Assessing Head Morphology Dimorphism in the Midland Painted Turtle (Chrysemys picta marginata) Using a Photographic Questionnaire
Abstract
The ability to identify the sex of animals accurately is important in population studies. Emydid turtles (Testudines: Emydidae) demonstrate a number of sexually dimorphic characters, including head (cranial) size and structure. Field observations from a long-term study of midland painted turtles (Chrysemys picta marginata) in Algonquin Provincial Park, Ontario, Canada, suggested distinct differences in external head morphology between the sexes. We evaluated these putative sexual differences in C. picta head morphology by conducting a visual questionnaire involving human observers of varying levels of experience (novice, beginner, intermediate, and advanced). Observers were capable of distinguishing the sexes based solely on head morphology with a high degree of accuracy (between 79% and 86% success) across experience levels. Observers identified head shape as a defining character distinguishing the sexes. We suggest that visual questionnaires are a quantifiable method of assessing dimorphic characters that can be used in addition to traditional morphometrics or geometric morphometrics to demonstrate a visual, rather than simply statistical, difference among characters and sexes. Despite the breadth of research conducted on C. picta, our study is among the first to describe, assess, and discuss the functional significance of head dimorphism in this model species.
For wildlife biologists, the ability to identify individuals within a population accurately and to identify organism characters (e.g., sex) is essential for demographic study. Color, pattern, deformities, and injuries, among other external features, may be reliable markers for the identification of individuals. Similarly, secondary sexual traits are obvious characters distinguishing the sex of study organisms. Studies in (social) psychology and perception have examined the ability of human observers to correctly identify the sex of different animal species based on diagnostic features of the head or face. Observers correctly differentiated the sex of rhesus macaques (Macaca mulatta) with 61% accuracy on the basis of sexually dimorphic traits of the face, including jaw size, nose size, eye size, and face length and width (Franklin et al. 2013). These authors concluded that observers were capable of accurately differentiating the sex of macaque faces by generalizing sexual dimorphisms found in human faces. In species with no obvious external sexually dimorphic characters, such as the domestic cat (Felis domesticus), participants performed poorly in identifying sex even with sex-identification training (46% to 58% among treatments; Quinn et al. 1999). In contrast, Biederman and Shiffer (1987) showed that professionals could identify the sex of day-old chicks (Gallus gallus) with > 98% accuracy based on minute, although defining, morphological characters. Naïve observers averaged 61% success while trained observers scored 84% success (Biederman and Shiffer 1987), illustrating the role of training and instruction for the recognition of morphometric characters. In applied fieldwork, melon profile photographs of bottlenose whales (Hyperoodon ampullatus) have been used with high success to categorize individuals according to age–sex categories (female, immature male, subadult male, mature male) and produced results consistent with molecular sexing techniques (Gowans et al. 2000). Similarly, photographs of the sexually dimorphic dorsal fin in bottlenose dolphins (Tursiops sp.) have been used to noninvasively identify sex in the field (Rowe and Dawson 2009). From video recorded at migration monitoring stations, the sex of adult chinook salmon (Oncorhynchus tshawytscha) has been evaluated with up to 92% accuracy based on body proportions (Merz and Merz 2004).
Sexually dimorphic traits are common in Testudines and often reflect the challenges associated with mating, such as male mounting and/or maintenance of a secure copulatory position. For example, males often demonstrate a concavity of the plastron (Ernst and Lovich 2009) that aids in mounting the domed carapace of the female. Modifications of the forelimb and hind-limb claws may be present, either as nuptial structures or for assistance in gripping females during mating. Soft tissue modifications, such as a rough tissue patch in the inner hind limb or a tail spur, are seen in mud and musk turtles (Kinosteridae) and assist with mounting and mating (Mahmoud 1967; Berry and Shine 1980; Ernst and Lovich 2009).
