Sexual
dimorphism in morphology and bite forceinthe mountain dragon,Diplodermabatangense (Squamata:
Anguidae), from the
Hengduan Mountains in western China
Ling Li1,2, Gang Wang3, Yuhao
Wen2, Yihua Xiang2, Peng
Guo2, Bingjun
Dong1, Yayong
Wu2
- College of Life Sciences,
Shenyang Normal University, Shenyang, Liaoning 110034, China
- Faculty
of Agriculture, Forest and Food Engineering,
Yibin University, Yibin, Sichuan
644000, China
- College of Chemistry and Life Sciences, Chengdu Normal University,
Chengdu, Sichuan 611130, China
Correspondence
Bingjun Dong, College of Life Sciences, Shenyang Normal University,
Shenyang, Liaoning 110034, China. Yayong Wu, Faculty of Agriculture,
Forest and Food Engineering, Yibin University, Yibin, Sichuan 644000,
China.
Email:dongbingjun@synu.edu.cn(DBJ); cdwyy201101@163.com(WYY)
Abstract: Lizards exhibit sexual dimorphism in various
phenotypic traits and capability performance (e.g. bite force), while
the patterns of sexual dimorphism vary among genera and species.
The relationship between morphology
and function is dynamic, and selective pressures can differentially
affect bite performance in different
sexes. Here, we
analyzed sexual dimorphism in
morphological traits and bite force and explored the determinants of
bite force in the mountain dragon (Diploderma batangense ) from
western China. A total of 135 individuals were collected, and nine
morphological characteristics and bite force were measured.
We
found a significant male bias in morphology and bite force performance,
with males found to be larger than females in regard to head size, body
size, limb length, tail length and bite force, which could be explained
by sexual selection. Results also showed that bite force was positively
correlated with head size, with the
determinant affecting bite force differing between males (mouth length)
and females (head width), which may be related to their different roles
in reproduction. This study provides a reference for understanding the
ecological adaptability of mountain dragons in the field and lays a
foundation for revealing the mechanism of bite force variation between
the sexes.
Keywords: Bite force; Lizards; Sexual dimorphism; Morphology;
Sexual selection
Introduction
Sexual
dimorphism, including differences in size, color, and capability
performance between the two sexes of a species, is widespread in
amphibians (Schäuble, 2004), reptiles (Zhong et al., 2017), mammals
(Mitani et al., 1996), birds (Fairbairn & Shine, 1993), and
invertebrates (Jozet-Alves et al., 2008).
Three
main hypotheses have been
proposed to explain the evolution of sexual dimorphism in animals
(Cruz-Elizalde et al., 2021). The first proposes that male-biased sexual
dimorphism can be explained by
sexual selection, in which larger males gain greater advantages in
resource allocation, territorial defense, and mate choice
(Endler & Houde, 1995;
Cruz-Elizalde et al., 2021). In contrast, the female-biased fecundity
advantage hypothesis suggests that larger females are selectively
favored by natural selection and tend to produce larger litter
clutches/size or egg production/offspring size (Cox et al., 2003;
Pincheira‐Donoso & Hunt, 2017; Cruz‐Elizalde et al., 2021). The third
hypothesis of intraspecific niche
divergence proposed by Rand (1967) and Schoener (1967) suggests that
differential use of one or several resources (e.g., food, microhabitats,
and territory) can reduce male-female competition when resources were
limited (Bolnick & Doebeli, 2003; Pincheira‐Donoso et al., 2018).
Sexual dimorphism in morphological traits and performance is prominent
in many lizard species, which can be explained by the above hypotheses
(Cruz-Elizalde et al., 2020, 2021). Indeed, sexual selection, fecundity
advantage, and niche divergence are not mutually exclusive and may have
combined effects on body size and other sexually dimorphic
characteristics (Ji et al., 2006; Dashevsky et al., 2013; Zhu et al.,
2022). In general, males are larger in body size (e.g., body length and
mass) but smaller in reproductive-related characteristics (e.g., body
cavity) than females (Olsson et al., 2002). Consequently, it can be
difficult to determine the causes of sexual dimorphism associated with
each hypothesis (Cox et al., 2003; Kuo, 2009).
