Sexual dimorphism in body traits is widespread across animal species. Lizards serve as models for investigating the evolutionary and ecological consequences of sexual size dimorphism (SSD) across traits including body size, head dimensions, and limb length (Tinkle and Dunham 1986; Jiménez-Arcos et al. 2017; Liang et al. 2021), providing insights into relative growth and form of morphological traits (Scharf and Meiri 2013; Goldberg et al. 2024). Males often exhibit proportionately larger heads due to male–male agonistic behaviours (Shine 1989), while females may show longer trunk lengths related to offspring production (Scharf and Meiri 2013; Pérez-Quintero et al. 2019). These variations can lead to hyperallometric or hypoallometric relationships between traits and body size, favouring intraspecific phenotypic divergence driven by different reproductive roles between sexes (Cox et al. 2007).

The alligator lizards of the subfamily Gerrhonotinae currently comprise five genera distributed across North and Central America (Vitt and Caldwell 2014; Blair et al. 2022). Historically, the genus Abronia Gray, 1838, was characterized as an exclusively arboreal lineage endemic to Mesoamerican highland forests, ranging from central Mexico to western Panama (Clause et al. 2024). However, recent genomic evidence reveals that Abronia is nested within, or closely allied with, terrestrial species of Mesaspis Cope, 1877, rendering both genera non-monophyletic under their current definitions (Gutiérrez-Rodríguez et al. 2021). Consequently, the expanded circumscription of Abronia now includes 39 species; while the majority remain arboreal, several taxa exhibit terrestrial habits (Uetz et al. 2025). Most of the arboreal species are characterized by a very striking colour pattern, characterized by bright colours in shades of blue, green, cream, brown, and black, which allow them to blend in with the epiphytic plants in their habitat (Campbell and Frost 1993). At present, a comprehensive understanding of ecological and life-history traits of Abronia populations remains limited and anecdotal, where ecological studies are mainly concentrated on Abronia taeniata (Wiegmann, 1828) (Villamar-Duque et al. 2019; Fierro-Estrada et al. 2023). Overall, there has been little analysis of morphological traits in Abronia species, particularly regarding sexual size dimorphism have been conducted in less than 15% of the 39 described Abronia species (Lemos-Espinal et al. 2001; González-Porter et al. 2015; Cruz-Elizalde et al. 2020).

In this study, we analysed the sexual size dimorphism and caudal autotomy of Campbell’s Alligator Lizard, Abronia campbelli Brodie & Savage, 1993, a Critically Endangered arboreal lizard micro-endemic to the pine–oak and cloud forests of southeastern Guatemala. The species inhabits old-growth oak forests dominated by Quercus peduncularis Née, 1801 and Q. brachystachys Bentham, 1842 where it shows a strong association with the epiphytic bromeliad Tillandsia usneoides (Linnaeus, 1753) which serves as its primary microhabitat (Ariano-Sánchez and Torres-Almazán 2010). We predicted sexual dimorphism in A. campbelli, with allometric differences between sexes. Specifically, we expected males to have broader heads than females, consistent with previous studies (Lemos-Espinal et al. 2001; Villamar-Duque et al. 2019; González-Porter et al. 2015; Cruz-Elizalde et al. 2020).

We conducted diurnal surveys near Potrero Carrillo, Department of Jalapa, Guatemala (Fig. 1). Between 2014 and 2016, we captured 54 individuals (29 adult males, 25 adult females) throughout the year. Fieldwork and specimen collection were performed under Research Licenses No. 036/2014 and No. 010/2015, and Collection Permits Serie A No. 000972 and No. 002013. After capturing the individuals, we recorded the following morphological measurements with an electronic caliper: snout–vent length (SVL) of individuals measured from the tip of the snout to the anterior margin of the cloaca; head width (HW), the maximum width of the head, and body mass (BM) measured using an analytical balance (± 0.01 g; Cruz-Elizalde et al. 2020). To ensure reproductive maturity, only adult individuals with a snout–vent length (SVL) ≥ 105 mm were included in the analyses. This threshold is conservative compared to the original species description, which reports adult sizes beginning at approximately 93–94 mm SVL (Brodie and Savage 1993). The sex of individuals was determined by manually everting hemipenes. We also recorded whether the individual had undergone caudal autotomy. Finally, each lizard was individually identified utilizing a passive integrated transponder tag (12.5 × 2.1 mm, 118 mg; Avid FriendChip PIT tags, Avid Identification Systems) before releasing it at the capture site. On each new capture, we first passed the PIT tag reader over different parts of the body to avoid duplicate measurements.

We examined the allometric relationships between SVL and morphological traits (HW and BM) for each sex. To test the isometric relation (null hypothesis, Charnov 1993), we considered a slope with a 95% confidence interval (CI) including the expected value of 1.0 to show isometry, while slopes with a 95% CI below or above the expected value were interpreted as hypoallometric or hyperallometric relationships, respectively (Fairbairn 1997). To estimate sexual size dimorphism, we initially conducted a t-student test to compare SVL between sexes. Subsequently, an analysis of covariance (ANCOVA) was used to assess sexual dimorphism adjusted to body size using SVL as the covariate. We used a chi-squared test to determine differences in the caudal autotomy between sexes. We present mean values ± 1 standard deviation (SD). All analyses were performed using PAST 5.1 (Hammer et al. 2001).

