Reptiles vary in terms of reproductive output, such as the clutch size (CS), egg dimensions, time that reproduction occurs during their lifespan, age, and relative clutch mass (RCM). Normally in many species of reptiles, these life history traits show complex relationships to both female body size (Radder and Shanbhag 2003) and intra- and inter-population variation (Castilla and Bauwens 2000; Kiefer et al. 2008). Thus, when these parameters are determined, information is provided on reproductive strategies that are utilized by different populations of a single species (Shine and Greer 1991). These variations of reproductive outputs have been attributed to both (1) abiotic factors that mainly include temperature and rainfall (Fitch 1985; Ji and Brana 2000), and (2) biotic factors such as body size, morphology, and age (Goodman et al. 2008). Thus, the study of life history traits may increase our understanding of how individuals of each species interact with the environment and the environment’s role in reproductive output.

The relative clutch mass has been considered one of the most important life history traits in reptiles (Tinkle et al. 1970; Vitt and Price 1982), and studied as the ratio of clutch mass with respect to the female mass (Pianka and Parker 1975; Gerald and Miskell 2007) or as the proportion of energy destined for reproduction (Vitt and Congdon 1978), and is considered an indirect estimate of reproductive effort in squamates (Tinkle 1969; Seigel and Fitch 1984). Several hypotheses have been proposed from the study of life history traits in reptiles that suggest that female body morphology and both clutch size and relative clutch mass have coevolved with foraging mode (Vitt and Congdon 1978). Therefore, sit-and-wait foragers tend to have robust bodies, high clutch size and relative clutch mass values, and low escape velocities; while active foragers show cylindrical bodies, low clutch size and relative clutch mass values, and high escape velocities (Vitt and Congdon 1978; Vitt and Price 1982; Rodríguez-Romero et al. 2005; Mesquita et al. 2016).

Basiliscus vittatus Wiegmann, 1828 is an oviparous lizard of the family Corytophanidae and is widespread in tropical regions of America. In Mexico, the Brown Basilisk has been recorded in Jalisco, Michoacán, Puebla, Tamaulipas, Veracruz, and also throughout Central America down to Panama (Campbell 1998; Castañeda-Hernández et al. 2011). The Brown Basilisk is also now registered in Florida as an introduced and invasive species (Krysko et al. 2006). For its habitat preferences, B. vittatus can be semi-aquatic, terrestrial, and a semi-arboreal lizard. According to previous studies, B. vittatus neonates show a habitat preference close to water bodies, while juveniles and adults tend to move away from permanent aquatic sources and have perch sites more elevated (Hirth 1963; Fitch 1973).

Particularly within the family Corytophanidae, studies about life history characteristics are scarce. In the genus Basiliscus, there are a few studies detailing aspects of natural history, which provide characteristics by habitat preferences, thermoregulation, and spatial movements (Hirth 1963; Fitch 1973; Laerm 1974), and others have related ecological and life history data (Van Devender 1982; Vaughan et al. 2007; Lattanzio and LaDuke 2012; Hernández-Córdoba et al. 2012). Unfortunately, much of the information obtained in these studies does not correspond to recently oviposited eggs and most samples were taken from different populations (Ortleb 1965; Lieberman 1980); in addition the limited samples size could promote variations in life history traits. The Brown Basilisk is classified as a type of passive forager (Cooper et al. 2013), however the wide range of microenvironments it can occupy (aquatic, terrestrial and arboreal) may possibly promote restrictions in its relative clutch mass. Recently, in a Mexican B. vittatus population localized in Ejido López Mateos, Catemaco, Veracruz, characteristics in regard to nesting site choice and dimensions of eggs were recorded for the first time (Suárez-Varón et al. 2016). It is now possible, using this new data, to make comparisons with populations of basilisk lizards distributed more to the south of this nesting description, for example, in Costa Rican populations (see, Hirth 1963; Fitch 1973), and to observe the effects of biotic/abiotic factors on reproductive output among different populations of B. vittatus in Mexico and other Central American locations. Thus, the purpose of the present study is to provide clutch traits and relative clutch mass data for a single population of B. vittatus that inhabits an open riverbed juxtapositioned to tropical rainforest habitat in Catemaco, Veracruz, Mexico. This study may help: (1) to elucidate if a relationship exists between life history traits (i.e., CS and RCM) and different biotic/abiotic factors (including morphological traits of females), (2) allow a comparison of these female reproductive traits within different populations of Mexican B. vittatus, and other lizards especially within the family Corytophanidae.

