Chemical discrimination among predators by lizards: Responses of three skink species to the odours of high‐ and low‐threat varanid predators

Animals must balance the benefits of predator avoidance with costs. Costs of predator avoidance, such as being forced to spend long periods inactive, should select for careful discrimination among predator species. Although prey responses to multiple predators have been well researched across many taxa, no studies have tested whether lizards discriminate among larger lizard predators. We examined the responses of three species of skink to two species of predatory goanna, one that occasionally consumes skinks, and the other a skink specialist. Three litter‐dwelling, tropical skink species, Carlia rostralis, C. rubrigularis and C. storri, were given a choice between a retreat site treated with the odour of one of the goanna species, and an odourless control. The two goanna species used for stimulus scents were: Varanus tristis, a species that consumes skinks as a major proportion of its diet, and Varanus varius, a species that consumes skinks occasionally. Both goannas are broadly sympatric with all three skink species. Carlia rostralis and C. storri both avoided the scent of V. tristis, whereas C. rubrigularis did not. However, no skink species avoided the odour of V. varius. Prey are clearly able to avoid predators based on chemical cues, and can discriminate among similar predators that pose different levels of threat.     

Growth and survivorship as proximate causes of sexual size dimorphism in peninsula dragon lizards Ctenophorus fionni

Males and females differ in body size in many animals, but the direction and extent of this sexual size dimorphism (SSD) varies widely. Males are larger than females in most lizards of the iguanian clade, which includes dragon lizards (Agamidae). I tested whether the male larger pattern of SSD in the peninsula dragon lizard, Ctenophorus fionni, is a result of sexual selection for large male size or relatively higher mortality among females. Data on growth and survivorship were collected from wild lizards during 1991–1994. The likelihood of differential predation between males and females was assessed by exposing pairs of male and female lizards to a predator in captivity, and by comparing the frequency of tail damage in wild‐caught males and females. Male and female C. fionni grew at the same rate, but males grew for longer than females and reached a larger asymptotic size (87 mm vs. 78 mm). Large males were under‐represented in the population because they suffered higher mortality than females. Predation may account for some of this male‐biased mortality. The male‐biased SSD in C. fionni resulted from differences in growth pattern between the sexes. The male‐biased SSD was not the result of proximate factors reducing female body size. Indeed SSD in this species remained male‐biased despite high mortality among large males. SSD in C. fionni is consistent with the ultimate explanation of sexual selection for large body size in males.     

Activity patterns and reproductive behavior of the Critically Endangered Bermuda skink (Plestiodon longirostris)

The study of rare or cryptic species in zoos can provide insights into natural history and behavior that would be difficult to obtain in the field. Such information can then be used to refine population assessment protocols and conservation management. The Bermuda skink (Plestiodon longirostris) is an endemic Critically Endangered lizard. Chester Zoo's successful conservation breeding program is working to safeguard, increase and reinforce skink populations in the wild. A key aim of this program is to develop our understanding of the behavior of this species. In this study, using 24 h video recordings, we examined the daily activity patterns, basking behavior and food preferences of four pairs of Bermuda skinks. The skinks displayed a bimodal pattern of activity and basking, which may have evolved to avoid the strength of the midday sun in exposed habitats in Bermuda. Captive Bermuda skinks appear to prefer a fruit-based diet to orthopteran prey. We also documented their reproductive behavior and compared it against two closely related species. Although there were many similarities between the courtship and mating behaviors of the three species, there was a significantly shorter period of cloacal contact in the Bermuda skink. Oophagia was also documented for the first time in this species. This knowledge has enabled the evaluation of the current ex-situ management practices of this species, filled gaps in knowledge that would be challenging to obtain in the field, and enabled the enhancement of both animal husbandry and reproductive success for the conservation breeding program.     

