Hypothesis 2: Brain Region Sizes
Next, we evaluated if changes in brain size were region-specific or if changes involved all regions observable at the gross morphology level. When using a LM with fork length as a covariate to correct for variation in body size, the main effect of lake was significant: fish from Two Rivers had relatively larger telencephala (lake: F2, 134= 250, P < 0.001), cerebella (lake: F2, 134 = 260, P < 0.001), optic tecta (lake: F2,134 = 370.6, P > 0.001), and olfactory bulbs (lake: F2,134 = 205.4, P > 0.001) than those from Opeongo. There was a significant interaction between fork length and lake for hypothalamus size (F3, 133 = 109, P = 0.003), where different allometric slopes produced Two Rivers fish with larger hypothalami than Opeongo fish at smaller fork lengths (Appendix S1: Figure S1).
The main effect of season and lake was also significant for all five brain regions, yet none of the regions demonstrated a significant interaction between season and lake. Thus, the seasonal patterns observed in brain regions were not population-specific (Table 2). Telencephalon size followed a trend similar to that of the whole brain (smaller in spring and summer, larger in fall and winter; Figure 3a), while cerebellum size was smallest during the spring, with no significant differences between the fall, winter, and summer (Figure 3b, Appendix S1: Table S3). Although season was a significant factor in the LM, the optic tecum did not differ between any seasons based on the Tukey tests (P > 0.06; Appendix S1: Table S4), however the general trend in this region was for lower size in the spring (Figure 3c). Like the cerebellum, both the olfactory bulbs and the hypothalamus were smallest during the spring, with no significant difference between summer, fall, and winter (Figure 3d,e, Appendix S1: Table S4).
Acoustic Telemetry
Model selection indicated that each of the movement and habitat use metrics from Lake of Two Rivers trout were best modelled using a GAMM with a year-specific weekly smoother rather than a common weekly smoother (Appendix S1: Table S4). During the summer, lake trout occupied greater depths (~12 m; Figure 4a) and were located well over 300 meters away from shore (Figure 4b). Pronounced vertical movement in the water column were characteristic of summer months (high SD of depth, Figure 4c), while horizontal movements were lowest at this time (Figure 4d). Conversely, lake trout occupied shallower depths during the coolest weeks of the year (fall, winter, early spring: Figure 4a). Rapid depth changes occurred around weeks 20 and 40 (mid-May and early October), with fish moving offshore to deeper water in the late spring (immediately following the Spring sample collected in May) and back nearshore into shallow water in early October (Figure 4a). Fish remained in generally shallow water throughout the winter. These periods of low depth occupancy in the spring, fall, and winter coincided with shorter distances from shore (Figure 4b) and a low SD in depth (Figure 4c), indicating that the fish were not regularly changing position in the water column during that time. Horizontal movement rates were lowest in the summer and increased dramatically in the fall (Figure 4c). Horizontal movement rates were higher in the second compared to the first winter, however this did not seem to be due to differences in system positioning error between winters.
Discussion
Our data suggest that the brain size of a temperate freshwater fish changes within an annual timespan, mirroring seasonal changes in habitat use and movement. Larger relative brain size in Two Rivers lake trout, which do not have access to an offshore forage fish and as a result forage more frequently in nearshore habitats (Vander Zanden and Rasmussen 1996), is consistent with previous observations that fish species and populations with a higher reliance on the nearshore habitat have larger brains (Edmunds et al. 2016b, Axelrod et al. 2018). Seasonal variation in relative brain size was apparent across both lakes, with brains being smallest during the spring and summer and largest during the fall and winter. It is unlikely that lake temperature alone was responsible for these trends. Though larger brain sizes have been associated with higher temperatures (and possibly metabolism; Gillooly and McCoy 2014, Yu et al. 2014, Závorka et al. 2020), we observed that lake trout brain size was smallest during the warmest season (summer). Previous work has suggested that increases in brain size reflect increased performance in cognitively demanding tasks (Kotrschal et al. 2013, Buechel et al. 2018). Our findings therefore support our first hypothesis that fish alter their brain size seasonally, potentially in accordance with seasonal variation in cognitive demands.