Multiple turtle species of the families Emydidae and Geoemydidae, among other taxa, demonstrate pronounced sexual dimorphism in head (cranial) size and structure (Moldowan et al. 2016b), notably among the map turtles and sawbacks (Graptemys spp.; Bertl and Killebrew 1983; Lindeman 2000, 2006) and cooters and red-bellied turtles (Pseudemys spp.; Bever 2008, 2009). These differences in head morphology are commonly attributed to dietary partitioning, reproductive roles, and other ecological selection pressures (Shine 1989; Stephens and Wiens 2009; Underwood et al. 2013; Moldowan et al. 2016a, 2016b). Another taxonomically diverse group of emydid turtles, the sliders (Trachemys spp.), demonstrates a gradient of head dimorphism (Moldowan et al. 2016b). Snout elongation in male Chrysemys and Pseudemys (= Trachemys) ranges from mildly expressed (i.e., not noticeable but statistically demonstrable) to spectacular (Moll and Legler 1971). Among Antillean Trachemys there are slight but consistent sexual differences in head shape and proportions of the snout (Barbour and Carr 1940), differences not seen in Trachemys taxa from the mainland United States (Seidel 2002; Legler and Vogt 2013). The degree of elongation, pointedness, and upturning of the snout is highly variable among male Mesoamerican Trachemys species (Seidel 2002; Legler and Vogt 2013). Modifications to the prefrontal bone of males progress during ontogeny and result in a bulbous, boss-like modification of the snout that gives larger (and/or older) individuals a very distinctive head profile (Legler and Vogt 2013).
Painted turtles (Chrysemys picta) are closely related to cooters (Pseudemys) and sliders (Trachemys) and together these taxa are collectively recognized as pond turtles (Emydidae: Deirochelyinae; Seidel and Smith 1986; Stephens and Wiens 2003, 2009). Numerous sexual dimorphisms have been formally described for C. picta, including a larger female body size, a relatively greater female carapace height, elongate male forelimb claws, and relatively greater male precloacal tail length (Ernst and Lovich 2009). Observational evidence from a long-term study (1978−present) of midland painted turtles (C. p. marginata) in Algonquin Provincial Park, Ontario, Canada, has suggested that head shape and size dimorphism exists (Moldowan et al. 2016a). Variation in rostrum length, tomiodont length (tooth-like cusps medial on the upper jawline; Moldowan et al. 2016a), and overall head shape between the sexes has been noted during handling and processing of Algonquin Park C. picta (Fig. 1). Given a putative head shape dimorphism in Chrysemys, external head morphology may serve to identify the sex of individuals. Our objective was to use a photographic questionnaire to test whether participants of differing experience levels were capable of accurately distinguishing female and male C. p. marginata based solely on lateral head characters. This assessment of head shape dimorphism in C. p. marginata complements a previous quantitative analysis of sexual dimorphism in cranial morphology (Moldowan 2014; Moldowan et al. 2016b).
Citation: Chelonian Conservation and Biology 16, 1; 10.2744/CCB-1235.1
METHODS
Photo Collection.
Turtles were sampled from multiple long-term study sites in Algonquin Provincial Park, Ontario (45°34′N, 78°41′W), where monitoring of population demographics and reproduction has taken place annually since 1978 (R.J.B., pers. comm., January 2017). Turtles were captured by dip net from canoe or baited hoop trap and transported to a field laboratory at the Algonquin Wildlife Research Station (WRS) for processing. Individual sex was assigned based on morphological characters (e.g., body size, foreclaw length, precloacal tail length) and known reproductive history. High-quality digital photographs were taken of the right side of the head of sexually mature C. p. marginata (> 100-mm midline plastron length) using a Canon Rebel XTi digital SLR camera equipped with an 18−55-mm lens. The heads of all turtles were photographed in the same visual plane to allow direct comparisons among individuals.
Visual Questionnaire of Head Morphology.
A visual questionnaire involving the evaluation of C. p. marginata head morphology was conducted with participants of varying experience levels. In a self-guided questionnaire (Supplemental File 1; available at http://dx.doi.org/10.2744/CCB-1235.1.s1), participants viewed slides of paired lateral head photographs of sexually mature C. p. marginata from Algonquin Provincial Park. Each slide showed 2 different adult turtles: 1 belonging to Group A (female) and 1 to Group B (male). The biological significance (i.e., sex) of the 2 groups was not revealed to participants. The placement of Group A and Group B photos on the left or right side of questionnaire slides was randomized.
Prior to beginning the questionnaire, participants were provided with reference images with 3 morphological traits identified for Group A and Group B turtles. Group A turtles (females) were described as having a short rostrum (distance between eyes and nostrils), often short tomiodonts (tooth-like cusps medial on the upper jawline; Moldowan et al. 2016a), and a robust, stocky head. Group B turtles (males) were described as having a comparatively elongate and pointed rostrum, tomiodonts ranging from short to long, and a slender, anterior-tapering head. Participants were then provided with 3 reference slides to familiarize themselves with the format of the exercise. In these reference slides, the Group A and Group B turtles were identified. Last, prior to beginning the questionnaire, participants were asked to identify their level of experience with C. picta from 4 categories: novice, no experience or prior observation of painted turtles; beginner, minimal experience or observation of painted turtles; intermediate, some experience or observation of painted turtles; or advanced, extensive experience or observation of painted turtles.
