Phenotype-dependent schooling behaviour across hierarchical levels in three-spined sticklebacks
Year: 2026
Type: preprint
Abstract: Schooling is a widespread behaviour among pelagic fish that reduces locomotion costs of migration and predation risk. It is characterised by coordinated movements, speed and orientation of group members. Although schooling can vary both between populations and among individuals of the same population, these levels of variation are seldom considered together. Here, we investigated whether schooling tendencies in three-spined sticklebacks ( Gasterosteus aculeatus ) vary between populations and among individuals within-populations depending on their phenotype. We first compared the schooling behaviour of anadromous (‘migrants’) and recently landlocked (‘residents’) populations and tested whether individual sociability explained within-population variation in schooling. We then studied whether schooling behaviour is modulated when migrants and residents are in mixed groups. Migrant groups displayed better schooling performance than residents with higher alignment. Within migrant groups, less social individuals displayed more leadership, were better aligned, swam closer to others and fatigued less than more social fish. These patterns were absent in residents. When the two populations were mixed, overall schooling performance declined, particularly in migrants, with schools exhibiting reduced compaction and alignment, resembling resident groups. Thus, individuals did not modulate their schooling behaviour to the group social composition and phenotype-dependent schooling patterns were disrupted in migrants when mixed. These findings suggest that migration-related behaviours can disappear quickly post-isolation. They also highlight the importance of individual sociability in self-organisation and shaping collective behaviour, especially in populations that retain migratory behaviour. Considering behavioural variation across multiple hierarchical levels is thus key to understanding complex social systems. Significance statement Many animals exhibit complex social behaviours that can benefit group members. In fish, schooling behaviour enables to migrate with reduced costs of locomotion and predation risk. Yet, the benefits of schooling and schooling behaviour itself are shaped by the environmental conditions, and thus are strongly influenced by human disturbance. We conducted a study in three-spined sticklebacks to test whether man-made barriers that prohibit some fish populations to undergo their seasonal migration (‘residents’) altered schooling behaviour compared to their ‘migrant’ anadromous ancestors. Migrants were indeed better at schooling than residents, and their schooling patterns were shaped by individual variation in sociability. Residents did not improve their schooling behaviour when mixed with migrants. This suggests that ∼ 50 years of isolation are sufficient to losing migration-related behaviour and that river reconnection will likely promote the emergence of partial migration in this system.
Institutions: University of Bern, University of Groningen
Language: English
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W7214099504