In reptiles, bite force is a comprehensive measure of
performance across multiple
ecological and behavioral activities, including
resource allocation, territorial
defense, aggressive interactions, and mate acquisition (Huyghe et al.,
2005; Kaliontzopoulou et al., 2012; Naretto et al., 2022). In general,
adults with a greater bite force are more likely to win intrasexual
contests (Wegener et al., 2019; Taverne et al., 2020). Furthermore,
during copulation, male lizards often bite the abdomen or neck of the
female to restrict movement or escape, thereby increasing their mating
success rate (Noble & Bradley, 1933; Herrel et al., 1996; Herrel et
al., 2010). In most reptile species, males tend to exhibit larger biting
capacity than females (Herrel et al., 2007; Sagonas et al., 2014;
Naretto et al., 2022). For example,
male Achala copper lizards (Pristidactylus achalensis )
demonstrate stronger bite force than females,
including lateral and frontal bites
(Naretto et al., 2022). Furthermore,
in species showing no differences in body size between males and
females, head size may still exhibit dimorphism due to the intrinsic
differences in energy requirements between males and females, leading to
dimorphism in bite force (McBrayer & Anderson, 2007). In this context,
females may evolve specific physical structures (e.g., feeding
structure) to enhance energy acquisition under selective pressure
(Bulté et al., 2008). For example,
female northern map turtles (Graptemys geographica ) can consume
larger snails and access more food resources than males due to their
stronger biting capacity (Bulté
et al., 2008).
Investigating the association between sexual dimorphism in
morphology
and bite force has been a topic of research in reptiles for many years.
Notably, several studies have reported that head morphology directly
affects bite force (Herrel et al., 2007; Sagonas et al., 2014; Naretto
et al., 2022) and differences in head size generate different functional
consequences (McBrayer & Anderson, 2007). In general,
taller and wider heads are
associated with larger bite force, resulting in species, age, and sex
differences (Taverne et al., 2020). As such, differences in
bite performance between males
and females are expected due to sexual dimorphism in head size, which is
common in lizards (Herrel et al.,
2007). However, the best predictors of bite force tend to differ between
the sexes due to differences in bite use (e.g., intrasexual competition,
intersexual interactions, and diet) (Herrel et al., 2007; Naretto et
al., 2022). For example, the traits that best predict bite force differ
between male and female tuatara (Sphenodon punctatus ), which is
likely due to sexual selection
(Herrel et al., 2010). Variation in
bite force between the sexes is often caused by differences in overall
size, especially variation in bite force driven by specific head traits
(e.g., head width and depth) (Lappin et al., 2006; McBrayer & Anderson,
2007). Despite the assumption that observed differences in head size
lead to parallel differences in bite force, which are dependent on the
bones and muscles of the head, the relationship between head size and
bite force remains unclear (Herrel et al., 2007; Huyghe et al., 2009).
Differences in jaw muscle architecture and physiology, including
skeletal lever system, pennation angle, muscle arrangement, and relative
size, may result in differences in bite force
(Herrel et al., 2007; Fabre et al.,
2014). Consequently, exploring the link between bite force and
morphology in sexual dimorphism may help to disentangle the potential
mechanisms related to the evolution of sexual dimorphism
(Kaliontzopoulou et al., 2012). A significant association between
morphology and bite force in sexually dimorphic species may provide
empirical evidence for the co-evolution of both traits and may point to
the joint effect of selection on form and function.
In the current study, we explored
sexual dimorphism in morphological traits and bite force inDiplodermabatangense ,
an oviparous lizard species endemic to small areas of brush habitat
along the upper Jinsha River near Batang County, Sichuan Province, China
(Wang et al., 2021). The species has been listed as a “National
Protected Key Wild Animal” in China since 2021, primarily due to
anthropogenic disturbance and habitat loss caused by hydropower and road
construction, resulting in fewer thermoregulatory sites. At present,
little is known about the biology and ecology of D. batangense ,
thereby limiting species-specific management strategies. Here, we
investigated a D. batangense population in the type locality and
addressed two main questions: (1) Do males and females differ in
morphological traits and bite force? (2) Is there a relationship between
head morphology and bite force in this species?
Materials and Methods
Study site
The study was carried out in the upper Jinsha Valley in Batang County
(29.46°N, 99.06°E, 2 497 m a.s.l.) on the Sichuan-Tibet border in
western China (Fig. 1). The area is characterized by a typical
dry-hot valley-plateau climate, with
a dry spring, cool summer, rainy autumn, and warm winter. Annual average
temperature is 13.8 °C, with
minimum and maximum monthly means of 3.9 °C and 19.9 °C in December and
June, respectively. Annual mean precipitation is 500 mm, with more than
80% falling between June and September. The region shows sparse
vegetation habitat, low vegetation cover (less than 50%), and sandy and
rocky soil substrate. The vegetation is dominated by shrubs (e.g.,Indigofera lenticellata , Sophora moorcroftiana , andRumex hastatus ) and herbs (e.g., Incarvillea arguta andArtemisia stechmanniana ).
