Discussion
We applied 3D geometric morphometrics on humerus and cranial osteology
in crocodile newts, a group of largely understudied Asian salamandrid
salamanders with polymorphic reproductive strategies, in order to reveal
patterns of SD and how it is linked to the different mating behaviours.
Our study does add new insights into the evolutionary forces leading to
morphological differences between the sexes in urodeles. We confirmed
our hypothesis that the different reproductive ecologies of crocodile
newts lead (at least partly) to different patterns of SSD and SShD, as
well as evolutionary trajectories of SD.
In salamandrids, it has been shown that allometric trajectories differ
between taxonomic units allowing higher flexibility for shape evolution
(Ivanović et al., 2007, Cvijanović et al., 2014, Ivanović & Arntzen,
2017). Even within populations of a species, differences in allometries
can be traced (Ivanović & Kalezić, 2012). Most of the pleurodeline newt
species showed similar allometric trajectories but some were able to
evolve into a different direction. Although there was a high flexibility
in allometries across different genera, similar allometric shape changes
were observed throughout the pterygoid, quadratum and the decrease of
the occipital region (Ivanović et al., 2012). Apparently the anterior
skull shape was therefore more flexible. We found similar allometric
trajectories between males and females in all species. In some
populations of selected European newts, allometries differ between males
and females (Ivanović et al., 2008) whereas in the most basal true
salamander this is not the case (Pogoda et al., 2020). In general put on
record, if selection acts on size in one sex, this can lead to shape
differences due to allometric shape changes during growth (Ivanović &
Kalezić, 2012, Pogoda & Kupfer, 2020). Complex interactions of
different allometric trajectories between species and sexes make it
complicated to pinpoint individual selective mechanisms. The different
allometric trajectories between species likely lead at least partly to
differences in shape as well as size differences in cranium and humerus
occur in the different newt species. Thus, SShD-patterns can vary based
on the allometric shape changes and the degree of SSD in a species.
Further, some body parts in urodeles are rather less strongly ossified
right after metamorphosis e.g., including the cranium, metacarpals and
–tarsals but they ossify during ontogenetic growth. This could explain
further shape variation in larger crania. Crocodile newts exhibit
elaborated bony ridges and ornamentation on the cranial skeleton which
may increase due to ongoing ossification during aging and growth,
leading to more elaborated squamosal bony ridge at the posterior part.
Increasing connection with size of the maxillary with the quadratum and
pterygoid can likely be attributed also to the ongoing ossification
during life.
Sexual shape dimorphisms in the cranium of other newts and salamanders
comprise differences in vomer length, quadrate position, occipital
region size, skull width and snout shape (Ivanović et al., 2008,
Ivanović & Kalezić, 2012, Alarcón‐Ríos et al., 2017, Pogoda et al.,
2020). These shape changes were also present in crocodile newts, but
they additionally differ in the extent of the squamosal bone. Different
shape changes from males to females between populations were already
indicated in other salamandrids like Lissotriton (Ivanović &
Kalezić, 2012), Ichthyosaura (Ivanović et al., 2009),Salamandra (Alarcón‐Ríos et al., 2017) and Salamandrina(Romano et al., 2009, Pogoda & Kupfer, 2020). Different ecological
selective forces each population of a species endures at its specific
site, likely play a major role on the maintenance of subtle shape
differences between species (Kalezić et al., 1992, Schäuble, 2004,
Angelini et al., 2015). Crocodile newts inhabit a large distribution
area from Nepal to Japan, including a variety of habitats from lowland
rainforest to temperate, mountainous grasslands (Hernandez, 2016, Wang
et al., 2018) forcing specific adaptations to those contrasting
environments. On the other hand, this group of salamanders showed high
conservatism in their macro-ecological differentiation (Hernandez et
al., 2018) likely leading to little shape differentiation between
species as well (Pogoda et al., 2020).
Sexual selection and sex roles during reproduction are a main force
forming body shapes (see Darwin, 1871, Fairbairn et al., 2007).
