3.3 | Correlated evolution of morphology and behavior
Of the morphological features that we tested for correlation with
behavior (see Table 1), we identified significant patterns of
correlation in eleven (Table 2, Fig. 3). Analysis using Pairwise
Comparisons (PC) and Maximum Likelihood (ML) were largely corroborative,
with strongest hypotheses of correlation returning the strongest
significance values in the PC analysis, and the highest delta-AICc
values for the independent model in the ML analysis (indicating the poor
performance of this model relative to models of correlation). Because
only cases that returned significant correlation in the PC analysis were
reanalyzed using ML, in the ML analysis the independent (uncorrelated)
model always performed the worst, returning the highest delta-AICc
values. Delta-AICc values were usually low for all dependent models,
signifying little difference in model-fit between different dynamics of
correlation.
Patterns of correlation between the spinnerets and behavior were as
expected: elongate posterior lateral spinnerets, widely-spaced
spinnerets, and pseudo-segmented apical segments of the posterior
lateral spinnerets were strongly positively correlated with the
construction of a web at the retreat entrance and to a lesser extent
with opportunist retreat construction (almost all web-builders are
opportunists), and were negatively correlated with burrowing and
entrance modification. In contrast, short apical segments of the
posterior lateral spinnerets were most strongly correlated with
structural modification to the burrow entrance, and negatively
correlated with opportunism and web-building (opportunists virtually
never structurally modify their retreat entrance). The rastellum
returned strong positive correlation with both burrowing and
entrance-modification, however the second pairwise comparison analysis
(PC2) revealed many cases of ‘neutral change’ with respect to the
rastellum and both of these behaviors. The serrula returned the opposite
pattern to the rastellum, being positively correlated with web-building
and opportunism, and negatively correlated with burrowing and entrance
modification, with negative correlation with burrowing returning the
strongest correlation. ‘Digging spines’ did not return a significant
correlation with burrowing, but instead with entrance modification
(positive). The presence of scopulae showed no correlation with any of
the behaviors tested. All modifications to the posterior legs –
enlargement relative to the anterior legs, dorsal bias in macrosetation,
and presence of a thorn patch on prodorsal patella III – showed a
strong positive correlation with both burrowing and burrow-entrance
modification. Finally, the presence of a common eye tubercle was
positively correlated with opportunistic burrowing (although there are
many cases of neutral change, see PC2), and a compact rectangular eye
group was negatively correlated with burrow-entrance modification,
however the two specific modifications to the eye group that were tested
for correlation (widening of the eye group and anteriorly-positioned
anterior lateral eyes) did not return significant correlation, despite
each only occurring in taxa with modified burrow entrances.