Fig. 1 Distribution (a),
habitat (b), and a male specimen (c) of Diploderma batangense .
Red dot represents specific study site.
Animal sampling
A total of 135 D. batangense lizards (77 males and 58 females)
were captured by lasso during daylight hours (10:00am–17:00pm) in
August 2020. The capture location was marked using a uniquely coded
plastic card. Each lizard was sexed by hemipenis eversion and dorsal
color pattern. Bite force and body temperature (BT) were measured
immediately after capture. All lizards were numbered and taken to a
makeshift laboratory in cloth bags. All lizards were released to their
original capture site after collection of data. All animal procedures
were carried out in accordance with and approved by the Animal Care and
Use Committee at Yibin University (YBU2020005).
Morphometrics
For all lizards, the following data were collected using a digital
caliper (DL91200, Deli, Ningbo,
China) to the nearest 0.01 mm: snout-vent length (SVL); tail length (TL,
distance from vent to tail tip); abdomen length (AL, distance from
posterior base of forelimb to anterior base of hindlimb); head length
(HL, distance from anterior tip of rostral scale to posterior aspect of
parietal); head width (HW, maximum width of head taken at posterior end
of mandible); head height (HH, distance from gular region to highest
part of head); mouth length (ML, distance from tip of snout to end of
opening); forelimb length (FLL, distance from forelimb to wrist joint);
and hindlimb length (HLL, distance
from hindlimb to ankle joint). Body mass (BM) was determined using a
digital scale (I-2000, Mengfu, Dongguan, China) to the nearest 0.01 g.
Bite force
To explore the relationship between
bite force and BT, anal temperature was collected before
each bite using an ultra-thin
catheter electronic thermocouple (UT323, +0.01° C; Victor, Shenzhen,
China) in the cloaca. In vivo bite force was measured using a
piezoelectric force transducer
instrument (VXT500, 0–50 N;
Viste, Shenzhen, China), consisting of a pair of bite plates (2 mm
thick, accurate to 0.1 N), hand-charged amplifier, and digital display.
To reduce damage to teeth, the end of each steel plate was covered with
non-toxic rubber. The instrument was calibrated before each bite. The
bite plate was placed at the unilateral molar, a posterior bite position
where maximum bite force can be recorded. During measurement, bite force
data were collected by gently pinching the head and tail. Bite force was
measured three times per individual,
and the maximum value was the maximal bite force for each lizard
(McBrayer & Anderson, 2007). Bite
force trials were separately conducted three times in the field and
indoors to minimize variation. Based on the six tests, only the maximum
bite force value and the corresponding anal temperature were retained
for further analysis.
Statistical analysis
To investigate differences in
morphological traits and bite
force between males and females, we first performed analysis of variance
(ANOVA) for morphological measurements (SVL, BM, HL, HW, FLL, and HLL)
and bite force values. As some samples (TL, AB, HH, ML, and bite force)
showed non-normality, we also used the nonparametric Mann-Whitney (U)
test. To determine the effect of individual SVL on morphology and bite
performance, we further examined the relative differences in
morphological traits and bite force between the sexes using similar
methods while controlling for the
effect of SVL (covariate).
To test whether morphological traits could explain the observed sexual
dimorphism in bite performance, we first examined the relationship
between bite force and head morphology for both sexes using univariate
linear regression. Next, we performed stepwise multiple regression
analysis to confirm which morphological traits were the best predictors
of variation in bite force for males and females.
Significance level was set to P < 0.05. All data
analyses were performed using SPSS v26.0 (SPSS Inc., Chicago, IL, USA)
and graphs were generated using OriginLab Professional v2021b.
Results
Sexual dimorphism inmorphology
Marked morphological dimorphism was observed between the male and female
lizards. Males were significantly larger than females in all
morphological traits measured (SVL: F 1, 133=13.047, p = 0.001; BM: F 1, 133 = 18.797,p < 0.001; HL: F 1, 133 = 64.170,p < 0.001; HW: F 1, 133 = 53.253,p < 0.001; TL: Z 1, 133 = -7.656,p < 0.001; HH: Z 1, 133 = -5.810,p < 0.001; ML: Z 1, 133 = -6.585,p < 0.001; FLL: F 1, 133 =
63.407, p < 0.001; HLL, F 1, 133= 73.092, p < 0.001, Table 1, Figure 2), except for AL
(Z 1, 133 = -0.691, p = 0.498, Table 1,
Figure 2d). In addition, for a given SVL, males had greater head size
(HW: F 1, 133 = 44.112, p = 0.005; HL:F 1, 133 = 71.294, p < 0.001;
logHH: F 1, 133= 30.462, p <0.001; logML: F 1, 133 = 43.027, p
< 0.001), weight (F 1, 133 = 5.513,p = 0.020), TL (F 1, 133 = 79.160, p
< 0.001), and limb length (FLL: F 1,
133 = 49.567, p < 0.001; HLL:F 1, 133 = 54.447, p < 0.001,
Table 1) compared to females.