Previously, we found already that the mating mode partly explains shape
variation in crocodile newts (Pogoda et al., 2020). While the mating
mode is explaining the different SSD and SShD-trajectories of cranial
morphology between species, it does not apply to the humerus, although
they differ in shape between mating modes. As both sexes of amplecting
species respond differently in cranial shape changes if referenced
accordingly to the mean cranial shape but not in the humerus (Fig.9)
differences of SD-trajectories of the cranium but not humerus morphology
might be explained. Further, as the cranium is much more complex build
it provides more possibilities for shape variation than the humerus
does. Only amplecting species exhibit SSD in the body parts investigated
herein. Often it is tried to explain cranial shape differences with
differences in food niches (Shine, 1989, Shetty & Shine, 2002, Ivanović
& Kalezić, 2012, Reinhard & Kupfer, 2015) however our knowledge on
food niche differences in salamanders is incomplete and a definite
association of cranial shape differences with food niches is yet
untested. No intersexual differences in food allocation were found inT. podichthys , the only studied crocodile newt in terms of
trophic ecology (Phimmachak et al., 2015b). Size differences and slender
humeri shape likely facilitate clasping (compare to Pleurodeles )
while more robust and ossified fore limbs provide better standing during
circular dancing. This is contradictory to patterns found in European
newts also applying a stereotypic courtship behaviour. Among European
newts males regularly bear longer fore limbs (e.g., Malmgren &
Thollesson, 1999, Çiçek et al., 2011, Reinhard & Kupfer, 2015). We
demonstrated that different selective forces are acting differently on
the crocodile newt taxa inducing interspecifically different allometric
trajectories while the trajectories were constant between the sexes. We
are aware that our sample size of some of the studied species and/or sex
was relatively small, mainly attributable to the overwhelming male bias
of crocodile newts housed in natural history collections sometimes with
a male/female ratio of about 30/1 but GM was proved in being well
capable of revealing even quite subtle shape differences (e.g., Blanco
& Godfrey, 2006, Abdel-Rahman et al., 2009, Arendt, 2010, Ivanović &
Kalezić, 2012, Pogoda et al., 2020)
We conclude that morphological adaptations between species take place in
correlation with and probably adaptation to their occupied environments
which include large variation from the tropics to temperate climate
zones while the selective forces stay more or less constant in the
entire group as sex roles do not diverge, although some aspects of
reproduction and courtship adapted. The interaction of variable
allometric trajectories of species and sexes might partly explain the
interspecific differences in SD patterns observed. Additionally,
ecological parameters might influence strength and direction of SD
across populations. The mating mode is to some degree an explanatory
variable of the interspecific cranial shape variation in the
SD-trajectories. The humerus differed with mating mode but other
selective forces shape SD-trajectories of this specific body part.
However, to understand evolutionary processes future studies especially
on the ecology of the enigmatic crocodile newts are needed. Numerous
species were just recently identified based on molecular data leading to
an enormous increase in species numbers over the last decade (Stuart et
al., 2010, Shen et al., 2012, Nishikawa et al., 2013a, Nishikawa et al.,
2013b, Hou et al., 2014, Nishikawa et al., 2014, Yang et al., 2014,
Khatiwada et al., 2015, Le et al., 2015, Phimmachak et al., 2015a, Qian
et al., 2017, Grismer et al., 2018, Grismer et al., 2019, Zaw et al.,
2019, Bernardes et al., 2020, Pomchote et al., 2020, Poyarkov et al.,
2021). However morphological distinct characters are hard to identify
and often appear somehow descriptive summarising differences of
particular body part proportions based on a few specimens without any
accounting for SD. Unfortunately no information on ecology from the many
recently described species is available and observations in captivity
are often based on individuals of uncertain genetic identity. It will
become more crucial to focus more on the intraspecific morphological
differences applying an integrative approach for future taxonomic
research on crocodile newts. Especially the subgenus Yaotritonincludes genetically distinct lineages not accessed accurately so far
(e.g., Wang et al., 2018, Bernardes et al., 2020). Future research has
to take into account larger taxonomic units with broader ecological
niches to resolve evolutionary processes and mechanisms of SD on a
larger scale.