The Proximate Causes of Sexual Size Dimorphism in Phrynocephalus przewalskii

Sexual size dimorphism (SSD) is a common phenomenon and is a central topic in evolutionary biology. Recently, the importance of pursuing an ontogenetic perspective of SSD has been emphasized, to elucidate the proximate physiological mechanisms leading to its evolution. However, such research has seldom focused on the critical periods when males and females diverge. Using mark-recapture data, we investigated the development of SSD, sex-specific survivorship, and growth rates in Phrynocephalus przewalskii (Agamidae). We demonstrated that both male and female lizards are reproductively mature at age 10–11 months (including 5 months hibernation). Male-biased SSD in snout-vent length (SVL) was only found in adults and was fully expressed at age 11 months (June of the first full season of activity), just after sexual maturation. However, male-biased SSD in tail length (TL), hind-limb length (LL), and head width (HW) were fully expressed at age 9–10 months, just before sexual maturation. Analysis of age-specific linear growth rates identified sexually dimorphic growth during the fifth growth month (age 10–11 months) as the proximate cause of SSD in SVL. The males experienced higher mortality than females in the first 2 years and only survived better than females after SSD was well developed. This suggests that the critical period of divergence in the sizes of male and female P. przewalskii occurs between 10 and 11 months of age (May to June during the first full season of activity), and that the sexual difference in growth during this period is the proximate cause. However, the sexual difference in survivorship cannot explain the male-biased SSD in SVL. Our results indicate that performance-related characteristics, such as TL, HW, and LL diverged earlier than SVL. The physiological mechanisms underlying the different growth patterns of males and females may reflect different energy allocations associated with their different reproductive statuses.     

断尾对蓝尾石龙子能量储存和运动表现的影响

在许多蜥蜴种类中,尾自切是一种主要的逃避天敌捕食的防御性策略。虽然断尾使蜥蜴获得短期的生存利益,但同时也需为此承受多方面的代价。利用从丽水采集的117条蓝尾石龙子来评价该种动物断尾的能量和运动代价。81条(约69%)石龙子至少经历过1次尾自切。断尾个体中,原先断尾事件的发生频率在不同尾区间存在显著差别,但两性间无差别。将实验组17条具完整尾的石龙子依次切去3个尾段,然后测定断尾前后石龙子的运动表现以及每个尾段、身体各部分中的脂肪含量。另15条具完整尾的石龙子作为对照组,仅测量其运动表现。尾部的脂肪含量与尾基部宽呈正相关,说明具较粗尾部的石龙子一般具有相当较多的尾部储能。尾部脂肪含量随尾长呈非等比例分布,大部分脂肪集中于尾近基部端。断尾几乎不影响蓝尾石龙子的运动表现,仅当大部分尾部被切除时疾跑速有较小程度的降低。显示了蓝尾石龙子因遭遇天敌捕食攻击或其它因素作用而产生的部分断尾可能并不会导致严重的能量和运动代价。由于野外种群蓝尾石龙子个体的断尾情况主要发生在尾近基部或中部位置,因此可以认为自然条件下该种动物的尾自切通常会遭受明显的能量和运动代价。     

Handling-related tail loss in an endangered skink: incidence, correlates and a possible solution

Caudal autotomy (tail loss) during capture and handling is widely reported among several families of lizards. Autotomy causes elevated stress levels in lizards, and imposes a significant fitness cost on autotomized individuals. Despite these detrimental impacts, conservation and ethical issues associated with handling-related tail loss have received little attention. We assessed the incidence and correlates of tail autotomy during capture and handling in an endangered skink, the alpine she-oak skink Cyclodomorphus praealtus . A significant proportion (9.3%) of lizards autotomized their tails during capture and handling. Medium-sized lizards were more likely to lose their tails during handling, and this effect was exacerbated at intermediate body temperatures. Probability of autotomy had a complex relationship with cumulative observer experience, independent of other risk factors. Based on the modelled relationship of autotomy with body temperature, we propose that alpine she-oak skinks be cooled immediately after capture to reduce rates of autotomy during subsequent handling.     

Modelling exposure to selected temperature during pregnancy: the limitations of squamate viviparity in a cool-climate environment

Behavioural thermoregulation is important for the success of cool‐climate lizards, and a basis of the cold‐climate hypothesis for the evolution of viviparity in squamate reptiles. The temperature (T_sel) selected by pregnant females in a thermal gradient is considered to be optimal for embryonic development; however, exposure to T_sel throughout pregnancy has been difficult to estimate in small‐bodied lizards as temperature‐sensitive telemetry is impractical. In addition, the value of maternal thermophily during pregnancy is controversial: some studies have shown elevated T_sel, whereas others have found lowered T_sel or no change during pregnancy. We estimated indirectly the overall exposure to T_sel during the 4–5 months of pregnancy of the cool‐climate, sub‐alpine species Oligosoma maccanni (McCann's skink, 3–6 g) from southern New Zealand. The thermal environment available to skinks was modelled using temperature loggers inside validated copper models in basking and retreat sites. Pregnant skinks were able to achieve mean T_sel (28.9 °C) in the field very infrequently (4–15% of each month during the final 4 months of pregnancy). In field thermoregulatory studies, pregnant females did not bask more frequently and did not show altered field body temperature compared with non‐pregnant adults, suggesting that all skinks (whether pregnant or not) thermoregulate maximally whenever conditions allow. Further research on cool‐climate lizards should address the significance for offspring phenotypes of low and variable exposure to T_sel during pregnancy, as well as the significance of temperatures for embryos in maternal bodies (viviparity) versus nest sites (oviparity) arising from differences in maternal body size. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 96 , 541–552.     