In Part I of the questionnaire, participants were asked to evaluate all 3 traits to best identify the Group B turtle in 25 photo pairings. In Part II, participants were again asked to evaluate all 3 traits to best identify the Group B turtle, but were also asked to specify which of the 3 head traits featured most prominently in their decision. In total (Parts I and II), observers were asked to identify Group B turtles in 51 photo pairings. Following the photographic questionnaire, 3 supplementary questions were asked to gauge a participant's overall conclusions (Supplemental File 1).
Participants were contacted via e-mail and provided with a generic background description of the questionnaire (Supplemental File 2; available at http://dx.doi.org/10.2744/CCB-1235.1.s2), a slideshow questionnaire (Supplemental File 1), and a Microsoft Excel answer input sheet (Supplemental File 3; available at http://dx.doi.org/10.2744/CCB-1235.1.s3). The questionnaire audience was primarily composed of Laurentian University (Sudbury, Ontario, Canada) undergraduate and graduate students in biology, members of field naturalist organizations, members of professional organizations (e.g., Canadian Herpetological Society), and biologists affiliated with the WRS. The sampled population represented individuals with novice, beginner, experienced, and advanced knowledge of painted turtles. The questionnaire was also spread by word of mouth and via social media as shown by questionnaires returned from participants who were not contacted directly. As such, the overall response rate of the questionnaire could not be calculated.
Statistical Analyses.
Using a 2-tailed exact binomial test, we tested whether the proportion of correct group (sex) assignment was significantly greater than would be expected by chance (50%). All 51 lateral head photo pairings were grouped and scored (n/51) to evaluate the performance of participants in correctly identifying the group (sex) of C. picta. A 1-way ANOVA using within-group weighted means, followed by a Tukey's post hoc test, was conducted to test for differences in questionnaire performance among participant experience levels (4 categories). The mean score of each group was weighted to account for unequal samples sizes among questionnaire groups. Answers to the supplementary questions were summarized and are presented using descriptive statistics.
RESULTS
In total, 133 participants completed the C. picta head morphology questionnaire (nnovice = 31, nbeginner = 28, nintermediate = 36, nadvanced = 38). Participants of all experience levels performed better than chance (50%) in identifying the sex of C. picta based solely on head characters (p < 0.001; Table 1). The distribution of questionnaire scores and average questionnaire scores showed high performance across all experience levels (Table 1; Fig. 2). One novice and one intermediate participant, together representing 1.5% of individuals sampled, were outliers scoring < 50% (Table 1). Two participants (1.5%), both in the advanced experience category, correctly distinguished the sexes in 100% of photo pairings. The ability to discern the sex of C. picta based on head morphology was marginally different among experience levels (F3,129 = 2.63, p = 0.053). Tukey's post hoc analysis showed that mean questionnaire score did not differ significantly among beginner, intermediate, and advanced levels of experience (p > 0.05 in all pairwise comparisons); however, a significant difference in identification ability was found between novice and advanced participants (p = 0.030; Table 1).
Citation: Chelonian Conservation and Biology 16, 1; 10.2744/CCB-1235.1
Across all experience levels, participants consistently identified overall head shape as the most distinguishing character between groups (Table 2). Overwhelmingly, participants of all experience levels stated that there was a definitive difference in overall head shape and/or structure between male and female C. picta (Table 3). Participants at the novice and beginner experience levels demonstrated an approximately 10% higher acceptance of a definitive sex difference in overall head shape and/or structure compared with intermediate or advanced level participants (Table 3). In the general comments section of the answer sheet, 4 participants remarked on an additional putative head morphology dimorphism from their visual assessments: in Group A (females) the “forehead” (anterior orbit to distal rostrum distance) was noted as downward sloping, whereas in Group B (males) this orbit–rostral region was generally nonsloping or horizontal (refer to Fig. 1). Recently mature males, for which head dimorphism is less developed compared with older/larger individuals (P.D.M., pers. obs.), were the most prone to misclassification by questionnaire participants.
DISCUSSION
Questionnaire Performance.