Figure 2 Comparison of morphological characteristics and bite
force between male and female Diploderma batangense . a) SVL,
snout-vent length; b) BM, body mass; c) TL, tail length; d) AL, abdomen
length; e) HL, head length; f) HW, head width; g) HH, head height; h)
ML, mouth length; i) FLL, forelimb length; j) HLL, hindlimb length; k)
Bite force; l) Ventral surface of male and female. Gray filled is male
and no fill is female; * Significant differences (p <
0.001).
Sexual dimorphism in
bite force and itsdeterminants
No significant relationship was detected between bite force and BT inD. batangense (F 1, 133 = 0.22, p =
0.133). However, significant differences in bite force were found
between males and females. In comparison to females, males had
significantly larger absolute
bite force (F 1, 133 = -4.494, p
< 0.001, Table 1, Figure 2k) and relative bite force
(F 1, 133 = 10.610, p = 0.001).
Univariate linear regression
analysis showed that bite force
was positively correlated with HL, HW, HH, and
ML (Table 2, Figure 3). Multiple
stepwise regression analysis showed that the determinants of bite force
were ML in males
(R2 =
0.467, p = 0.002; Table 2) and HW in females
(R2 = 0.378, p < 0.001; Table
2).
Figure 3 Relationship between bite force and head morphology inDiploderma batangense . Relationships between bite force and head
length (HL) (a), head width (HW) (b), head height (HH) (c), and bite
force and mouth length (MF) (d). Red points represent females and black
points represent males.
Discussion
Lizards
exhibit sexual dimorphism in various
phenotypic traits and behaviors, such as size, shape, weight,
coloration, bite force, and locomotor performance (Cox et al., 2003).
Three patterns of sexual
dimorphism have been identified in lizards
(Cruz‐Elizalde et al., 2021),
including male-biased (Herrel et
al., 2010), female-biased (Cox et al., 2009), and unbiased (Schwarzkopf,
2005), which vary among different genera and families (Cox et al., 2007;
Cruz‐Elizalde et al., 2021). TheD. batangense population from the Hengduan Mountains
demonstrated obvious male-biased
morphology and bite force
performance. Notably, theD. batangense males were markedly larger (i.e., equivalent body
size) than females in terms of head size (HL, HW, HH, and ML), body size
(SVL and BM), limb length (FLL and HLL), and tail length (TL), with
stronger bite force, although no differences in AL were observed. This
male-biased pattern has been widely reported in the lizard family,
including in Tropidurus(Tropiduridae), Anolis(Dactyloidae), Varanus (Varanidae), Sceloporus(Phrynosomatidae), and Diploderma (Anguidae) (Cox et al., 2007;
Kuo et al., 2009; Jiménez-Arcos et al., 2017). Interestingly, Kuo et
al., (2009) reported that closely related species D. swinhonis(previously Japalura swinhonis ) shows marked male-biased
sexual dimorphism in all
morphological variables, but when controlling for body size, males had
proportionately longer and wider heads, but shorter body length and
limbs. Xiong et al., (2022) also found
that D. micangshanensisshows significant male-biased sexual dimorphism in all morphological
variables, except for trunk
length, but when controlling for SVL, only head shape (HL, HW, HH, and
ML) and trunk length differ significantly between the sexes. Differences
in sexual dimorphism between closely related species may be related to
differences in habitats (Da et al., 2014), as reported inPodarcis hispanica andP. bocagei (Kaliontzopoulou et al., 2012).
Male-biased sexual dimorphism is
usually explained by sexual selection, which predicts that males will
evolve morphological features and capacities that enhance mating
success, whether through improved fighting ability against other males
or a better chance of attracting females (Endler & Houde, 1995;
Cruz-Elizalde et al., 2021). Previous studies have shown that larger
males (e.g., larger body size and head shape) are more likely to
outcompete and gain access to better resources such as food, territory,
and mates during antagonistic encounters (Herrel et al., 2010; Hierlihy
et al., 2013). Food availability is a determining factor of
intraspecific competition in many lizard species (Pafilis et al., 2009;
Donihue et al., 2016).Diploderma batangenseinhabits dry-hot valleys characterized by relatively scarce food
sources. In the studied D. batangense population, aggressive
male-male interactions were relatively common during and outside the
mating season, with larger individuals more likely to win in aggressive
interactions for food (unpublished data).