Of the 54 A. campbelli analysed for this study, females exhibited a hypoallometric relationship for SVL–HW and BM–HW (slope < 1), while the SVL–BM showed a hyperallometric relationship (slope > 1; Table 1). Meanwhile, for males, SVL–BM were consistent with slope = 1; both SVL–HW and BM–HW exhibited hypoallometry (Table 1). The SVL of males ranged from 89.0 to 129.8 mm (mean 105.6 ± 10.7 mm SD), while females exhibited SVL ranging from 90.0 to 122.0 mm (mean 106.1 ± 8.6 mm SD). The sexes showed a similar body size (t_1,52 = 0.25, p = 0.79). Females and males showed similar BM (F_1,51 = 1.05, p = 0.30), while males exhibited broader heads than females (F_1,51 = 59.70, p < 0.01; Fig. 2). The HW of males ranged from 18.2 to 26.2 mm (mean 23.3 ± 1.9 mm SD), while females exhibited HW ranging from 16.0 to 24.0 mm (mean 18.8 ± 1.8 mm SD). The BM of females ranged from 13.5 to 39.1 g (mean 27.3 ± 6.1 g SD), and the BM of males ranged from 15.4 to 45.0 g (mean 28.6 ± 7.1 g SD). Both sexes showed a similar body mass (t_1,52 = 0.61, p = 0.53). The frequency of caudal autotomy was not different between males and females, 4.17% and 4.88%, respectively (χ² = 1.16, d.f. = 1, p = 0.28).

Females showed a positive relationship between body size and body mass, meaning that females have higher body weight than expected as body size increases, but we did not observe differences in weight compared to males. On the other hand, we not only observed a significant allometric relationship between body size and head width in males, but also wider heads compared to females, which suggests that head width in males is under intrasexual selective pressure. Our results support monomorphism in body size and body mass of A. campbelli. Based on the literature and our results, the sexual size monomorphism in Abronia species may maximize reproductive performance at similar body sizes (Villamar-Duque et al. 2019; Cruz-Elizalde et al. 2020).

Males of A. campbelli showed on average 19.3% wider heads than females. Some studies have also shown that males have wider heads than females; for example, 14.5% in Abronia monticola (Cope, 1878), 11.3% in Abronia graminea (Cope, 1864), and 10.2% in A. taeniata (Vial and Stewart 1989; Lemos-Espinal et al. 2001; Cruz-Elizalde et al. 2020). However, in available data for A. campbelli obtained prior to our study, Ariano-Sánchez and Torres-Almazán (2010) reported an HW mean of 26.0 ± 1.9 mm for males and 16.1 ± 2.1 mm for females, which showed males had on average 38.0% wider heads than females. These observations could be related to courtship behaviors such as neck and head bites during the courtship period. Gvozdík and Van Damme (2003) observed that sexual dimorphism in head size among adult lacertilians arises from differential growth in the postorbital region of the head, which is more pronounced in males. This area serves as the insertion point for the jaw abductor muscles, which are responsible for bite force. Some studies have shown that sexual selection may favour larger heads in males (resulting in greater bite force) if differences in male–male combat ability translate into the capacity to maintain territories and win intrasexual agonistic encounters, thereby increasing access to females (Vitt 1983; Lappin et al. 2006). For instance, Formanowicz et al. (1990) examined the interaction patterns between and within sexes of Abronia vasconcelosii (Bocourt, 1872) under laboratory conditions. The authors reported nine behaviour patterns, of which bites from head to tail were the most frequent in both male–male and female–female interactions. We observed antagonistic encounters in A. campbelli, mainly between individuals of the same sex, although occasionally also between sexes. This behavior is characterized by one individual chasing another, often culminating in biting between the contestants. These interactions were opportunistic observations in the wild, during which one individual was confirmed as a male; this consisted of a chase, and no biting was observed as the individuals quickly fled. Additional observations occurred during field handling in rare instances when individuals were temporarily placed together in the same collection bag, although individuals were normally kept separately. In these cases, encounters involved chasing and biting and appeared more common between individuals of the same sex. Animals were separated immediately to prevent injury. During mating, male Abronia typically restrain females with a firm bite on the left or right side of the head. For example, Schmidt-Ballardo and Mendoza-Quijano (1999) observed a male grasping a female on the temporal region in A. mixteca for 8 hours. Schmidt-Ballardo et al. (2015) observed that A. deppii copulated for approximately 18 h. Solano-Zavaleta et al. (2017) observed captive male A. antauges maintaining a mate-holding grip on the left side of the female’s head for nearly four hours. In comparison, similar mate-holding behavior in the genus Elgaria has been documented to persist for over 24 hours (Nguyen et al. 2020). This behavior suggests that larger-headed males may have a greater ability to control females, potentially increasing their reproductive success.

Some studies have shown that the incidence of tail loss in males is likely due to intrasexual agonistic encounters (Vitt et al. 1974). We observed a low frequency of caudal autotomy in A. campbelli, particularly when compared to species like A. vasconcelosii, which has been reported to exhibit regenerated tails in 72.2% of captured individuals (Formanowicz et al. 1990). The low frequency of caudal autotomy observed in A. campbelli may reflect several non-mutually exclusive factors, including reduced intraspecific aggression, variable population densities associated with its endangered status, or an inherently lower autotomy threshold, possibly because the prehensile tail represents a particularly valuable locomotor asset whose loss would be highly costly. Notably, low autotomy rates do not necessarily imply that antagonistic interactions are directed away from the tail. As Pafilis et al. (2019) and Brock et al. (2015) have shown, inducing tail loss in a competitor may itself be selectively advantageous, as it removes a valuable locomotor structure and could reduce the rival’s competitive ability. As Martin (1955) noted, the loss of this “fifth limb” significantly impairs arboreal performance, making its preservation essential for survival in the canopy. Ultimately, our findings highlight how morphological constraints and social behavior intersect in these endangered lizards, underscoring the urgent need for further natural history research to inform their conservation.