Size matters: Sexual dimorphism in the pelvic spurs of the Bahamian Boa (Chilabothrus strigilatus strigilatus)

Abstract

Determining gender in snakes almost always involves highly intrusive techniques that have the potential to harm or at least significantly stress the animal. Clear morphological differences of conspecific males and females, defined as sexual dimorphism, can provide an alternative strategy to determine the sex of a snake with little harm. The commonly used sexual size dimorphism (SSD), however, is not ubiquitously applicable and has its limitations for some taxa and different life stages. The size differences in pelvic spurs, a little investigated morphological feature in some snakes, could provide a minimally invasive method to determine sex in these snakes. We compared the pelvic spurs of 32 Bahamian Boas of different life stages and body sizes to find out if pelvic spurs provide a robust alternative to identify sex in these snakes. We found that male Bahamian Boas possessed spurs nearly twice as long and notably more curved than females and that the proportion of the spurs to body size differed significantly between the sexes. Furthermore, spur length increased considerably more with an increase in body size in males than in females. We conclude that the investigation of the pelvic spurs in these boas allows for a simple and minimally invasive approach to determine gender and may also represent an alternative strategy for other species.

Full text

Sexual dimorphism refers to differences in the
morphology, physiology or behaviour of conspecific
males and females (Mori et al., 2017). Knowledge
of such sexual dimorphisms can help researchers to
readily determine an individual’s gender. In snakes, sex
is typically determined by the presence or absence of
hemipenes - the male reproductive organs. Examining
for the presence of hemipenes on live specimen is
usually achieved by probing or physically everting the
male’s reproductive organs (Fitch, 1960). This process
is, however, highly intrusive and has the potential to
injure the snake if not conducted with care. Furthermore,
the probing of a snake requires access to specialised
tools and requires practice and finesse to conduct
appropriately. The use of sexual size dimorphism (SSD),
which refers to the differences in body weight and size
between males and females, allows for a less invasive
approach to determine a snake’s gender (Cox et al.,
2007). While valuable, SSD is not equally pronounced
across all taxa and is often not suitable for individuals
that are not fully grown. Instead, some studies have
suggested that differences in the size and shape of pelvic
spurs – small claw-like protrusions found laterally on
each side of the cloaca in members of the Boidae,
Pythonidae, Loxocemidae and Tropidophiidae, could
exhibit sexually dimorphic characteristics and thus be
used to determine gender (Stickel and Stickel, 1946;
Slip and Shine, 1988; Shine and Slip, 1990; Shine et
al., 1998; Díaz et al., 2014; Rodríguez-Cabrera et al.,
2016).
Pelvic spurs are vestigial remnants of hind legs that
remain internally connected to bones and muscle tissue
(Pough et al., 2003). These spurs show surprising
mobility and males use them during both copulation
(Gillingham and Chambers, 1982; Slip and Shine, 1988)
and male-to-male combat (Carpenter et al., 1978). Thus,
it is plausible that there is selective pressure towards
larger and more curved spurs in male boas. Females
have not been found to utilise their spurs and generally
possess smaller spurs. To date, this relationship between
sex and the size and shape of the pelvic spurs has
only been explored in a few species and currently no
information on sexual dimorphism in the Bahamian Boa
(Chilabothrus strigilatus) exists.
Here, we measured pelvic spurs in the Bahamian
Boa to determine if pelvic spur length could be used
Herpetology Notes, volume 14: 201-203 (2021) (published online on 26 January 2021)
Size matters: Sexual dimorphism in the pelvic spurs of the
Bahamian Boa (Chilabothrus strigilatus strigilatus)
Sebastian Hoefer1,*, Nathan Jack Robinson1,2, and Theodora Pinou3
1 Cape Eleuthera Institute, The Cape Eleuthera Island School,
Eleuthera, The Bahamas.
2 Fundación Oceanogràfic, Oceanogràfic De Valencia, 46013
Valencia, Spain.
3 Department of Biology, Western Connecticut State University,
Danbury, CT 06810, USA.
* Corresponding author. E-mail: sebastianhoefer@outlook.com
© 2021 by Herpetology Notes. Open Access by CC BY-NC-ND 4.0.
Abstract. Determining gender in snakes almost always involves highly intrusive techniques that have the potential to harm or
at least significantly stress the animal. Clear morphological differences of conspecific males and females, defined as sexual
dimorphism, can provide an alternative strategy to determine the sex of a snake with little harm. The commonly used sexual
size dimorphism (SSD), however, is not ubiquitously applicable and has its limitations for some taxa and different life stages.
The size differences in pelvic spurs, a little investigated morphological feature in some snakes, could provide a minimally
invasive method to determine sex in these snakes. We compared the pelvic spurs of 32 Bahamian Boas of different life stages
and body sizes to find out if pelvic spurs provide a robust alternative to identify sex in these snakes. We found that male
Bahamian Boas possessed spurs nearly twice as long and notably more curved than females and that the proportion of the spurs
to body size differed significantly between the sexes. Furthermore, spur length increased considerably more with an increase in
body size in males than in females. We conclude that the investigation of the pelvic spurs in these boas allows for a simple and
minimally invasive approach to determine gender and may also represent an alternative strategy for other species.
Key words. Boidae, Morphology, Pelvic spurs, Gender determination, Sexual dimorphism, West Indies