Forced copulation is an important
reproductive strategy in various territorial lizards (Rodda, 1992;
Herrel et al., 2010), in which females struggle physically and violently
in an apparent attempt to escape mounting males (Burghardt & Rand
1982). Diploderma batangense also showed aggressive and forceful
behavior during copulation, in which a male approached a female and
grasped her neck with his jaws. Therefore, the larger body size and bite
force in D. batangense may be related to actual mating success
and fitness (Lappin & Husak, 2005; Herrel et al., 2010).
Bite force is a comprehensive
measure of trait performance in multiple activities, including
courtship, conflict, and hunting, especially in males (Herrel et al.,
2007; Kaliontzopoulou et al., 2012;
Sagonas et al., 2014; Herrel et al.,
2018). Individual bite force capacity is directly related to head size
(McBrayer & Anderson, 2007; Herrel
et al., 2010; Kaliontzopoulou et al., 2012), generally increasing with
head size (Deeming et al., 2022; Naretto et al., 2022). This phenomenon
was found in present study, where bite force was positively correlated
with head size (HL, HW, HH and ML). Understandably, a larger head
results in a larger cross-sectional area of jaw muscles and more
developed jaw-cranial musculature, thus generating greater bite force
(Herrel et al., 2001). Therefore, increasing head height and
cross-sectional area of the jaw muscles may be a mechanism for
increasing occlusal force (McBrayer & Anderson, 2007). Head length,
head width, and mouth length can also affect bite force (Herrel et al.,
2001; Sagonas et al., 2014), not unexpectedly as bite performance can be
influenced by the size of the jaw musculature, ratio of the upper and
lower jaws, insertion point and direction of jaw musculature, and
microstructure of the muscle fibers (Herrel et al., 1998;
Kaliontzopoulou et al., 2012). Thus, external head morphology is a
readily accessible proxy for the joint action of structural components.
Interestingly, Lappin and Husak (2005) found only a weak correlation
between external head size and bite force in male collared lizards
(Crotaphytus collaris ) and suggested that internal skull
structure may be a key determinant of bite force.
Consequently, there is growing recognition that the
underlying musculature may drive changes in bite force, as demonstrated
in various Anolis lizard species, in which the adductor externus
and pseudotemporalis muscles are considered good determinants of bite
force (Wittorski et al., 2016).
Herrel et al., (2007) also proposed that testosterone levels can affect
the size of the adductor muscles of the outer jaw and may play an
important role in regulating bite force. However, further study is
necessary to fully elucidate the determinants of bite force.
Bite performance is expected to vary between males and females due to
differences in head shape and size (Herrel et al., 2007). Therefore,
the best predictors of maximal
bite force capacity tend to differ by sex. For example, lower jaw length
is the best predictor of bite force in Anolis carolinensis males,
whereas head width and jaw-closing in-lever are the best predictors in
females (Herrel et al., 2007). In our study, the best predictors of bite
force also differed between the males (ML) and females (HW). This
difference may be explained from a structure-function relationship:
i.e., with the relative increase in head width, the adductor cavity in
males is significantly larger than that in females, which may
accommodate larger mandibular adductor muscles and enhance interactions
between muscles and skull, thus improving bite force strength (Lappin et
al., 2006; Sagonas et al., 2014). The jaw musculature and skeletal
system are closely related to bite force, and it is clear from the
principle of leverage that the larger the male force arm (lower jaw
length), the greater the bite force generated (Herrel et al., 2007;
Herrel et al., 2010; Wittorski et al., 2016). Sexual dimorphism in
lizard bite force is generally thought to be related to fighting or
territorial defense in males, with an added advantage in intrasexual
competition (Naretto et al., 2022) and forced mating success (Sagonas et
al., 2014). Even among males of similar body length, success in combat
and forced mating appears to favor those with greater body robustness,
head size, and bite force (Lappin et al., 2006). Differences in head
size and performance may also be related to differences in reproductive
allocation between the sexes, with females investing more in
reproduction than males and requiring higher energy intake (Bulté et
al., 2008). Thus, head width, the best predictor of
bite force in females, may be related to energy requirements for
follicular development and offspring rearing (Naretto et al., 2022).
Although there was no advantage in head width between the D.
batangense females and males, females with wider heads had a larger
bite force compared to other females, and thus may be able to feed on
larger food items and provide more energy for their offspring.