2.3 The choice of months representing the three age categories
We compared skulls in three age categories of individuals that
characterize their entire lifetime: fully grown juveniles,
size-decreased subadults, and regrown adults (Figure 3). To test the
homogeneity of each age category, we used an ANCOVA model with the month
as a factor, the time (year) as a covariate, and the interaction between
month and year. The skull height of juvenile individuals differed
between July and August (F 1,201=8.08,P =0.0049), and the interaction was significant
(F 1,201=8.12, P =0.0048) in the period
between 1953 and 1981, when individuals from both months were measured.
Thus, we further analysed July and August juveniles separately.
The interaction between month and year was not significant in the
analogous ANCOVA model for skull height in subadults from December,
January, and February (F 2,82=0.75,P =0.476). However, skull height had not fully decreased in
December and was higher than in January and February when we removed the
interaction from the model (P =0.0159 and P =0.0426,
respectively). The skull height did not differ in the model that
included only the month and year for skull height in subadult shrews
from January and February (F 1,67=0.55,P =0.461), and we used only these months for further comparisons
(Figure 3). In these two months, the skull height was lowest in the
lifespan of S. araneus (Dehnel 1949; Pucek 1970). Moreover, the
lack of an interaction between month and year in December, January, and
February indicated that skull height reached a minimum in January
irrespective of the year. In adults (Figure 3), neither the year × month
interaction nor the month was significant in the model for skull height
(F 4,104=0.88, P =0.476;F 4,104=0.88, P =0.477). The month was also
nonsignificant (F 4,108=1.67, P =0.163) when
the interaction was removed from the model. Thus, we retained all months
from April to August in the final analyses of adults.
We ran the same tests for skull width and length, which varied much less
between seasons (Lázaro et al. 2017). The year × month interaction was
not significant in any age category, and there were no significant
differences between collection months. See Figure 4 for an overview of
analysed S. araneus data.