Discussion
The comparative analysis presented here documents both intra- and
interspecific differences in shape complexity within the amblypygid
pedipalp. A principal component analysis of Fourier harmonics shows
frequent overlap between species, with intraspecific shape variation
often exceeding shape differences between species, even in the
relatively low numbers of individuals representing each taxon in this
study. MANOVA does identify statistically significant pairwise shape
differences between most species. However, unsupervised k-means
struggles to define species clusters accurately. Of note, the twoDamon species, D. diadema and D. medius , could not
be separated on the basis of MANOVA, k-means or within the PCA
morphospace in either segment. Likewise, some species ofParaphrynus could not be differentiated on the basis of shape
complexity in either segment. As such, our results do not support
Hypothesis 1.
This result has potential implications for species recognition using
pedipalp characters. MANOVA, a ‘supervised’ method which knows which
specimens belong to each species a priori , could define
differences between species. However, the unsupervised k-means methods,
which has no prior knowledge of which species specimens belong to, could
not. K-means is arguably a better analogue for the placement of a
specimen within a species, as a taxonomist, like the k-means algorithm,
would have no prior knowledge of specimen’s placement within a species.
Notably, previous studies have identified species of Damon andParaphyrnus partially on the basis of the relative length of
spines on the femur and tibia pedipalp segments (Weygoldt, 2000;
Prendini, Weygoldt & Wheeler, 2005; Joya & de Armas, 2012). Although
our results do not invalidate the use of any specific pedipalp tibia and
femur characters to define species (as our analysis investigates overall
pedipalp shape), we do urge caution when differentiating between closely
related species using pedipalp traits.
In support of Hypothesis 2, a significant negative relationship between
complexity and pedipalp length was identified in both segments. Visual
inspection of Fourier outlines suggests that complexity predominantly
reflects the relative length of major pedipalp spines (Figure 2).
Interestingly, a similar pattern was revealed using GMM in D.
variegatus , with longer pedipalps having shorter spines. Longer
amblypygid pedipalps are therefore found to have relatively shorter
‘major’ spines once normalised to centroid size. Previous work has
demonstrated the importance of display in the evolution of pedipalp
morphology. Roughly, 80% of conflicts in P. longipes are decided
via display in the favour of the individual with longer pedipalps
(Chapin & Reed‐Guy, 2017). Pedipalp display has also been observed in
territorial contests in a number of amblypygid species (Fowler-Finn &
Hebets, 2006; Porto & Peixoto, 2013; Chapin & Hill-Lindsay, 2016) and
during the first stages of courtship across the group (Weygoldt, 2000;
Chapin & Hebets, 2016). This suggests that amblypygid pedipalps are
likely under the influence of sexual selection via mate choice, and that
resource holding potential may be correlated with pedipalp length.
It may therefore be possible that a trade-off between pedipalp length
and complexity exists, with some species taking a less complex form,
with relatively shorter spines, allowing for increased pedipalp length,
developing under the pressures of sexual selection and contest, to be
achieved at lower developmental cost. Hypothetically, a less complex
form with shorter spines may be less energetically costly to produce per
unit length, which may allow individuals to achieve longer pedipalps at
lower energetic cost, though as yet we know nothing of genetics of
development in amblypygids, and such inferences are therefore
limited.
There is some evidence for structures becoming less complex with
increasing size within arachnids, although little is known about
interspecific trends of shape complexity in arachnids. For example,
cheliceral dentition is known to be more pronounced in juveniles and
relatively decreased in the large chelicerae of adults (Solifugae),
though this may in part be a function of cheliceral wear (Bird, 2015).
Some pedipalp spines also appear reduced in male thelyphonids with large
pedipalps (Rajashekhar & Bali, 1982). However, it is often the case
that structures under the influence of sexual selection are larger and
more complex in arachnids. For example, male spider legs that are used
in courtship display are longer than female conspecifics and often
possess elaborations such as ridges of setae (Peckham & Peckham, 1889;
Kronestedt, 1990; Girard & Endler, 2014). Furthermore, the fourth legs
of male opiliones, used in male-male contest, are also longer and have
larger elaborated coxal apophyses not seen in females (Willemart et al.,
2009; da Silva Fernandes & Willemart, 2014). Thus, though a trade-off
may be possible, evidence from other arachnids provides mixed support.
