Comparing seasonal changes in brain organization in two
populations
As previously published for the Radolfzell population, the results from
Gugny confirm different patterns of decrease and/or regrowth (or lack of
change) in the volume of each brain region (Fig. 5). As each region
contributed to the overall decrease/regrowth of brain volume to a
different extent, we observed a seasonal change in the organization of
the brain. However, the pattern of decrease and regrowth in most brain
regions was similar in Radolfzell and Gugny. Olfactory bulbs of females
but not males significantly decreased from summer juveniles to winter
subadults at both locations (P < 0.05) reflected by a
significant interaction of age and sex (F = 4.2, P< 0.05), but we found no difference in seasonal olfactory bulb
size between Radolfzell and Gugny at neither factor nor interaction
level (P > 0.1). The only other brain region where
we found a different pattern between the sexes was the cerebellum but
only in Radolfzell (see also Lázaro et al. 2018b). The cerebellum did
not vary seasonally at either location (P > 0.5).
However, subadult winter females in Radolfzell had larger cerebelli than
males, while in Gugny we did not find this sexual dimorphism. Changes
for all other brain regions are discussed for males and females
together. Volume of the neocortex significantly declined from summer
juveniles to winter subadults at both locations
(P <0.001). We found that summer juveniles have a larger
neocortex in Gugny (P < 0.01), but we found no
difference in winter, meaning that there was a more pronounced decrease
in Gugny. We did not find a difference in neocortex volumes between
winter subadults and adults at either location. The rhinal and piriform
cortices decreased their volume from summer to winter (P< 0.01) and did not regrow in adults, again with no difference
between locations. Also, overall striatum volume decreased from summer
juveniles to winter (P < 0.001), but did not increase
in adults (P > 0.5). Again, there was no difference
between locations. Within the striatum, this pattern was repeated in the
caudoputamen (P (juv-sub) < 0.001; P (sub-ad)
> 0.5) and amygdala (P (juv-sub) < 0.05;P (sub-ad) > 0.1), while the nucleus accumbens did
not significantly change size at all. The overall volume of the
hippocampus decreased from summer to winter (P < 0.05)
and did not regrow in adults. There was no difference between locations.
Within the hippocampus, volume decrease was only found in CA2
(P (juv-sub) < 0.05). Both the thalamus and hypothalamus
decreased and regrew significantly, with no difference between locations
(thalamus: P (juv-sub) < 0.001; P (sub-ad)
< 0.05); hypothalamus: (P (juv-sub) < 0.001;P (sub-ad) < 0.001).
In summary, although each brain region makes a different contribution to
the seasonal changes in brain size, giving rise to a marked
re-organization of the brain structure along individuals’ life. The
seasonal changes in each brain region in Gugny are remarkably similar to
the variation observed in Radolfzell, with the exception of a slight
difference in neocortex winter decrease – more emphasized in Gugny –
and a quite different pattern in the cerebellum. However, the lack of
overall brain mass increase in spring in Gugny may be due to early
sampling and the results in Lázaro et al. (2018b) from Radolfzell might
describe these patterns better.
Interestingly, the structural changes described in Russia (Yaskin, 1994)
largely differ from both Radolfzell and Gugny. The only brain region
with a similar pattern is the neocortex, which is the structure that
shows the greatest winter decline in all three populations – 37%
decline in Russia and Gugny, 28% in Radolfzell – although this is
followed by a 18% spring regrowth in Russia, which we did not observe
in the other populations. The paleocortex of Russian shrews
shrinks/regrows 28/12% in mass respectively, more pronounced than the
intermediate values in the corresponding regions – rhinal and piriform
cortices – in Gugny (21/6%), and the less pronounced changes in
Radolfzell (18/4%). This is the only brain structure that matches our
expectation of a geographic and/or environmental gradient, with the
Polish population intermediate between Russia and Germany. Hippocampus
changes are much greater in Russia (29/33%) than in Gugny (10/5%) and
Radolfzell (10/8%), while the olfactory bulbs, which did not change
seasonally in Russia, showed strong changes both in Radolfzell (14/14%)
and Gugny (24/12%). These inconsistencies refute the hypothesis of a
simple linear geographic trend. Instead, the differences in brain
structure between populations and seasons might reflect local
adaptations to specific climatic or habitat features. Nevertheless, we
must also point out differences in the used methods. We used volumetric
estimations derived from tracing brain regions in fixed, sectioned, and
stained sections, while Yaskin (1994) weighed dissected tissue.