Sexual dimorphism in the alligator lizard Gerrhonotus infernalis (Sauria: Anguidae): implications for sexual selection.
The principal causes of sexual dimorphism in lizards are often hypothesized to be related to sexual selection, niche segregation, and nonadaptive processes related to behavioral or physiological differences between the sexes (Butler and Losos, 2002; Cox et al., 2003; Molina-Borja and Rodriguez-Dominguez, 2004). Sexual selection may generate dimorphisms as a consequence of intrasexual and intersexual competition whereby a reproductive advantage is gained by one sex. For example, males may increase fighting ability by attaining greater body size or weaponry used in male-male combat. Presumably, winners of these combative encounters have increased access to mates, and these males would have a fitness advantage over smaller males, or males with smaller weapons (e.g., jaws; Lailvaux et al., 2004; Lappin and Husak, 2005). Sexual dimorphism also may evolve to decrease niche overlap. Many species are known to segregate important resources such as preferred prey or occupancy of micro-sites to optimize use of individual resources (Butler et al., 2007). Finally, sexual dimorphism may evolve as a byproduct of other behavioral or physiological processes such as patterns of differential growth or longevity (Stewart, 1985; Johnson et al., 2005; McBrayer and Anderson, 2007; Ljubisavljevic et al., 2008).
Within genera or other lower taxonomic groups, sexual dimorphism can manifest itself in varying degrees in different characteristics (Molina-Borja, 2003; Butler et al., 2007). In anguid lizards (Barisia and Elgaria), sexual dimorphism is present in size of the head and body (i.e., snout-vent length; Vial and Stewart, 1989; Rutherford, 2004; McBrayer and Anderson, 2007). Stewart (1985) found annual variation in the degree of sexual dimorphism in Elgaria coerulea. Such variation could be due to year-to-year climatic and ecological conditions at particular study sites (e.g., temperature, rain, abundance of prey, etc.; Schauble, 2004). Therefore, when assessing the presence and magnitude of sexual dimorphism, many extrinsic (e.g., temperature, precipitation, and humidity) and intrinsic (e.g., physiologic and genetic) factors must be considered.
Gerrhonotus infernalis is commonly known as falso escorpion or Texas alligator lizard; it is distributed in North America from central to western Texas in the United States through Coahuila, entering into Nuevo Leon, Tamaulipas, and north of San Luis Potosi in Mexico (Smith, 1979; Good, 1994). It is most often found in humid, shady, rocky areas. It is a relatively large lizard (180-200 mm snout-vent length; SVL), with short limbs and small claws. Males seem to have larger heads than do females of the same size. Adults can be found throughout the year but are less active during hot and cold months (summer and winter; Greene et al., 2009).
Like other anguid lizards (Smith, 1979), G. infernalis is a secretive species in temperament and can be found moving slowly on leaf litter or hidden under rocks. Due to their secretive habits, insufficient information exists on the ecology, natural history, and potential for sexual dimorphism in this species, especially in the Mexican portion of its distribution. The objective of this study was to evaluate the sexual dimorphism of G. infernalis in morphological traits and to compare these data with those from other lizards to examine patterns of sexual dimorphism.
Materials and Methods--We conducted this study in Parque Ecologico Chipinque (25[degrees]34'-25[degrees]35'N, 100[degrees]18'-100[degrees]23"W), in the municipalities of Monterrey and San Pedro Garza Garcia in the state of Nuevo Leon, Mexico. The park occupies 1,791 ha of a protected area, "Parque Nacional Cumbres," within the Sierra Madre Oriental mountain range; elevation is between 600 and 2,200 m at Chipinque. The climate at Chipinque is semidry (300500 mm annual precipitation) with a noticeable rainy season in the summer. The average annual temperature ranges from 1822[degrees]C (Garcia, 1981). Five different types of vegetation are present at Chipinque: oak; oak-pine; pine-oak forest; mountain scrub; desert scrub.
From April 2008 to December 2009, we captured 89 specimens of G. infernalis by hand or noose along various footpaths at Chipinque. To reduce the minimum difference in size between males and females, we retained only individuals >100 mm in SVL (adults). We determined sex by eversion of hemipenes. We marked each individual by toe-clipping (Waichman, 1992) to ensure no recapture was included. All lizards were released at the site of capture after all data were gathered. To include measurements from other years, we examined 13 specimens (3 males, 10 females) from the Herpetological Collection of the Universidad Autonoma de Nuevo Leon collected in previous years at Chipinque: females 317, 318, 1616, 1921, 3882, 5734, 5788, 6951, 6960, 7104; males 1809, 3881, 6232. Data from field and museum specimens were included in the statistical analysis.
