3.4 Change in the magnitude of Dehnel’s phenomenon from 1955 to
1985
The magnitude of Dehnel’s phenomenon increased over the tested time
period (Figure 6a). The skull height of August juvenile and subadult
shrews decreased significantly over time
(F 1,50=11.83, P =0.0012;F 1,68=19.52, P <0.0001,
respectively) but did not change in July juvenile and adult shrews
(F 1,149=1.34, P =0.249;F 1,72=0.00, P =0.985; Figure 6b). This is
also illustrated in Figure 6a: the standard errors of juvenile and adult
skull height overlapped considerably, while the subadult skull height
decreased strongly over time (years). There was no significant change in
the skull length of August juveniles, subadults or adults
(F 1,50=0.00, P =0.978;F 1,68=0.05, P =0.829;F 1,72=0.07, P =0.942, respectively), but
the change was significant in July juveniles
(F 1,149=5.00, P =0.0269).
In a second approach to analyse the magnitude of Dehnel’s phenomenon, we
compared the slopes of the regression lines of skull height over time
(years) between juveniles vs. subadults and subadults vs. adults (Figure
6b, scatterplots in Figure S3, see also Figure 4a). A lack of
interaction between the year and age (parallel lines) would indicate no
change in Dehnel’s phenomenon. We explored this in an ANCOVA model of
the skull height with the skull length, year, age, and year × age
interaction (Table 4). Year and skull length were highly significant in
the comparison of July juveniles with subadults, August juveniles with
subadults, and subadults with adults (Table 4). The interaction between
year and age was highly significant in two comparisons (Table 4): the
decline in skull height from July juveniles to subadults was greater
with time (Figures 6b and S3a), and the regrowth in skull height from
subadults to adults increased over time (Figures 6b and S3c). In both
cases, this was due to the very steep line in subadults. In contrast,
the interaction was nonsignificant in the comparison of August juveniles
with subadults (Table 4, Figures 6b and S3b). The lines of July and
August differed significantly in slope (the interaction between month
and year: F 1,198=12.60, P=0.0005 in ANCOVA with
year: F 1,198=14.72, P=0.0002; month:F 1,198=12.55, P=0.0005; skull length:F 1,198=27.57, P<0.0001; Figure 6b). The
skull width of juveniles (July, August) and adults did not change
significantly over time (Table S2). The year × age interaction was not
statistically significant in any of the three models for
length-corrected skull width (Table S3). When these interactions were
removed from the models, age was significant only in the comparison of
subadults and adults, and the year was never significant (Table S3).
To discuss the impact of weather on Dehnel’s phenomenon, we additionally
tested the regressions of all weather parameters on time (years) in
July, August and January between 1955 and 1985. The only significant
regression was that of the August mean daily temperature (positive
relationship; F 1,29=6.36, P =0.0175).
However, the mean daily temperature in January decreased significantly
during the first 60% of the tested period (1955 to 1972;F 1,16=8.16, P =0.0114, andF 1,15=19.9, P =0.0005 when one clear
outlier was removed; Figure S2c).