The mean clutch size of B. vittatus in this study is 6.2 eggs (range 5–9 eggs/female), which is within the known range for this species (range 3.9–12 eggs), for the genus (range 3.9–18 eggs), and for the family Corytophanidae (Table 3). Although in squamates clutch size varies between congeners distributed in regions with different requirements (Hamilton et al. 2008), it is also affected by strategies of reproduction within populations. For example, if there are one or more clutches per season, variation in clutch size should occur in later clutches (Vitt 1977). In the present study, we do not know if there are multiple clutches in our study population; however, according to previous evidence B. vittatus shows multiple clutches (Fitch 1973). In this study, we did not consider the effect of multiple clutches on clutch size and relative clutch mass since we are unsure if multiple clutches occur in our population. A future study will investigate whether single or multiple clutches occur within our population at our study site.

Our results show that females of B. vittatus with larger body sizes have larger clutch sizes compared to smaller females (Fig. 2). Although there are some lizards that do not show this relationship of body size to clutch size (Wen-San 2010; Ljubisavljević et al. 2012), this pattern is common in many squamates (Radder and Shanbhag 2003), and includes other genera of tropical lizards such as Tropidurus (Vitt and Goldberg 1983), Eurolophosaurus (Galdino et al. 2003), and Emoia (Hamilton et al. 2008). Also, the association between SVL with inter-limb distance is important, since larger abdominal cavities allow the storage of more eggs (Williams 1966; Castilla and Bauwens 2000; Kratochvil and Kubicka 2007). Thus, the body size of the female allows for an increase in clutch size and maximum width of the eggs (Table 2). In addition, the increase in body size of squamates has been closely related to its reproductive response; females with higher SVL tend to maximize their fitness by increasing the numbers of eggs (Rutherford 2004). However, their chances of reproducing in future events may be restricted due to their increased susceptibility to predation because of decreased locomotor capacity and escape velocity (Galdino et al. 2003). Likewise, the increase in body size should be an evolutionary advantage that allows females not only to fill the larger abdominal space, but also increase the capacity to store energy reserves (e.g., food reserves, fat bodies) allocated towards reproduction (Williams 1966; Qualls and Shine 1995). Energy reserves in some reptilian species are reduced considerably during the gestation period by a cessation of food intake, which has been observed to affect growth and physical condition of the female (Shanbhag et al. 2000). In the present study, the effect of robustness (female body condition) on the variations of clutch size and clutch mass was not supported because no significant association between these variables was observed as recorded in previous studies of others lizards (Shanbhag et al. 2000). Thus, much of the available energy stores within reproducing females may be allocated immediately to reproduction (Shine 1980; Wilkinson and Gibbons 2005; Warner et al. 2008).

A central theory in vertebrate life histories is the trade-off between clutch size and dimensions of eggs (Stearns 1992). In regards to this theory, the clutch size of B. vittatus was related to maximum width and volume of the eggs regardless of the SVL of the female. Our data suggest that the variations present in the dimensions of the eggs show limitations imposed by the clutch size supporting this trade-off hypothesis. Thus, the volume of eggs is more variable in comparison with consistent and less variable measures of maximum length and width of the eggs. This supports the adaptative canalization hypothesis, which suggests that maternal investment is related to body size, promoting a higher clutch size, but not larger eggs (Brockelman 1975). In contrast, clutch mass and volume of egg were traits that displayed more variation within reproductive traits, which have been strongly correlated to female body size (Fig. 3) or body mass because these traits can limit/enhance the amount of available abdominal space for carrying eggs (Shine 1992). In contrast, egg size is invariant to different selective pressures, such as physiologic mechanisms, predation, climatic tolerance, as well as the agility of the females during gravidity (Shine and Greer 1991). Then, the size of egg is most likely limited by: 1) the length and arrangement of eggs within the oviduct (Ford and Seigel 1989), and 2) by the size of both the pelvic girdle (Congdon and Gibbons 1987) and vent of the females to facilitate the oviposition process.

Furthermore, clutch size is molded by environmental characteristics attributed to abiotic factors such as precipitation, seasonality, or temperature (Ji and Brana 2000; Lepage and Lloyd 2004) because these factors modify the availability of resources (e.g., food) and thus, the energy that females can acquire prior to reproductive events (Ballinger 1977; Seigel and Fitch 1984). Consequently, clutch size is a trait that has evolved as an adaptive mechanism to environment pressures (Fitch 1985), especially in those species that are widely distributed and whose adult body size shows a range of variability (e.g., B. vittatus). Thus, changes in clutch size are expected (Fitch 1970) as a result of different selective pressures to local environment constraints (Shine and Downes 1999), which is balanced by maintaining an optimal number of eggs that allows greater fitness in females (Fitch 1970; Shine and Greer 1991). The variation in clutch size among populations probably reflects differences in the availability of energy resources that females obtain and which are influenced by the biotic and abiotic conditions of each local study area.

Within our data, female B. vittatus raise their relative clutch mass at the same time that their clutch size increases; the inversion towards reproduction can increase or decrease with respect to the age, weight, and size of the females, in such a way that the energy available for reproduction is allocated in an optimum amount within each egg (Doughty and Shine 1998; Radder and Shanbhag 2004; Orrell et al. 2004).