Photographic questionnaires indicated that perceptible differences in head morphology exist between male and female C. picta. Regardless of a participant's level of experience with C. picta, participants accurately distinguished between the 2 sex-based groups with, on average, 79%−86% success (Table 1) based on head morphology alone. These findings provide strong support that there is a (human) perceptible difference in head (cranial) structure between male and female C. p. marginata from Algonquin Park. Overall head morphology, rostrum length, and tomiodont length were key traits distinguishing the sexes, supporting a priori field observations that these traits differed between the sexes (Moldowan et al. 2016a). In addition, the distinct male and female C. picta head morphology findings reported here from photographic assessments are corroborated by analyses of cranial morphometric data for C. picta in Algonquin Park (Moldowan 2014; Moldowan et al. 2016b). Our findings suggest that overall cranial size/shape and associated head characters (e.g., tomiodont and rostrum length) can be used in combination with other identified sexual dimorphisms (Ernst and Lovich 2009) to identify the sex of C. picta in the field.
The ability of observers to sex-differentiate C. picta based on head morphology was substantially greater than that reported for similar studies involving mammalian taxa (Quinn et al. 1999; Franklin et al. 2013). This result suggests that the difference between male and female C. picta head morphology is visually quite pronounced. Training has been shown to have a negligible effect (Quinn et al. 1999) to strong positive effect (Biederman and Shiffer 1987) on the accuracy of identifying sex in nonhuman species through visual surveys. The basic training provided in our questionnaire, namely the 3 introductory reference slides distinguishing Group A and Group B, may also have improved the ability of observers to distinguish the sex of C. picta. Future researchers may wish to improve upon our survey design by employing a completely randomized photo pairing questionnaire (e.g., AA, AB, BA, BB) rather than the more simplistic photo pairing (AB or BA) we conducted.
Head (Cranial) Dimorphism in Testudines.
The importance of visual cues in the mating behavior of turtles is unclear. In Testudines, body shape may allow individuals to distinguish conspecifics, but does not appear to inform about the sex or suitability of potential partners. For instance, male turtles will direct reproductive behavior toward inanimate objects that resemble the body shape and size of conspecifics (Hidalgo 1982; Baker and Gillingham 1983). In addition, many interspecific and intrasexual sexual pairings have been reported in diverse testudine taxa (Eglis 1962; Davis and Jackson 1973; Arndt 1977; Baker and Gillingham 1983; Karl et al. 1995), suggesting that although body shape may be a relevant signal for mate recognition, this trait likely functions alongside other cues (i.e., tactical, chemical, and auditory; Liu et al. 2013) in sex recognition.
Multiple alternative hypotheses have been proposed to explain the observed head dimorphism in C. picta (Moldowan 2014; Moldowan et al. 2016a, 2016b). A sexual selection hypothesis suggests that head shape dimorphism may serve as a visual cue for mate recognition. For example, the elongated, bulbous snout and cranial dimorphism of some male Mesoamerican Trachemys may act in mate recognition or serve as an “erotic prod” (Legler 1990; Legler and Vogt 2013). The larger relative size and gripping arrangement of the tomiodonts in male C. picta may assist with coercive reproductive tactics (Moldowan 2014; Moldowan et al. 2016a, 2016b). The elongate rostrum of male C. picta could serve to enhance olfaction, either for food or mate searching. Alternatively, head shape dimorphism might be explained by intersexual dietary specialization. However, this trophic dimorphism hypothesis does not appear to be supported for C. picta on account of no known sex-specific feeding specializations (Ernst and Lovich 2009; Padgett et al. 2010), although explicit testing is still necessary.
In summary, the findings reported here show a (human) perceptible difference in head morphology between male and female C. picta. In combination with other dimorphic characters, head morphology can serve as a useful tool for distinguishing the sexes in C. picta. We suggest that visual questionnaires serve as a qualitative and quantifiable method of assessing dimorphic characters that can be used in addition to traditional morphometrics and geometric morphometrics to demonstrate a visual, rather than simply statistical, difference among characters.
Right lateral head profile of midland painted turtles (Chrysemys picta marginata) from Algonquin Provincial Park, Ontario, Canada. Note blunt snout and blocky head morphology of the adult female (a) compared with elongate rostrum and angular, anterior tapering snout of the adult male (b).
Histograms of survey scores (expressed as percentage) from midland painted turtle (Chrysemys picta marginata) head morphology questionnaire across 4 levels of participant experience.
Contributor Notes
Handling Editor: Peter V. Lindeman
