, however, negative results are a crucial piece of the puzzle in our quest to advance human knowledge. Finding out that something does not work is also a relevant discovery (even though it will generate less publicity).

Most scientists are working extremely hard behind the scenes, and only a few will get the attention that they deserve. Indeed, most media attention and awards for the happy few are often the result of favoritism and knowing the right people. That is why I try to direct the spotlight to lesser-known scientists and the fruits of their unseen labor. Even if – or perhaps especially if – their results are negative.


Sottas, C., Schmiedov√°, L., Kreisinger, J., Albrecht, T., Reif, J., Osiejuk, T. S., & Reifov√°, R. (2021). Gut microbiota in two recently diverged passerine species: evaluating the effects of species identity, habitat use and geographic distance. BMC Ecology and Evolution21(1), 1-14.

Featured image: Common Nightingale (Luscinia megarhynchos) © Marcel Burkhardt | Wikimedia Commons

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