Keywords
Acoustic telemetry, behavior, brain regions, brain size, habitat complexity, lake trout, phenotypic flexibility, seasonality.
Introduction
Teleost fishes exhibit exceptional flexibility in their behavioral and physiological responses to changing environments, which has allowed them to colonize an impressive number of habitats at different latitudes (Dill 1983, Armstrong and Bond 2013). Previous work on freshwater fish has highlighted the need to better document and resolve the role of neural flexibility in underpinning life history strategies and ecology among different species and populations (Gonda et al. 2013). Fish often exhibit large variation in the proportional size of their brain regions in association with ecological and sensory specialization (Kotrschal et al. 1998, Gonzalez-Voyer and Kolm 2010). There is, however, a growing body of experimental literature suggesting that novel environments can influence fish brain size within the span of a few weeks or months (Park et al. 2012, Herczeg et al. 2015, Turschwell and White 2016, Fong et al. 2019, Závorka et al. 2020). If fish are capable of rapidly adjusting their brain size to cope with new environments, then perhaps fish can exhibit changes in brain size that allow them to succeed in the face of seasonal and interannual changes in natural environments.
Seasonality in temperate lake ecosystems generates dramatic declines in temperature and light levels during the late fall and winter due to shifting daylight cycles and ice/snow cover. Species time the phenology of many key life history events (e.g. Salvelinus namaycushreproduction; Martin 1957) and activity patterns with these seasonal cycles. Some species, for example, suppress their activity in response to winter conditions and wait to reproduce until the spring, while others reproduce in the fall and remain active all winter (Shuter et al. 2012). In either case, predictable seasonal habitat shifts are commonplace among temperate fish species (e.g. to reach spawning sites or overwintering areas or to access prey; Hanson et al. 2008, Shuter et al. 2012). It is possible then, that coupled seasonal shifts in abiotic conditions and fish behavior could increase cognitive demands during particular seasons.
Brain and brain region size is correlated with performance in cognitively demanding tasks (Kotrschal et al. 2013, Buechel et al. 2018) and with variation in neuron numbers (Marhounová et al. 2019). As such, brain size variation is generally conceived as relating to variation in cognitive demands (i.e. sensory, motor, and integrative functions). To date, only a single study has explored seasonal variation in the size of one brain region (the telencephalon) in a wild fish, the round goby (Neogobius melanostomus ). This study found larger telencephalon sizes during the reproductive season, which was associated with the increased spatial processing demands of mating (McCallum et al. 2014). It is still unknown however, whether whole brain size is seasonally flexible in fishes, or if the size of individual brain regions can change independently of one another across seasons.
Here, we investigated the seasonal variability in total brain mass and individual region volumes (Figure. 1) of lake trout (Salvelinus namaycush ) from two lakes in Ontario, Canada. We first hypothesized that seasonal changes in cognitive demands would drive changes in lake trout brain size over a seasonal timescale. Second, we hypothesized that any changes in overall brain size were either the result of: a) seasonal demands for region-specific processing (i.e. a mosaic change in the size of some brain regions and not others), as certain regions might be of more or less utility depending on season-specific life history, or b) ubiquitous changes in the size of each brain region (i.e. concerted change in brain size; Finlay and Darlington 1995, Striedter 2005). Brain size data were collected in both lakes over six consecutive seasons. Trends in brain size variation were compared to habitat use and movement rates data obtained by acoustic telemetry from one of our study lakes to assess the relationship between seasonal patterns in brain size and behavior.