To assess the existence of sexual dimorphism in the population at Chipinque, we measured the following traits on each individual: snout-vent length (SVL, from the tip of the snout to the posterior margin of the anal scale); head length (HL, from the tip of the snout to the posterior margin of the ear); head width (HW, maximum head width); head height (HH, maximum head height); right forelimb length (RFLL, from the forelimb armpit to the wrist); right hindlimb length (RHLL, from the hindlimb axilla to the ankle); fourth toe length on the right forelimb (RFFTL); fourth toe length on the right hindlimb (RHFTL); trunk length (TrL, the distance from the posterior margin of the forelimb insertion to the anterior margin of the hindlimb insertion); tail length (TL, from the posterior margin of the anal scale to the tip of the tail); tail width (TW, immediately posterior to anal scale). These measurements were taken with a vernier caliper (0.01-mm precision). Total weight was measured with a Pesola spring scale (0.5-g precision). We quantified only one meristic character, the number of transverse dorsal lines (TDL). We also counted bite marks present on each live individual and classified them as bites (teeth lines were easily observed), scars (defined by the presence of regenerated tissue), or wounds (defined by the presence of blood).
To determine if the number of males was equal to the number of females in this population, we used chi-square analysis. Because males and females appeared to be visually different in body size, we size-adjusted all morphological data by regressing each variable on SVL and collecting the residuals. We performed a principal-component analysis (PCA) on the size adjusted residuals to explore which variables were most influential in shaping the variation in body shape between males and females. By interpreting the loadings of each variable on each principal-component axis, we selected four variables upon which to test specific hypotheses of male-female differences in morphology.
We examined variation between male and female morphological characteristics using one-way analysis of variance (ANOVA). To evaluate possible differences between size of the head and length of the body between the sexes, we performed an analysis of covariance (ANCOVA; covariate = SVL; Sokal and Rohlf, 1995). To evaluate differences in the size of the head between sexes, we performed a reduced-major-axis regression (RMA; Sokal and Rohlf, 1995). Each linear dimension of the head (HL, HW, and HH) was entered as a dependent variable, and SVL as the independent variable. The null hypothesis of the isometry was supported if the 95% confidence intervals encompassed a slope of 1.0. Positive allometry was interpreted as slopes (and confidence intervals) >1.0. Slopes and confidence intervals <1.0 were interpreted as negative allometry. To evaluate whether the frequency of bites and scars on males and females was equal, we performed chi-square analysis.
Except for chi-square tests, all variables were [log.sub.10] transformed prior to analysis. All statistical analyses were performed using SPSS 17.0 software.
RESULTS--The sex ratio of males to females was 1.6:1 which was significantly different from 1:1 ([x.sup.2.sub.0.05,1] = 5.94, P = 0.015). We measured 59 males and 43 females. In general, males were larger than females (Table 1). The coefficients from the PCA indicated that TrL, HL, HW, and HH were important in shaping PC axes 1 and 2 (Table 2). TrL loaded the negative end of PC1, while HL loaded heavily on the positive end of PC1. Thus, individuals with long trunks loaded the negative end of the axis whereas individuals with long heads loaded the positive end. HL, HW, and HH were important in shaping PC2; males tended to have lower scores for each of these variables and, thus, were shifted toward the negative end of PC2. Only measurements of limbs showed high coefficients for PC3, indicating that variation in neither forelimb length nor hindlimb length contributes substantially to variation in the shape of the body.
Females exhibited a negative allometric growth pattern for HL but presented isometric growth for HW and HH. Males exhibited isometric growth for HL and positive allometric growth for HW and HH (Table 3). ANOVA showed significant differences in the subset of variables selected from the results of PCA. Males were larger in SVL ([F.sub.1,100] = 4.85, P < 0.05), HL ([F.sub.1,100] = 13.41, P < 0.05), HW ([F.sub.1,100] = 28.00, P < 0.05), and HH ([F.sub.1,100] = 46.90, P < 0.05). ANCOVA revealed that males had larger heads with respect to body size than did females of the same body size ([F.sub.3,97] = 8.37, P < 0.05). The effect of the sex was significant for HW and HH but not for HL (Fig. 1).
