3. Results
3.1 The change of skull
size from 1953 to 2004
The skull height and width, but not the length, were larger in adult
males than in females in S. araneus , as confirmed in the ANCOVA
models with sex and year (F 1,110=7.20,P =0.0084; F 1,110=11.25, P =0.0011;F 1,110=1.00, P =0.320; the interaction
between the sex and year added to the models was nonsignificant:F 1,109=0.14, P =0.711;F 1,109=0.36, P =0.552;F 1,109=0.37, P =0.543). Nonetheless, as we
were mainly interested in relative change and because otherwise sample
sizes would have been too small, we pooled the sexes for further
analyses.
The skull height and width of juvenile and adult S. araneus , as
well as the skull length of juveniles, decreased significantly over the
sampling period. In contrast, the relationship between skull length and
time was not significant in adults (linear regression models; Table 1;
regression equations are in supporting information, Table S1).
Characteristically, all these relationships (except skull height in
adults) revealed a common pattern when smoothed with LOESS: little
change until the early 1970s and decreases thereafter (Figure 4;
scatterplots in supporting information, Figure S1). The decrease in
skull width of adults from 1970 to 2004 was even sharper than that in
the entire time period (F 1,70=11.46,P =0.0012). The decrease in the skull length of adults was
significant when restricted to this time period
(F 1,70=8.79, P=0.0041 andF 1,69=4.99, P=0.0288 when one outlier in 2004 was
removed).