Tracing the geographic origins of behavioural variation using genomics

Abstract

Consistent behavioural differences among individuals can have significant ecological and evolutionary consequences, yet their origins and persistence within populations have proven challenging to determine. In newly established populations at expanding range margins, behavioural variation may result from natural selection and spatial sorting, with behavioural types that promote dispersal being more common. While such behaviours are expected to decline in the generations following population establishment, ongoing immigration may nevertheless maintain this variation at the range edge. However, this dynamic remains largely unexplored, likely due to the difficulty of quantifying levels of ongoing immigration in the wild. Here, I combine genome-wide sequencing data with behavioural assays to examine whether individual behavioural variation in a range-edge population of common reed warblers (Acrocephalus scirpaceus) relates to differences in genetic ancestry. Over the past century, reed warblers have expanded into Finland and Estonia, where they exhibit higher aggression and explorative behaviour than individuals from the species’ European breeding range core. However, it remains unclear whether this reflects the persistence of dispersal-enhancing traits from founders or continued immigration to the range front. I found genomic admixture from the range core in 23.17% of range-edge individuals (n = 19/82, proportions: 0.12 to 0.44), with demographic modelling providing some evidence of ongoing gene flow into the expanding range front population. Among range-edge individuals, those exhibiting more exploratory behaviour were also more likely to carry range-core genetic ancestry, whereas no such association was found for aggression. These findings suggest that ongoing immigration of highly exploratory individuals may contribute to behavioural patterns at the range edge, supporting the idea that exploration facilitates successful dispersal, including settlement in the new breeding population. However, the possibility that the observed admixture reflects retained ancestral variation from founders could not be ruled out by coalescent-based demographic modelling, leaving room for the alternative explanation that these traits persist from the original colonisers. More generally, this study highlights the value of using genomic data to determine the role of genetic or geographic ancestry in shaping behavioural variation at species range margins.

Popular Abstract

Where do exploratory reed warblers at the edge of their range come from? Insights from their genes

We often think of behaviour as a flexible trait that individuals can adjust to suit their environment. But many animals also show behaviours that are surprisingly consistent across different situations. Some individuals are consistently more aggressive or more exploratory than others – and these patterns can influence where individuals go and even whether species spread into new areas.

When a species expands into a new region, individuals which are more exploratory or aggressive may be more likely to be among the first colonisers. These behaviours are thought to not only promote departure from original sites, but may also facilitate establishment in new places. But what happens after colonisation? Are the behaviours we observe in these populations still shaped by ongoing movement of individuals from elsewhere?

To find this out, I studied common reed warblers (Acrocephalus scirpaceus), small songbirds that have expanded their European breeding range northward into Finland and Estonia over the past century. Interestingly, reed warblers at the northeastern edge of their range are more exploratory and aggressive than those in Central Europe. But it is unclear why. Are these behaviours retained from the original colonisers, or do they reflect more recent immigration from further south?

To investigate this, I combined behavioural assays with genetic data from across the genome. I looked at whether birds in Finland and Estonia showed signs of mixed genetic ancestry with the range core – in other words, whether their DNA was partly similar to birds from central Europe – and whether this ancestry was linked to their behaviour.

I found that about a quarter of the birds at the range edge had mixed genetic backgrounds, with up to 44% of their genome resembling that of birds from the range core, which may be an indication of ongoing movement into the range edge. Moreover, the more exploratory birds at the range edge also tended to be more likely to carry Central-European genetic ancestry. Exploration has previously been linked to individual movement in other species. This suggests that new, exploratory reed warblers from the range core may still be arriving in the northeast, and influencing local behavioural patterns.

However, it is also possible that the observed genetic similarity between range-edge individuals and the range core stems from shared ancestry, rather than from immigration introducing those genes into the range edge. While demographic modelling provided some support for ongoing gene flow, it could not rule out this alternative, leaving open the possibility that the observed link between genetic ancestry and behaviour at the range edge reflects the persistence of traits from the original colonisers.

Overall, this study demonstrates that combining behavioural assays with genetic data can shed light on the origins of behavioural differences within populations. It suggests that movement between populations may help maintain behavioural traits like exploration at range margins, which can play an important role in facilitating further range expansion.

• author: Klumpp, Malin Viviane
• supervisor: Bengt Hansson
• organization: Degree Projects in Biology
• course: BION02 20242
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9215058
• date added to LUP: 2025-11-07 10:56:05
• date last changed: 2025-11-07 10:56:05