Geographic differences in seasonal brain mass changes
Literature on seasonal changes in mammalian brain size is scarce (see
Supporting Information), but average brain mass of S. araneusdecreased by 20.9 ± 5.6% from summer juveniles to winter subadults and
regrew by 10.0 ± 4.2% to adult size. This is the most remarkable aspect
of Dehnel’s phenomenon. The size of the mammalian brain, once fully
grown, is usually more or less fixed and changes of this magnitude are
unparalleled. Only the song brain of some birds reversibly changes by
similar magnitudes (Nottebohm, 1981; Tramontin et al. , 1998).
Experimentally induced changes in brain size, by starving or other
environmental manipulations, result in changes of less than 5% (Bedi &
Bhide, 1988). In humans, brain size increases during ontogeny, reaching
a peak at the age of 20; then, after 45-50 years of age it undergoes a
progressive, unidirectional decline of 11% over the next 40 years, as a
result of ageing (Dekaban & Sadowsky, 1978).
When we analyzed the compiled shrew literature data with regards to
geographic variation in the intensity of seasonal size change we found
only little variation between populations. However, in contrast to
braincase height, we found no correlation between any geographic
variable and the intensity of both decrease and regrowth of brain mass
(Tables 1 and 2). When we looked at our own data from Gugny and
Radolfzell in more detail, we again found no significant effect of sex
on the variation of corrected brain mass (AIC(M1) = -509.6, AIC(M2) =
-509.7; ANOVA, P > 0.1) and excluded it from the
models. We did find significant variation between seasons at the factor
level in the final model M2, but not between locations and their
interaction (d.f . = 50, adj. R2 = 0.45,F (seas.) = 22.5, F (loc.) = 0.5, F (seas.:loc.) =
2.1, P (seas.) < 0.001, P (loc.) >
0.5, P (seas.:loc.) > 0.1). However, there was no
difference in absolute brain mass at any of the age stages that could be
explained by geographical variables either (P > 0.1
for all models). At both locations there was a significant but similar
decrease from summer juvenile to winter subadult (Tukey test, P< 0.001). Surprisingly we found no significant regrowth from
winter subadult to adult (P > 0.1). Thus, corrected
brain mass was similar at every stage in both populations (Table 3, Fig.
4). This lack of spring regrowth in brain mass contradicts previous
research, where a significant regrowth in brain mass from winter to
summer was found in all studied populations of S. araneus (Bielak
& Pucek, 1960; Pucek, 1970; Yaskin, 1994; Lázaro et al. , 2018a).
In fact, in our own previous study of the Radolfzell population we found
a significant regrowth of brain mass, with a maximum during July-August
(Lázaro et al. , 2018a). The adult brains used for the present
analyses were collected earlier (May-July). Especially in Gugny, we
collected most adults in May when body mass regrowth peaks and which is
when the regrowth peak is commonly measured. Thus, the adults we
included here probably had not completed brain regrowth yet. This
emphasizes the importance of timing data collection correctly and
defining the size stages for studies of Dehnel’s Phenomenon carefully.
An accurate estimation of the maxima and minimum of the cycle is
critical for the quantification of the change. Probably most studies of
Dehnel’s Phenomenon do not report exact maxima and minima, and
consequently all values on change intensity presented here are timed
with spring body mass maxima and thus underestimates of the actual
change in brain mass. This must be considered when studying Dehnel’s
Phenomenon intensity as well as the cycle phenology.
Striking in our combined results is the discrepancy between variables.
Changes in braincase height did not reflect changes in brain mass,
unlike in previous studies (Bielak & Pucek, 1960; Lázaro et al. ,
2018a). The geographical patterns we found in braincase height decrease,
were not reflected by patterns in brain mass changes. This is probably
due to the small sample size of brain mass data. Only few studies (10
publications about the common shrew, the most intensively studied
species) have investigated the seasonal changes in brain mass.