A total of 141 bites were counted on 39 of the 56 males (1-13/individual). A total of 23 scars were counted on 16 of the 56 males (range of 1-3/individual). On nine males, a total of 11 wounds were counted (range of 1-2/ individual). A total of 50 bites were counted on 21 of the 33 females (range of 1-9 bites/individual). On nine females, there were 17 scars (range of 1-5/individual). No female had wounds. The frequency of bites and scars differed significantly by sex ([x.sup.2.sub.0.05,2] = 8.813, P = 0.012).
Discussion--Sexual dimorphism is common in lizards, although variation exists among different taxa (Molina-Borja, 2003; Butler et al., 2007). In our study, we show that G. infernalis exhibits sexual dimorphism at Chipinque; males possessed larger heads and bodies than did females. Males were heavier than females as well. In many anguid species, sexual dimorphism in morphology of the head has been related to combat or mating. In Elgaria coerulea, males have higher heads than do females (McBrayer and Anderson, 2007), but females have greater SVL than do males (Stewart, 1985; Rutherford, 2004; McBrayer and Anderson, 2007). In Barisia monticola, males have higher, longer, and wider heads than do females (Vial and Stewart, 1989). For B. monticola (Vial and Stewart, 1989) and E. coerulea (McBrayer and Anderson, 2007), it has been suggested that males with bigger heads have advantages in combat or copulation rituals, because the head plays a pivotal role in both behaviors (Bowker, 1988; Formanowicz et al., 1990). In many species of lizards, the characteristics that determine sexual dimorphism are dimensions of the head and size of the body (Pinto et al., 2005). Dimensions of the head have been suggested to be more important than size of limbs in certain performance attributes (Huyghe et al., 2005).
The presence of bites is associated with agonistic behavior. In Abronia vasconcelosii, nine aggressive, interspecific and intraspecific, behavioral patterns have been recorded. Bites between males are more frequent than those between males and females. Interestingly, neither size of the body nor residency status influenced the frequency of these behavioral patterns (Formanowicz et al., 1990). Hence, this finding suggests that combat is not related to territoriality in this species. In natural field-conditions, we observed combat lasting 2-4 min for two pairs of G. infernalis. One was male-male; for the other, we could only determine that one individual was a male. In both, the first individual was subjugating the second individual with a bite on the neck, without releasing. After various strangling movements, the individual being bitten escaped. Bites by the winning individual left marks on the losing individual, and blood was apparent at the wounds.
These events and the variation in the frequency of bites, scars, and wounds between sexes in G. infernalis are likely the consequence of agonistic behavior among males (Bowker, 1988; Formanowicz et al., 1990; McBrayer and Anderson, 2007), suggesting this is one of the origins of sexual dimorphism in G. infernalis.
Sexual dimorphism in the size of the head can lead to sexual differences in the force of bites (Herrel et al., 1996, Herrel et al., 1999; Husak et al., 2006; Lappin et al., 2006a, 2006b; McBrayer and Anderson, 2007). This is likely due to increased size, orientation, or both of the musculature of the jaw (Herrel et al., 1996). It is suggested that larger heads in males provide an advantage in male-male combat or during copulations with females (Herrel et al., 1996; Herrel et al., 1999; Husak et al., 2006). Sexual dimorphism often favors larger males or larger weapons (Cox et al., 2003; Lappin et al., 2006b) in species that present aggressive and territorial behavior because both are advantageous during combat (Husak et al., 2006). Sexual dimorphism may be generated by differential mating success of the larger males, or males with larger and stronger (or both) heads (i.e., weapons; Lailvaux et al., 2004). Intrasexual selection likely directs evolution toward exaggerated, or performance-enhancing, structures like heads of males because these yield advantages in agonistic encounters while establishing and maintaining territories or in securing mates (Herrel et al., 1996; Lappin and Husak, 2005; Lappin et al., 2006a).