Variation in whole brain size reflects underlying variation in individual brain regions, which were also found to vary seasonally. The telencephalon tracked the observed changes in brain size best (larger in the fall and winter compared with spring and summer). Larger telencephala have been associated with higher utilization of nearshore habitats (Gonzalez-Voyer and Kolm 2010, Edmunds et al. 2016a). In their investigations of the round goby Neogobius melanostomus, McCallum et al (2014) also found larger telencephalon sizes prior to the spawning season. As lake trout move nearshore onto spawning shoals in the fall, they navigate more structurally heterogeneous environments, while also coordinating complex social behaviors during mating (Johnson et al. 2018). Nearshore habitat use was maintained throughout the winter (at least in telemetered Two Rivers lake trout), which would also be associated with navigating under reduced light conditions (due to reduced photoperiod and snow and ice cover; Blanchfield et al. 2009). Larger telencephala in the fall and winter could therefore reflect increased cognitive demands associated with spawning in the fall and nearshore habitat use during the fall and winter. Nearshore foraging is also known to occur in spring (Guzzo et al. 2017) however, and telemetered lake trout in our study had just begun to move away from shore at the time of spring sampling. Smaller telencephala in the spring and summer could, therefore, be associated with movement into offshore, deeper water.
Unlike the telencephalon, seasonal changes in size of the other brain regions were limited to a decrease in size in the spring. Nearshore habitat use is higher in fall and winter compared to summer, and reproduction occurs in the fall (Guzzo et al. 2017). Habitat use and spawning alone are likely insufficient to explain the size of these brain regions, which were of similar size in fall, summer, and winter. The smaller size of the cerebellum, olfactory bulb, optic tectum, and hypothalamus in spring vs. fall may potentially be driven by increased lake trout foraging and growth during the spring (Fry 1939, Guzzo et al. 2017). Large increases in total body length during spring months may reduce the relative brain size to body length ratio; such a dilution would be expected if brain growth lags behind patterns in global/systemic growth. Alternatively, reduced brain sizes during the spring may be a product of energy trade-off mechanisms that mediate energetically expensive tissues to optimize growth potential. Trade-offs in brain and gut size, for example, have been observed in fish and other vertebrates (Aiello and Wheeler 1995, Kotrschal et al. 2013, Monnet et al. 2020, Rosenfeld et al. 2020). Investment in large, energetically expensive digestive organs in support of growth in the spring could entail temporary reduction in brain size to allow a more effective allocation of resources to different parts of the organism (Armstrong and Bond 2013). Whether such organ system trade-offs happen on a seasonal scale is unknown. Future work is required to explore how seasonally changing cognitive and energetic demands might govern the size of the brain and its regions.
Previous work has identified seasonal brain size flexibility. The size of the telencephalic hemispheres and whole brain of a benthic fish and a shrew, respectively, have exhibited distinct seasonal patterning (McCallum et al. 2014, Lázaro et al. 2018). Seasonal variation in the size of a specific brain region, the hippocampus, has also been noted in birds and mammals (Yaskin 2011). However, the inability to follow the record brain metrics from the same individuals over time represents a central limitation to our study (and previous studies on this topic), given that, at this time, animals must be sacrificed to measure brain sizes. We therefore cannot discount the possibility that individuals captured during fall and winter tended to be those individuals within the population that had larger brains. We have also included a limited set of ecological traits (i.e., habitat use, movement rates, fall reproduction by using mature individuals), making our inferences about the role of brain size in these behaviors tentative until a larger suite of traits can be explored. Additionally, we echo previous concerns about the uncertain role of brain size in cognition, and stress caution when interpreting its relationship with complex behaviors (Healy and Rowe 2007). However, the fact that we observed the same seasonal trend in both lakes, and the observation that larger relative brain masses coincided with increased nearshore habitat use and movement rates in telemetered Lake of Two Rivers lake trout suggest that brain size of lake trout is seasonally flexible in support of, or in response to, changes in behavior. More work will be needed to elucidate which specific behaviors are impacted by seasonally flexible changes in brain size.