Sebastian Hoefer
et al.
202
as a non-invasive method to distinguish between sexes.
The Bahamian Boa is a large arboreal boid snake
that inhabits mesic and dry forests as well as various
other shrubs, trees and bushes across several islands
in the Bahamian archipelago (Henderson and Powell,
2009; Powell and Henderson, 2012). Currently, five
subspecies of C. strigilatus exist across the Bahamian
islands with the subspecies Chilabothrus strigilatus
strigilatus inhabiting the eastern Great Bahama Bank,
excluding Cat Island (Fig. 1; Reynolds and Henderson,
2018).
From August 2019 to May 2020, we opportunistically
sampled individuals of C. s. strigilatus on the island of
Eleuthera, The Bahamas. In total, we found 32 Bahamian
Boas, 28 dead-on-the-road and 4 live individuals. For
each boa, we measured the pelvic spur length to the
nearest 0.5 mm and snout-vent length (SVL) to the
nearest mm. In addition, sex of each individual was
determined by assessing for the presence of hemipenes
via dissection or probing. Pelvic spurs in males were
on average almost twice as long as in females, with the
mean pelvic spur length in males being 2.6 mm (n =
16; range: 0.5 – 5.0 mm) compared to the 1.5 mm in
females (n = 16; range: 0.5 – 3.0 mm). Controlled for
body length, the proportion of spur length relative to
SVL was 0.3% in males and 0.1% in females. We used a
generalised linear model explaining spur length by sex
while controlling for size and the interaction of sex and
size. We scaled size in the model to help with model
conversion and the interpretation of model estimates.
The model revealed that males had significantly larger
spurs (GLM, estimate = 2.12981, SE = 0.16605, t-value
Figure 1. Male and female pelvic spur length in relation to
snout-vent length. Dashed lines represent the regression lines
and the grey area the 95% confidence intervals.
Figure 2. Pelvic spurs of a similarly sized male and female Bahamian Boa (Chilabothrus strigilatus strigilatus). Dorsolateral view
of the cloacal area of a female (A) and a male (B); white arrows mark the pelvic spurs. Female (C) and male (D) pelvic spurs on
a metric ruler for size reference. Photographs © Sebastian Hoefer.

= 12.826, P < 0.001***) and that the spur length relative
to body size differed significantly between the sexes
(GLM, estimate = 1.30894, SE = 0.18087, t-value =
7.237, P < 0.001***). In juvenile boas (< 700 mm SVL,
R.G. Reynolds (pers. comm., March 2020) found that
spur length did not differ between the sexes, however
adult males possessed spurs at least twice as long
compared to females of the same size (Fig. 2). Overall,
the slope of the regression in males was notably steeper,
showing that spur length in males increased more
significantly with an increase in body size compared to
females. Moreover, we found distinct differences in the
shape of the pelvic spurs between males and females
with male pelvic spurs being notably curved whereas
female spurs appeared relatively straight (Fig. 3). These
clear size and shape differences of the pelvic spurs in
male and female C. s. strigilatus allow for an easy, safe
and fast method to distinguish between sexes without
requiring any tools or practice and offer an alternative
to probing or everting of hemipenes. Furthermore, this
clear sexual dimorphism found in the Bahamian Boa
might also be present in other spurred snakes and thus
the external examination of pelvic spurs could be used
to easily determine sex across various taxa.
Acknowledgements. We would like to thank The Cape Eleuthera
Island School for providing the means to conduct research in
The Bahamas. We also thank Sophie Mills, Samuel Tabbutt and
A.A. Ron Cogger for helping with the snake dissections and data
collection as well as Birgit Szabo for statistical advice.
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Size matters: Sexual dimorphism in the pelvic spurs of the Bahamian Boa 203
Accepted by Graham Walters