It is suggested that larger heads in males also provide an advantage during copulation with females (Herrel et al., 1996; Herrel et al., 1999; Husak et al., 2006); bigger heads and the presence of marks from bites have been associated with mating in which the male uses its mouth to grip a female during copulation (Bowker, 1988; Formanowicz et al., 1990; McBrayer and Anderson, 2007). We did not test this hypothesis directly, but the presence of bites and scars and the lack of wounds in females may be the result of males biting females during copulation, which suggests that intersexual competition could promote sexual dimorphism in this species.
Many species are known to segregate important resources such as preferred prey or microsite occupancy to optimize use of resources by individuals (Butler et al., 2007). It has been proposed that cranial morphology may be related to diet (Herrel, 1996; Husak et al., 2006). Although we did not specifically test this hypothesis, variation in diet also could provide a sufficient selection pressure to affect the evolution of the head in male and female G. infernalis.
Patterns of differential growth also have been proposed as a possible mechanism to explain sexual dimorphism. Patterns of differential growth arise as a consequence of nonadaptive processes between sexes (Johnson et al., 2005; Ljubisavljevic et al., 2008). For example, at sexual maturity, females invest substantial energy into reproduction rather than additional growth (Liu et al., 2008). If so, we might expect females to exhibit isometric or negatively allometric growth in body regions not associated with reproduction (e.g., weapons like the head). Several important allometric differences were observed in G. infernalis; females were generally negatively allometric or isometric, and males were isometric or positively allometric in dimensions of the head (Table 3). This result, plus the presence of male-male combat, supports the idea that the cause of sexual dimorphism is male-male combat rather than differential growth. If differential growth were the sole cause, we would not expect males to show positive allometry in their fighting structures.
It is often difficult to exclude the possibility that patterns of differential growth might contribute to the generation or maintenance of sexually dimorphic characteristics. Differential growth is frequently present in lizards (Butler and Losos, 2002; Cox et al., 2003; Molina-Borja and Rodriguez-Dominguez, 2004). In our study, we found the same pattern of differential growth as in Hemidactylus turcicus (Johnson et al., 2005) and Dinarolacerta mosorensis (Ljubisavljevic et al., 2008). These patterns are probably common in lizards. Here, the differences in dimensions of the head were considerable; on average, HW in males was 16% greater than that in females, and HH in males was 20% greater than that in females. The magnitudes of these differences are unlikely to have evolved solely by differential growth. It is impossible to know if such differences arose by differential growth, but it seems likely that selection would be required to generate such large differences between males and females in morphology. Hence, we conclude that sexual dimorphism in this population is most likely the result of sexual selection on structures that enhance male-male combat. However, studies examining courtship and copulatory behavior are needed to verify this conclusion. Like most studies, we cannot refute the possibility that mating and nonadaptive processes, such as differential growth, may have been involved in the origin of sexually selected traits. In addition to courtship behavior, future research should focus on quantifying the degree of sexual dimorphism in this and other populations of G. infernalis and in additional anguid species. This will allow us to understand the role of ecological versus phylogenetic contributions toward dimorphic traits and how they are maintained.
We would like to thank Parque Ecologico Chipinque and San Antonio Zoo and Aquarium for financial support to conduct this research. This research also was supported by Consejo Nacional de Ciencia y Tecnologia (CONACyT) grant number 190512 to MGB. Collection permits OFICIO NUM. SGPA/ DGVS/01253/08 and OFICIO NUM. SGPA/DGVS/02370/09 were issued by the Secretaria de Medio Ambiente y Recursos Naturales.
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Submitted 4 February 2011. Accepted 23 March 2013. Associate Editor was Felipe de Jesus Rodriguez Romero.