Structure of the femora and autotomous (postpygal) caudal vertebrae in the three-toed skink Chalcides chalcides (Reptilia: Squamata: Scincidae) and its applicability for age and growth rate determination

The limbs of the three-toed skink Chalcides chalcides are reduced to such a degree that the three digits are too small for skeletochronology. This study, performed on animals collected near Isernia (central Italy), describes the structure of the caudal vertebrae, which are often naturally lost due to autotomy, in order to determine whether they can be used to obtain data on age and growth with skeletochronological techniques. The reliability of the autotomous caudal vertebrae for skeletochronology was verified by performing skeletochronological analyses also on femora. Although the identification of the lines of arrested growth (LAGs) was easier in femora than in autotomous caudal vertebrae, a high correspondence of the LAG count between the two bones was observed. Females were larger and lived longer than males (4 and 3 years, respectively). For both sexes, the snout vent length (SVL) was significantly correlated with age. For both sexes, sexual maturity was attained after two hibernations from birth, beginning at the 20th month of age. At first reproduction, males had an SVL of 91–106 mm and females one of 111–150 mm. Von Bertalanffy growth curves of age versus SVL showed that females had slower growth rates than males for attaining their asymptotic SVL (females: 197 mm; males: 143 mm). The results provide the first data on age and growth of C. chalcide, and show that autotomous caudal vertebrae are reliable alternatives for obtaining such data for limb-reduced reptiles, avoiding the need to sacrifice or disable animals, as occurs when long bones are used.     

Influence of woodland remnant edges on small skinks (Richmond,New South Wales)

Abstract Habitat fragmentation due to clearing often results in an increase in edge areas compared with overall remnant size, but there are limited data on the influence of increased edge areas on generalist species. Therefore, the abundance of small (<55 mm) generalist scinid lizards in small woodland remnants of the Cumberland Plain (western Sydney, Australia) was compared. Using pitfall trapping and points of transect survey techniques, three species were encountered: Lampropholis guichenoti, Lampropholis delicata and Cryptoblepharus virgatus. The abundance and diversity of these skinks differed significantly between edge and core areas; fewer individuals were encountered on the edge of the remnants than in core sites. Cryptoblepharus virgatus was only observed in edge sites where L. guichenoti made up the greatest skink abundance. In contrast, L. delicata abundance was greatest at core sites. There was also a significant difference between the edge and core with respect to the size structure of the L. guichenoti population. Larger numbers of subadults were observed in the edge sites, whereas there were significantly fewer adults and juveniles in these areas. Habitat characteristics did not account for the skink distribution observed.     

Growth and annual survival estimates to examine the ecology of larval lamprey and the implications of ageing error in fitting models

This study used existing western brook lamprey Lampetra richardsoni age information to fit three different growth models (i.e. von Bertalanffy, Gompertz and logistic) with and without error in age estimates. Among these growth models, there was greater support for the logistic and Gompertz models than the von Bertalanffy model, regardless of ageing error assumptions. The von Bertalanffy model, however, appeared to fit the data well enough to permit survival estimates; using length‐based estimators, annual survival varied between 0·64 (95% credibility interval: 0·44–0·79) and 0·81 (0·79–0·83) depending on ageing and growth process error structure. These estimates are applicable to conservation and management of L. richardsoni and other western lampreys (e.g. Pacific lamprey Entosphenus tridentatus) and can potentially be used in the development of life‐cycle models for these species. These results also suggest that estimators derived from von Bertalanffy growth models should be interpreted with caution if there is high uncertainty in age estimates.     

Size-dependent predation by snakes: selective foraging or differential prey vulnerability?