However, it must be stressed that these examples are all intraspecific
and thus may not be a good analogue for the interspecific trends we
report in this work.
Another possibility is that that spine length is constrained by
selective regimes or developmental factors, meaning spines have simply
remained at a similar absolute size across species. In this case,
species with longer pedipalps would have relatively shorter spines. For
example, spine length could be limited by developmental constraints
relative to pedipalp length. Additionally, it is possible that
absolutely longer spines may provide limited additional functional
benefits to prey capture, and are not important in display-based
contest, meaning there is little advantage to spine length increasing
with isometry relative to pedipalp length. Pedipalp spines are thought
to primarily function in prey capture, with a number of species forming
pedipalp ‘catching baskets’, which are hypothesised to help capture and
secure prey items. Little is known about amblypygid diets, but they are
thought to be largely composed of primary consumer arthropods,
especially of the orders Orthoptera and Blattodea (Chapin & Hebets,
2016). If the primary function of spines is indeed in prey capture,
spine size may scale more closely with prey size than pedipalp size.
Therefore, should prey size remain similar or equal across the group,
spine size may also remain similar across species leading to relatively
smaller spines in species that have attained longer pedipalps due to
pressures of sexual selection and contest. However, this hypothesis can
only be properly investigated by addressing the current paucity in data
related to diet in amblypygids.
An analogue to the palpal form in Amblypygi is found in scorpion
pedipalp chelae, which increase in size at rates much lower than
isometry with relation to body size (Van der Meijden, Herrel & Summers,
2010). This may be because species with vastly different body sizes feed
on similarly sized prey (Polis & McCormick, 1986), suggesting there is
little added benefit to larger species producing equivalently sized
chelae. If a similar overlap in prey size exists across amblypygid
species, spines (which are hypothetically under the most direct
selection for prey capture) may also scale at rates lower than isometry
relative to pedipalp length. More work is needed into amblypygid life
history and social dynamics to determine the relative importance of
sexual selection, selection via territorial contest, and natural
selection via prey capture.
As part of this study, sexual shape dimorphism was quantified across
amblypygids for the first time. Significant sexual dimorphism in shape
complexity is identified in the amblypygid pedipalp tibia, with female
tibiae found to be characterised by greater complexity than their male
counterparts. As previously discussed, complexity appears to be
influenced by the length of the major spines, which are relatively
larger in the female tibia. This may be due to the increased length of
the male pedipalp, which develops under the pressure of sexual
selection. A similar pattern has already been identified using GMM
analysis of the pedipalps of D. variegatus , with the observed
sexual shape dimorphism being thought to arise due to female optimising
for prey capture while males optimise for pedipalp display (McLean,
Garwood & Brassey, 2020). The occurrence of this form of dimorphism in
numerous species across the group may suggest that these driving forces
behind sexual dimorphism in size and complexity are pervasive across the
group. Interestingly, contest and courtship both involve display and are
very similar in behaviour across amblypygid species, suggesting this
could be a common selection pressure (Weygoldt, 2000). However, more
work into amblypygid life history is needed to fully understand the
drivers of pedipalp morphology.
Conclusion
In conclusion, our results highlight
the importance of considering intra- and interspecific variation in
terms of shape, by highlighting a number of previously undocumented
shape relationships across amblypygid pedipalps. Here we find that
within-species shape complexity variation in pedipalps can occasionally
exceed differences between species, and thus caution needs to be taken
when defining species on the basis of pedipalp characters. We also find
that gross complexity decreases with increasing pedipalp length,
potentially uncovering a trade-off between investment in pedipalp total
length (for use in sexual selection and territorial contest) and
pedipalp spine length (primarily for use in prey capture). Sexual
dimorphism in gross complexity is also present in the tibia and follows
similar patterns seen intraspecifically in other studies, once again
highlighting the pressures of display based contest and courtship, and
trophic niche partitioning. Future studies that look to address the
paucity of data on amblypygid life history, ecology and social dynamics
will be valuable in further understanding the functional morphology in
this unique system.