MARGARITA GARCIA-BASTIDA, * DAVID LAZCANO, LANCE D. MCBRAYER, AND ROBERTO MERCADO-HERNANDEZ
Laboratorio de Herpetologia, Facultad de Ciencias Biologicas, Universidad Autonoma de Nuevo Leon, A.P. 513, San Nicolas de los Garza, Nuevo Leon, Mexico, C.P. 66450 (MGB, DL) Department of Biology, P.O. Box 8042, Georgia Southern University, Statesboro, GA 30460-8042 (LDM) Departamento de Ciencias Exactas, Facultad de Ciencias Biologicas, Universidad Autonoma de Nuevo Leon, San Nicolas de los Garza, Nuevo Leon, Mexico, C.P. 66450 (RMH)
* Correspondent: firstname.lastname@example.org
TABLE 1--Mean ([+ or -] 1 SE), range, and sample size of each morphological variable (millimeters) in Gerrhonotus infernalis. M = males, F = females (field and museum specimens). Abbreviations for variables are defined in the text. Variable Mean [+ or -] 1 SE M F SVL 132.89 [+ or -] 1.57 127.66 [+ or -] 1.67 HL 31.16 [+ or -] 0.60 27.84 [+ or -] 0.44 HW 20.07 [+ or -] 0.44 16.91 [+ or -] 0.34 HH 13.94 [+ or -] 0.30 11.14 [+ or -] 0.25 RFLL 22.95 [+ or -] 0.35 21.72 [+ or -] 0.25 RHLL 25.80 [+ or -] 0.27 24.79 [+ or -] 0.35 RFFTL 8.32 [+ or -] 0.12 7.92 [+ or -] 0.14 RHFTL 9.93 [+ or -] 0.14 9.38 [+ or -] 0.17 TrL 72.75 [+ or -] 0.90 70.91 [+ or -] 1.14 TL 237.87 [+ or -] 7.50 228.28 [+ or -] 7.39 TW 9.45 [+ or -] 0.16 10.15 [+ or -] 0.35 Weight 35.44 [+ or -] 1.36 30.35 [+ or -] 1.41 TDL 8.07 [+ or -] 0.12 8.27 [+ or -] 0.15 Variable Minimum Maximum M F M F SVL 104.50 102.80 155.00 158.00 HL 18.00 21.30 40.00 33.00 HW 13.50 11.60 29.00 21.50 HH 10.00 7.30 19.00 16.00 RFLL 16.00 17.60 30.00 26.40 RHLL 20.50 21.00 31.10 29.80 RFFTL 6.50 6.50 10.50 10.00 RHFTL 7.20 7.10 13.00 11.80 TrL 58.50 59.00 87.60 93.50 TL 99.00 126.00 298.00 281.00 TW 7.00 7.50 12.30 18.00 Weight 17.00 17.00 58.00 52.00 TDL 6.00 7.00 10.00 10.00 Variable n M F SVL 59 43 HL 59 43 HW 59 43 HH 59 43 RFLL 59 43 RHLL 59 43 RFFTL 59 42 RHFTL 59 42 TrL 59 43 TL 24 22 TW 59 43 Weight 56 33 TDL 56 33 TABLE 2--Eigenvalues for the first three principal components from principal-component analysis of nine morphological variables for male and female Gerrhonotus infernalis. Abbreviations for variables are defined in the text. Principal component Variable 1 2 3 HL -0.1458 -0.7124 -0.4196 HW -0.0887 -0.4540 -0.0260 HH -0.0668 -0.3417 -0.0164 RFLL 0.0071 -0.2349 0.6327 RHLL -0.0275 -0.2444 0.6305 RFFTL -0.0360 -0.0365 0.0383 RHFTL -0.0371 -0.0971 0.0936 TrL -0.9805 0.1842 0.0436 TW -0.0365 -0.0986 0.1128 Percentage of variance explained 30.78 15.99 4.69 TABLE 3--Results of reduced-major-axis regression for patterns of growth in head length, head width, and head height of male and female Gerrhonotus infernalis. Parameter Mean [+ or -] 1 SE Slope Head length Males 31.16 [+ or -] 4.61 1.263 Females 27.84 [+ or -] 0.44 0.570 Head width Males 20.07 [+ or -] 3.39 1.463 Females 16.91 [+ or -] 0.34 0.985 Head height Males 13.94 [+ or -] 2.33 1.404 Females 11.14 [+ or -] 0.25 0.524 Parameter Confidence interval [R.sup.2] Head length Males 0.924-1.601 0.495 Females 0.219-0.921 0.208 Head width Males 1.166-1.760 0.630 Females 0.593-1.377 0.386 Head height Males 1.080-1.719 0.583 Females 0.012-1.037 0.094 Parameter Allometry Head length Males Isometric Females Negative Head width Males Positive Females Isometric Head height Males Positive Females Isometric
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|Author:||Garcia-Bastida, Margarita; Lazcano, David; McBrayer, Lance D.; Mercado-Hernandez, Roberto|
|Date:||Jun 1, 2013|
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