I staged replicate encounters between unrestrained lizards andsnakes in outdoor enclosures to examine size-dependent predationwithin the common garden skink (Lampropholis guichenoti). Yellow-facedwhip snakes (Demansia psammophis) forage widely for activeprey and most often consumed large skinks, whereas death adders(Acanthophis antarcticus) ambush active prey and most oftenconsumed small skinks. Small-eyed snakes (Rhinoplocephalusnigrescens) forage widely for inactive prey and consumed bothsmall and large skinks equally often. Differential predationmay reflect active choice by the predator, differential preyvulnerability, or both. To test for active choice, I presentedforaging snakes with an inert small lizard versus an inertlarge lizard. They did not actively select lizards of a particularbody size. To test for differential prey vulnerability, I quantifiedvariation between small and large lizards in behavior thatis important for determining the outcome of predator—prey interactions. Snakes did not differentiate between integumentarychemicals from small and large lizards. Large lizards tendto flee from approaching predators, thereby eliciting attackby the visually oriented whip snakes. Small lizards were moremobile than large lizards and therefore more likely to passby sedentary death adders. Additionally, small skinks were more effectively lured by this sit-and-wait species and less likelyto avoid its first capture attempt. In contrast, overnightretreat site selection (not body size) determined a lizard'schances of being detected by small-eyed snakes. Patterns ofsize-dependent predation by elapid snakes may arise not becauseof active choice but as a function of species-specific predatortactics and prey behavior.     

Can protective attributes of artificial refuges offset predation risk in lizards?

Artificial refuges are often used to supplement habitat in areas where natural shelters have been degraded or removed. Although artificial refuges are intended to support particular species, they may be equally attractive and accessible to others, including predators. We explored the influence of snake predation risk and shelter attributes on the overnight use of different artificial refuges (timber, tiles, and iron) using the predator‐prey relationship between Boulenger's skink, Morethia boulengeri and the curl snake, Suta suta. We collected adult M. boulengeri from two bioregions in south‐eastern Australia: the Riverina, where the two species co‐occur, and the South Western Slopes, where S. suta does not occur. Two adult S. suta were collected for use as chemical donors. We conducted four experiments on overnight refuge choice to determine: (i) predator‐scent avoidance, (ii) artificial refuge preferences, (iii) a trade‐off between a preferred refuge and predator‐avoidance, and (iv) the effect of gap height on refuge preference. We found that skinks avoided predator‐scented refuges in favour of identical, but unscented refuges. Skinks preferred timber refuges, and most skinks maintained this preference when predator‐scent was added. However, when gap height was manipulated, skinks shifted to the refuge with the smallest gap. Skinks displayed complex regional variation in behaviour; skinks from both bioregions avoided predator‐scent, but in the trade‐off experiment, skinks from the South Western Slopes were less likely to avoid predator‐scented timber refuges than those from the Riverina. Our findings suggest that protective refuge attributes, such as small gap height, can offset the risk implied by predator‐scent within a refuge. This study highlights the need to consider predator‐prey interactions when designing and using artificial refuges for habitat restoration or biological monitoring.     

Structure of the femora and autotomous (postpygal) caudal vertebrae in the three-toed skink Chalcides chalcides (Reptilia: Squamata: Scincidae) and its applicability for age and growth rate determination

The limbs of the three-toed skink Chalcides chalcides are reduced to such a degree that the three digits are too small for skeletochronology. This study, performed on animals collected near Isernia (central Italy), describes the structure of the caudal vertebrae, which are often naturally lost due to autotomy, in order to determine whether they can be used to obtain data on age and growth with skeletochronological techniques. The reliability of the autotomous caudal vertebrae for skeletochronology was verified by performing skeletochronological analyses also on femora. Although the identification of the lines of arrested growth (LAGs) was easier in femora than in autotomous caudal vertebrae, a high correspondence of the LAG count between the two bones was observed. Females were larger and lived longer than males (4 and 3 years, respectively). For both sexes, the snout vent length (SVL) was significantly correlated with age. For both sexes, sexual maturity was attained after two hibernations from birth, beginning at the 20th month of age. At first reproduction, males had an SVL of 91–106 mm and females one of 111–150 mm. Von Bertalanffy growth curves of age versus SVL showed that females had slower growth rates than males for attaining their asymptotic SVL (females: 197 mm; males: 143 mm). The results provide the first data on age and growth of C. chalcide, and show that autotomous caudal vertebrae are reliable alternatives for obtaining such data for limb-reduced reptiles, avoiding the need to sacrifice or disable animals, as occurs when long bones are used.     

