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.