Introduction
Sexual dimorphism (SD) subscribing marked differences between sexes such
as in morphology is a common phenomenon in the animal kingdom (e.g.,
Fairbairn et al., 2007). Different theories try to explain the causal
relationship between selection and morphology, namely i. fecundity
(e.g., Kupfer et al., 2004), ii. sexual selection (Shine, 1979) and iii.
ecological niche portioning (Hedrick & Temeles, 1989, Shetty & Shine,
2002). In context with body architecture, SD can be expressed as size
dimorphism (SSD) or shape dimorphism (SShD) underlying different
selection processes (e.g., Schwarzkopf, 2005, Kupfer, 2007, Pogoda &
Kupfer, 2018). While SSD is better studied and known in many vertebrates
(e.g., Cox et al., 2007, Lindenfors et al., 2007), SShD needs much more
attention as it substantial influence species ecology (e.g., Shetty &
Shine, 2002, Alcorn et al., 2013, Pogoda & Kupfer, 2018, Pogoda et al.,
2020).
To understand the processes leading to different morphologies between
males and females, comparative studies are needed allowing conclusions
from species ecology to its morphology. Amphibians are a vertebrate
group with heavily female-biased SSD (e.g., Shine, 1979, Kupfer, 2007).
Nevertheless, only approximately 61% of known salamander species
exhibit female-biased SSD, while about 19% exhibit a male-biased SSD
(Kupfer, 2007, Amat, 2019). Although less diverse in terms of species
numbers, urodeles evolved various reproductive modes and behaviours
including diverse life history strategies (Sparreboom, 2014, Kieren et
al., 2018). As Salamandridae or true salamanders includes most of the
variability in reproductive biology known from urodeles (Sparreboom,
2014, Frost, 2018), making the group well-suited for the investigation
of SSD and SShD.
Pleurodeline newts, named Pleurodelini, comprise a diverse monophyletic
group of salamandrid salamanders (e.g., Veith et al., 2018, Wang et al.,
2018) thus, being important in the reconstruction and understanding of
the evolutionary processes forming SD among Salamandridae (Pogoda &
Kupfer, 2018). Among Pleurodelini monophyletic crocodile newts evolved a
variety of reproductive modes and strategies (Salvador & García-París,
1999, Hernandez, 2016, Kieren et al., 2018). For mating either a ventral
amplexus or a circular mating dance is performed either taking place in
aquatic or terrestrial habitats. Also, female crocodile newts deposit
egg clutches either in water or on land (Igawa et al., 2013, Phimmachak
et al., 2015b, Pasmans et al., 2017, Gong et al., 2018, Hernandez,
2016). These differences in terms of mating and reproductive ecology can
even be observed within one genus Tylototriton comprising most
crocodile newt species. Some of the reproductive traits correlate with
phylogeny e.g., oviposition site, while others do not e.g., mating mode.
Nevertheless, both phylogeny and phylogenetic independent ecological
traits leading to coevolution of similar cranial shapes among crocodile
newts (Pogoda et al., 2020). So far, SD was rarely investigated in
crocodile newts (Seglie et al., 2010, Phimmachak et al., 2015b). Many
studies are regularly descriptive and limited to a few body measurements
such as snout-vent length, cloacal size and some others (e.g., Khatiwada
et al., 2015, Fei & Ye, 2016, Hernandez & Hou, 2018) often with low
sample sizes. A variety of SSD and SShD patterns such as longer and
wider trunks in females and longer limbs and crania in males are known
in many other salamandrids (e.g., Malmgren & Thollesson, 1999, Romano
et al., 2009, Amat et al., 2015, Reinhard et al., 2015, Reinhard &
Kupfer, 2015, Altunışık, 2017). Consequently, it is likely that also a
variety of dimorphic traits are undetected so far in crocodile newts.
There is even less known about SD in the osteology of urodeles in
general (Ivanović & Kalezić, 2012, Pogoda & Kupfer, 2018). Researchers
just started exploring this field of morphology research accessed mainly
by modern non-invasive CT technology (Broeckhoven & du Plessis, 2018).
In a terrestrial salamandrid salamander, it was shown that the same
patterns of SSD can be detected in the osteology as in the external
morphology (Pogoda & Kupfer, 2018). Further, excluding soft tissue
leads to an enormous increase of morphological structures which can be
used in studying especially SShD otherwise covered. In ventral
amplecting Pleurodeles , the sister taxon to the crocodile newt
genera Tylototriton and Echinotriton , differentially
shaped humeri between sexes are known for long time (e.g., Herre, 1952).
The common ancestry of ribbed and crocodile newts and the
interspecifically different reproductive strategies may imply variable
SD patterns tightly linked to ecology. Understanding SSD and SShD
patterns in context with phylogeny and ecology will aid understanding
the evolutionary biology of salamanders. Thus, rather then just
providing intersexual comparisons of measurements or shape data, these
must be linked to other traits of the investigates system in order to be
able to reveal potential selection sources leading to observed
morphological differences.
The aim of our study was to investigate SD in crocodile newts, linking
patterns to the different reproductive ecologies of the species. Our
focus was laid on the cranium and fore limb morphology. Male cranial
morphology was different among amplectant and dancing species (Pogoda et
al., 2020). Thus, we assumed different interspecific patterns of SShD of
crocodile newt cranial morphology applying different mating modes. Fore
limb morphology is of special interest as the fore limb likely
experiences different mechanical needs when a ventral amplexus is
applied or not. We hypothesize that species which apply an amplexus
during mating have a more pronounced SSD of their fore limbs than
species mating without physical contact and that the different mating
patterns lead also to differences in SShD between these reproductive
groups. We used µCT scans of crania and humeri of crocodile newts and
employed 3D geometric morphometrics (GM) to test our hypotheses.