At home with the birds: Kalahari tree skinks associate with sociable weaver nests despite African pygmy falcon presence

The way in which animals use habitat can affect their access to key resources or how they are buffered from environmental variables such as the extreme temperatures of deserts. One strategy of animals is to modify the environment or take advantage of structures constructed by other species. The sociable weaver bird (Philetairus socius) constructs enormous colonial nests in trees. These nests are frequented by Kalahari tree skinks (Trachylepis spilogaster) and the two species coexist over a large portion of the Kalahari Desert in South Africa. We tested whether skinks were more abundant in trees containing sociable weaver nests and asked whether the physical features of trees were important predictors of skink abundance. We then focused on potential costs of this association by examining the relationship between skink abundance and the presence of a potential predator, the pygmy falcon (Polihierax semitorquatus), which exclusively uses weaver nest colonies for roosting and nesting. Finally, we simulated a predatory threat to determine if skinks assess risk differently if a weaver nest is present. We found a significant positive association between the presence of weaver nests and skink abundance. In the absence of nests, the type of tree did not influence skink abundance. Skinks used weaver nests and were more likely to perch on the nest than the tree. When threatened with predation, skinks preferred to take refuge in nests. Surprisingly, the presence of nesting pygmy falcons in nests did not influence skink abundance, perhaps because of the abundance of nearby refuges within nests, tree crevices, or in debris at the tree base. We suggest that sociable weaver nests provide multiple benefits to skinks including lowered predation risk, thermal refuges and greater prey availability, although this requires experimental testing. In the current era of global climate change, sociable weaver nests may become a crucial resource for skinks seeking refuge as the Kalahari climate warms.     

The impact of plant associations on Macrosteles quadrilineatus management in carrots

Habitat diversification can influence the interactions of insects with plants and this can be used in agroecosystems for the management of pest populations. Plant diversification can be achieved through planting crops, such as trap crops, or by adjusting weed management. Aster leafhopper, Macrosteles quadrilineatus Forbes (Hemiptera: Cicadellidae), is a polyphagous species that uses cereals, vegetables, and weeds as host plants. The influence of weeds on M. quadrilineatus abundance was investigated experimentally in carrot [Daucus carota L. cv. Canada (Apiaceae)] field plots by adjusting the level of management of two groups of weeds (broadleaf and grass) and by comparing it to weed‐free plots. The preference of M. quadrilineatus for different cereal and weed species relative to carrots was tested in choice test assays. Habitat context influenced the abundance of M. quadrilineatus in the field experiments. The presence of border crops such as oat, rye, barley, wheat, and triticale did not significantly attract or repel this insect to carrot plots compared to the no‐border treatment. However, spelt‐bordered plots had 42% fewer M. quadrilineatus than three treatments, triticale, wheat, and barley, that had the highest insect abundance. The type of weed management affected M. quadrilineatus abundance in carrot plots, but not the frequency of herbicide application. Plots that had carrot growing with broadleaf‐weeds had about 59% fewer M. quadrilineatus compared with those growing with crabgrass or carrot alone. In the greenhouse choice tests, grasses (e.g., cereals) attracted and broadleaf‐weeds repelled M. quadrilineatus relative to carrots. In summary, carrot growers may be able to manage this pest by reducing the interaction of cereal cover crops with carrots and eliminating grassy weeds in commercial production fields.     

Energetic and locomotor costs of tail loss in the Chinese skink, Eumeces chinensis

Caudal autotomy is a defense mechanism used by numerous lizards to evade predators, but this entails costs. We collected 294 adult Chinese skinks (Eumeces chinensis) from a population in Lishui (eastern China) to evaluate energetic and locomotor costs of tail loss. Of the 294 skinks, 214 (c. 73%) had previously experienced caudal autotomy. Neither the proportion of individuals with regenerated tails nor the frequency distribution of locations of the tail break differed between sexes. We successively removed four tail segments from each of the 20 experimental skinks (adult males) initially having intact tails. Lipid content in each removed tail segment was measured, and locomotor performance (sprint speed, the maximal length traveled without stopping and the number of stops in the racetrack) was measured for each skink before and after each tail-removing treatment. Another independent sample of 20 adult males with intact tails was measured for locomotor performance to serve as controls for successive measurements taken for the experimental lizards. Caudal lipids were disproportionately stored along the length of the tail, with most lipids being aggregated in its proximal portion. Tail loss significantly affected sprint speed, but not the maximal length of, or the number of stops during the sprint. However, the adverse influence of tail loss on sprint speed was not significant until more than 51% of the tail (in length) was lost. Our data show that partial tail loss due to predatory encounters or other factors may not severely affect energy stores or locomotor performance in E. chinensis. As tail breaks occurred more frequently in the proximal portion of the tail in skinks collected from the field, we conclude that caudal autotomy occurring in nature often incurs substantial energetic and locomotor costs in E. chinensis.     

