Differences in seasonal skull dimensions and body mass between
four populations of the common shrew
In our own previous work we used tooth row length measured from X-ray
images as a factor to correct for individual size variation as it
remained constant throughout the shrews’ lifespan once summer juveniles
were fully grown at our study site in Southern Germany (Lázaro et
al. , 2017). Here, to use a calliper measurement in collection specimens
(in some collections X-rays were not possible) we measured mandible
length as a proxy to that correction factor. However, when looking at
three additional populations (Žofín, Gugny and Białowieża) we found that
mandible length varied between seasons (d.f. = 186, adj.R2 = 0.19, F = 5.3, P (seas.)
< 0.05, P (loc.) < 0.001, P (seas.:loc.)
> 0.1). Results for size-corrected and absolute values did
not significantly differ in Radolfzell. Consequently, we compared
absolute values between the four populations. First we tested for the
effect of sex, but found no significant effect of sex and its
interactions on braincase height for all locations in the model
comparisons (AIC(M1) = -79.2, AIC(M2) = -67.8; ANOVA, P> 0.5). Thus, we excluded sex from further comparisons of
skull dimensions. This is interesting, as even though differences in
behavior and energetic pressure should exist particularly during
reproduction in the adults, and some sexual dimorphism was found in
mandible morphology of S. araneus (Nováková & Vohralík, 2017).
In the final model M2 (d.f . = 200, adj.R2 = 0.78, F (season) = 155.7,F (location) =146.6, F (interaction seasons X location) =
1.3), we found a difference between seasons and locations at the factor
level (P < 0.001 both), but not their interaction
(P > 0.1). The Tukey test revealed a decrease in
braincase height from summer juveniles to winter subadults (P< 0.001) and an increase from winter subadults to adults
(P < 0.001) at each location (Fig. 2, Table 3).
Braincase height values for all seasons combined were highest in Gugny,
followed by Radolfzell, Žofín and Białowieża (P < 0.05
in all pair-wise comparisons, Table 3). Thus, shrews from the four
populations differed in size, but the magnitude of Dehnel’s Phenomenon
did not. Our analyses of data from the literature confirmed a more
pronounced decline in braincase height towards Northeastern populations
at a large geographical scale. However, our review also revealed large
levels of variation in winter size decline between populations within
small areas (e.g. northern Germany (Schubarth, 1958)) and similar
decline values in widely separated populations (e.g. southern Germany
and central Finland (Skaren, 1964; Lázaro et al. , 2018a)). Our
four focal populations did not follow the predicted pattern but they are
all situated in central Europe and habitat differences might not be
strong enough to cause the variation observed at a larger scale.
Interesting is that size, as measured by braincase height did not follow
the expected pattern either. Sorex araneus is smaller with
increasing latitude in direct contradiction to Bergmann’s rule. However,
the two neighboring Polish populations differed more in size than Gugny
(Northeastern Poland) and Radolfzell (Southern Germany), which were
almost identical.
As previous studies had also found (Dehnel, 1949; Lázaro et al. ,
2017, 2018a), braincase height changed most strongly than other skull
metrics in all populations. Thus, we focus on results for braincase
height here. To summarize, we only found a slight winter decrease in
skull length and a spring increase in braincase width, only in
Radolfzell. More details of the results for skull length and braincase
width can be found in the Supporting Information (see Table S2).
Again similar to the results on braincase height, we found few
differences in body mass between the more closely investigated
populations (Radolfzell, Gugny and Žofín). Again we found no significant
effect of sex on body mass variation between seasons (AIC(M1) = 290.3,
AIC(M2) = 293.6 ; ANOVA, P > 0.1) and pooled data of
males and females in all analyses. Body mass differed significantly
between seasons and locations at both factor and interaction levels (M2,d.f .=116, adj. R2 = 0.88,F (seas.) = 424.2, F (loc.) = 13.8, F (seas.:loc.) =
2.8, P (seas.) < 0.001,P (loc.)>0.001, P (seas.:loc.) >
0.05). All three populations decreased from summer juvenile to winter
subadult followed by a pronounced regrowth to adult (Table 3, Fig. 3,
Tukey test, P < 0.001 for all populations). Body mass
was similar in juveniles and adults in all populations, but winter
subadults from Žofín were lighter (P < 0.001). Žofín is
the only high-altitude population in our dataset. Mountain populations
suffer harsher winter conditions and we expected Dehnel’s Phenomenon to
be stronger in shrews at higher altitudes. The stronger body mass
decline we found in Žofín supports Dehnel’s Phenomenon as a seasonal
adaptation. However, we do not see a matching difference in braincase
height decline. This might mean that changes in body mass are more
sensitive to local environmental differences and/or current conditions.
For example, there is the little evidence for winter body mass decline
in Norway (Frafjord, 2008), but a 27% decline found at similar
latitudes in Finland (Hyvärinen & Heikura, 1971). Alternatively, given
that data from the various sites were collected during completely
different years, seasonal changes in body mass may have resulted from
other causes independent from Dehnel’s Phenomenon, for example, winter
malnutrition or non-adaptive changes.