The fact that there is no observable relationship between relative clutch mass and SVL in B. vittatus suggests that larger females do not proportionally exhibit greater reproductive effort (Stuart-Smith et al. 2007). However, in B. vittatus clutch size and relative clutch mass are limited by body morphology (SVL and absolute mass respectively) rather than by foraging mode or escape velocity, thus the only way females can increase their reproductive responses are to be larger in order to produce more eggs. According to the regression model (CS = -5.07224 + 0.089843 * SVL, Fig. 2), which estimates the body size that allows the females to increase their clutch size; then, substituting our CS (= to 6.2 eggs) into this model equates to an increase in 8% female SVL for each egg added since the difference between our SVL (mean =126.3 mm) and the estimated SVL = 136.596 mm (for 7.2 eggs in the model equation) implies an increase of 10.2 mm of female SVL. This indicates that the reproductive response in B. vittatus is strongly linked to the body morphology, which involves the SVL and absolute mass of the females. Future studies should incorporate abdominal-volume data (Shine 1992) to evaluate if body shape (more than inter-limb distance) restricts or promotes reproductive output. This should offer a model for the index of available space in the abdominal cavity and the egg carrying capacity of the female squamates.

The relative clutch mass of B. vittatus (0.17) in the present population is similar to other studied species within the family Corytophanidae, as reported by Scharf et al. (2015): genus Basiliscus (= 0.21, B. plumifrons Cope, 1875); (= 0.18, B. galeritus Duméril, 1851); (= 0.16, B. basiliscus Linnaeus, 1758), and genus Laemanctus (= 0.17, L. longipes Wiegmann, 1834), which contrast to the highest relative clutch mass recorded in Corytophanes cristatus Merrem, 1820 (= 0.27), an extreme passive foraging lizard (Andrews 1979). Mesquita et al. (2016) also obtained a lower relative clutch mass value (= 0.08) for B. vittatus; and it appears that the variation in relative clutch mass is more easily observed at the population level rather than at the species level (Du et al. 2005). It has also been hypothesized that relative clutch mass has coevolved with foraging mode (Vitt and Congdon 1978); however, in this study there is no supportive evidence for this hypothesis; since the value of relative clutch mass index obtained for B. vittatus is close to that of active foragers (RCM = 0.17, Mesquita et al. 2016). Therefore for this study, we suggested that both traits are under independent selective pressures regarding foraging mode for B. vittatus. Although B. vittatus is classified as a sit and wait forager based on low movements per minute and percentage of activity (Cooper et al. 2013), it has lower values in its reproductive investment when compared with other sit and wait species, such as: Phrynosoma orbiculare Linnaeus, 1789 (= 0.27, Suárez-Rodríguez et al. 2018), Sceloporus clarkii Baird & Girard, 1852 (= 0.30, Shine 1992), and Sceloporus grammicus Wiegmann, 1828 (= 0.28, Rodríguez-Romero et al. 2005). These lizards have been associated with high values in relative clutch mass, low escape velocities, and robust bodies (Vitt and Congdon 1978; Vitt and Price 1982). This contrast in B. vittatus versus other sit and wait squamates may be due to the fact that the relative clutch mass of B. vittatus is considerably limited by its body morphology, since it has an elongated body and tail such as those observed in active foragers (Teiidae) (Ramírez-Bautista et al. 2000) and longer posterior extremities that facilitate running to high speeds even across the surface of water (Hirth 1963). According to this, it is suggested that B. vittatusmay hold an intermediate position between both foraging modes, which may provide information on the evolution of more complex adaptations, such as escape mechanisms (Cooper 2005). In this way, the reproductive effort in B. vittatus has been molded to maintain its locomotor skills such as: velocity of escape, attachment to branches, submersion, etc., all making the Brown Basilisk less vulnerable to predation (Geral and Miskell 2007; Suárez-Varón unpublished results). Consequently, natural selection may favor individuals in this population of lizards with an optimal relative clutch mass that allows for escape performance in different microhabitats in which these lizards occupy, but still maintains little variation in the dimensions of the egg, which has been associated with low values in relative clutch mass index (Harvey and Pagel 1991). This implies that an optimization of the clutch size and clutch mass is in play to maximize fitness. Finally, the clutch size and relative clutch mass may show a response to multiple selective pressures caused by: 1) local environmental effects (Vitt 1981; Zamora-Abrego et al. 2007), 2) constraints associated to the body morphology (Vitt and Price 1982), 3) microhabitat (Vitt 1981), and 4) foraging and escape strategies, as well as the use of crypsis (Vitt and Congdon 1978; Rodríguez-Romero et al. 2005), which limit the reproductive output and should influence both clutch size and relative clutch mass.