Location of long‐term communal burrows of a threatened arid‐zone lizard in relation to soil and vegetation

The great desert skink (Liopholis kintorei) of the Egerniinae subfamily (Reptilia: Scincidae) is a communal burrowing lizard that inhabits arid spinifex grasslands in central Australia. Great desert skink activity is centred in and around the burrows which are inhabited for many years. However, it is not known whether skinks select burrow sites with specific attributes or how continuing occupancy of burrows is influenced by the surrounding habitat; especially post‐fire, when plant cover is reduced. Here, we test whether great desert skink burrows in areas burnt 2 years previously and in longer unburnt areas are associated with particular habitat attributes, and whether there are differences between occupied and recently abandoned burrow sites. Vegetation composition, cover and soil surface characteristics at 56 established great desert skink burrows, including occupied and recently unoccupied burrows, were compared with 56 random nearby non‐burrow control sites. Burrow sites had higher plant cover compared with the surrounding landscape in both recently burnt and longer unburnt areas and were more likely to be associated with the presence of shrubs. Soil stability and infiltration were also higher at burrow sites. However, we found no evidence that burrows with lower cover were more likely to be abandoned. Our results suggest that great desert skinks may actively select high cover areas for burrow construction, although differences between burrow and control sites may also partly reflect local changes to plant cover and composition and soil properties resulting from burrow construction and long‐term habitation of a site. Further research should determine if burrows with shrubs or higher plant cover provide greater protection from predators, more structural stability for burrow construction, increased prey abundance or other benefits. We recommend that maintenance of areas with relatively higher plant cover be prioritized when managing great desert skink habitat.     

The Effect of Human Presence and Human Activity on Risk Assessment and Flight Initiation Distance in Skinks

Antipredator behavior and risk assessment of many species are affected by the presence of humans and their activities. Previous studies have largely been conducted on birds and mammals and relatively less is known about human impacts on reptiles. We used flight initiation distance (FID) as a measure of risk assessment in inland blue‐tailed skinks (Emoia impar) and tested the direct and indirect effects of humans on risk assessment. We first examined the effects of varying levels and types of human disturbance and activity on skink FID. We found that skinks flushed at significantly longer distances in areas with the least human activity. We then tested the degree to which skinks are able to discriminate different numbers of humans by comparing FID across three different types of approaches. Skinks did not significantly differentiate between a single approacher and a single approacher coming from a group of two other people, but did flush at greater distances when approached by three people simultaneously. Although skinks are not directly harvested or harassed by humans, they have refined human discrimination abilities. Overall, skinks habituate to a variety of human activities and perceive a larger threat when the number of human approachers is greater.     

Ecology of the growth of Anolis nebulosus (Squamata: Dactyloidae) in a seasonal tropical environment in the Chamela region,Jalisco, Mexico

Juvenile growth rates are thought to be restricted by available food resources. In animals that grow throughout the year, such as tropical lizards, growth is therefore predicted to be faster during the rainy season. We test this prediction using a population of Anolis nebulosusby describing the growth trajectories of both sexes using nonlinear regression models, and we then correlate the growth rates of individuals with food available in the environment, precipitation, and temperature. The Von Bertalanffy model fits the growth rates of the females better, while the logistic‐by‐length model fits the males better. According to both models, the males grew faster than females, reaching slightly smaller sizes at adulthood. Males reached sexual maturity when 35 mm long, at an age of seven months, and females matured at 37 mm (SVL), taking nine months to reach this size. In 1989, juvenile males and females grew more in both seasons (rainy and dry) than adults; for 1990, there were no differences by season or between age classes. These results are interesting since in the 1989 and 1990 rainy seasons, practically the same orders of prey and the greatest abundance of prey available in the environment were registered. A possible explanation could be that predation was more intense in 1990 than in 1989. There is little evidence that food, temperature, and humidity affect growth rates of A. nebulosus, refuting our predictions. This is mainly due to the low variation in growth observed in 1990. Therefore we think that the growth of this species reflects a complex combination of ecological and genetic factors.