Seasonal variation of behavior and brain size in a freshwater fish

Teleost fishes occupy a range of ecosystem and habitat types subject to
large seasonal fluctuations. Temperate fishes in particular, survive large
seasonal shifts in temperature, light availability, and access to certain
habitats. Mobile species like lake trout (Salvelinus namaycush) can
behaviorally respond to seasonal variation by shifting their habitat
deeper and further offshore in response to warmer surface water
temperatures during the summer. During cooler seasons, use of more
structurally complex nearshore zones by lake trout could increase
cognitive demands and potentially result in a larger relative brain size
during those periods. Yet, there is limited understanding of how such
behavioral responses to a seasonally shifting environment might shape, or
be shaped by, the nervous system. Here we quantified variation in relative
brain size and the size of five externally visible brain regions in lake
trout, across six consecutive seasons in two different lakes. Acoustic
telemetry data from one of our study lakes was collected during the study
period from a different subset of individuals and used to infer
relationships between brain size and seasonal behaviors (habitat use and
movement rate).  Our results indicated that lake trout relative
brain size was larger in the fall and winter compared to the spring and
summer in both lakes. Larger brains coincided with increased use of
nearshore habitats and increased horizontal movement rates in the fall and
winter based on acoustic telemetry. The telencephalon followed the same
pattern as whole brain size, while the other brain regions (cerebellum,
optic tectum, olfactory bulbs, hypothalamus) were only smaller in the
spring.  These findings provide evidence that flexibility in
brain size could underpin shifts in behavior, which could potentially
subserve functions associated with differential habitat use during cold
and warm seasons and allow fish to succeed in seasonally variable
environments.

硬骨鱼类（Teleost fishes）栖息于各类易受大规模季节性波动影响的生态系统与栖息地中。温带鱼类尤需应对温度、光照可获得性以及特定栖息地可达性的大幅季节性变化。诸如湖鳟（Salvelinus namaycush）这类活动性较强的物种，可通过行为策略响应季节性环境变化：夏季表层水温升高时，它们会将栖息地向更深、更远的远岸区域迁移。在寒冷季节，湖鳟会利用结构更为复杂的近岸区域，这一行为会提升其认知负荷，并可能在该时期使其相对脑尺寸（relative brain size）增大。然而，目前学界对于这类响应季节性环境变化的行为如何塑造神经系统，或是受神经系统所塑造，仍缺乏足够的认知。本研究中，我们在两个不同的湖泊中，对连续六个季节的湖鳟相对脑尺寸以及五个肉眼可见的脑区尺寸进行了量化分析。研究期间，我们从其中一个研究湖泊的另一组个体中收集了声学遥测（acoustic telemetry）数据，并以此推断脑尺寸与季节性行为（栖息地利用与移动速率）之间的关联。结果显示，两个湖泊中的湖鳟相对脑尺寸均在秋季与冬季大于春季与夏季。基于声学遥测数据，脑尺寸更大的个体在秋季与冬季的近岸栖息地利用度与水平移动速率均更高。端脑（telencephalon）的变化模式与全脑尺寸一致，而其余脑区：小脑（cerebellum）、视顶盖（optic tectum）、嗅球（olfactory bulbs）以及下丘脑（hypothalamus）仅在春季尺寸更小。本研究结果证明，脑尺寸的可塑性或许是行为转变的基础，这或可辅助鱼类在冷暖季节实现与差异化栖息地利用相关的功能，帮助鱼类适应季节性波动的环境。