DIET OF THE LIZARD Mabuya agilis ( SAURIA ; SCINCIDAE ) IN AN INSULAR HABITAT ( ILHA GRANDE , RJ , BRAZIL )

We examined the stomach contents of 21 specimens of Mabuya agilis (Sauria; Scincidae) collected during February 2001 at the restinga habitat of Praia do Sul, in Ilha Grande, RJ, Brazil. Diet was composed of various types of small arthropods, with no plant material being eaten. Spiders were the most important items in the diet, followed by orthopterans. Apart from the absence of isopterans, the diet of Mabuya agilis from this insular area was similar to those of other conspecific mainland populations. This suggests that factors such as insularity and the absence of other small sympatric lizards may not have a significant effect on the feeding habits of the Praia do Sul population.


INTRODUCTION
The ecology of lizard species in the Atlantic Rainforest biome in eastern Brazil is still little studied, although the amount of information on the subject has increased considerably within the last ten years (Sazima & Haddad, 1992;Rocha, 1994, 2000, andincluded references).Most information has come from studies carried out in mainland coastal habitats of Rio de Janeiro, Espírito Santo, and São Paulo States in southeastern Brazil (Vanzolini & Rebouças-Spieker, 1976;Rocha, 1994Rocha, , 2000)), but the ecology of lizard populations living in insular habitats is mostly unknown, but for a few studies (e.g., Vanzolini & Rebouças-Spieker, 1976;Rocha et al., 2002).This lack of knowledge impedes understanding of how ecological differences may arise as a result of the insular environment.The community equilibria of islands are relatively fragile and their conservation depends on considerable knowledge, not only on species composition, but especially on the range of the many ecological aspects under which they operate (Rocha et al., 2002).
The lizard Mabuya agilis (Scincidae) is widespread along the coastal plains of southeastern, and part of northeastern Brazil (Rocha, 2000;Rocha et al., 2002).Information regarding its ecology, including diet, has recently been published for mainland populations inhabiting restinga habitats (Rocha & Vrcibradic, 1996, 1999;Vrcibradic & Rocha, 1995a, 1996, 2002).The only available data on ecological aspects of an insular population of M. agilis come from the recent study of Rocha et al. (2002) in the Abrolhos Archipelago, about 70 km off the coast of southern Bahia State, northeastern Brazil.At the restinga habitat of Praia do Sul in the island of Ilha Grande, Rio de Janeiro State, M. agilis is apparently the only small diurnal lizard species present, whereas in all other areas where the ecology of that species has been studied it shares its habitat with other similar-sized diurnal lizards, which may be potential competitors.
In this study we analyze the feeding ecology of an insular population of Mabuya agilis from Ilha Grande, Angra dos Reis municipality, Rio de Janeiro State, and compare with other populations of this species for which data on diet are available.

MATERIAL AND METHODS
The study was carried out during February 2001 at the restinga habitat of Praia do Sul (23 o 10'S, 44 o 18'W), on Ilha Grande, an island located in the south of Rio de Janeiro State, southeastern Brazil.Restinga habitats occur along most of the southern portion of the island (Araújo & Oliveira, 1988).Praia do Sul is currently a biological reserve.Annual rainfall in the area is about 2,300 mm and mean annual temperature is about 23 o C (Pereira & Araújo, 2000).The study area is located at sea level.
Lizards were collected using rubber bands and Victor ® mouse glue-traps.After collection, we recorded each lizard's body mass (to the nearest 0.1 g) using a Pesola ® spring balance, and fixed the animals with 10% formalin solution and later storing them in 70% alcohol.In the laboratory, the snoutvent length (SVL) of each individual was measured with a digital caliper (to the nearest 0.1 mm) and all lizards were dissected for stomach content analysis.After excision, lizard stomachs were opened and their contents were spread on a Petri dish and examined under a stereomicroscope.Arthropod prey were identified to the taxonomic level of Order.For each food item found we measured its length and width with a digital caliper (to the nearest 0.1 mm) and estimated its volume using the formula for a prolate spheroid: V = 4/3π(length/2)(width/2) 2 (Vitt, 1991).Mean prey size was calculated as the average length of the five largest identifiable prey items (or of all prey items, for individuals containing less than five items in the stomach).For each prey category, we also calculated an index of importance (I x ) (Powell et al., 1990) which represents the sum of the proportions of the number, volume, and frequency of occurrence of each prey in the diet divided by three.We also recorded the proportion of individuals of M. agilis with empty stomachs.We tested for size (SVL) differences between male and female M. agilis using one-way analysis of variance (ANOVA) (Zar, 1999).Sexual differences in head length (taken as the distance from the snout to the posterior border of the ear opening) was tested using analysis of covariance (ANCOVA), with lizard SVL as the covariate (Zar, 1999).Differences in mean prey length between males and females were tested by one-way ANOVA.The relationship between mean prey length and lizard SVL was analyzed using simple regression analysis.Descriptive statistics are presented throughout the text as arithmetic mean ± one standard deviation.

DISCUSSION
The diet of Mabuya agilis at Praia do Sul was characterized by the consumption of a relatively wide spectrum of food item categories (11 prey types) which characterizes the diet of an opportunistic predator.Also, the diet of the insular population did not contain plant material, an item not common and of low importance in the lizards of the genus Mabuya (Vrcibradic, 2001).Lizards of the genus Mabuya are, in general, non-specialized predators of arthropods (e.g., Huey & Pianka, 1977;Vitt & Blackburn, 1991;Castanzo & Bauer, 1993, 1998;Vitt, 1995;Vrcibradic & Rocha, 1995a, b, 1996, 1998;Vitt et al., 1997).The consumption of a wide array of prey types, including both relatively sedentary (e.g., larvae) and highly mobile preys (e.g., spiders, orthopterans), has also been found for some mainland populations of M. agilis in restingas of Rio de Janeiro and Espírito Santo States (Vrcibradic & Rocha, 1995a, 1996) and also for the insular population of Abrolhos Archipelago (Rocha et al., 2002).This relatively wide consumption of prey types has been argued to reflect the mixed (activeambush) foraging strategy of this species (Vrcibradic & Rocha, 1995a, 1996;Rocha et al., 2002).The consumption of spiders by M. agilis at Praia do Sul was remarkable (it was the most important prey category by number, volume, and frequency).But because we have no data on relative prey availability in the area, we do not know to what extent this high consumption of spiders reflects selectivity of this prey type in the insular habitat of Ilha Grande.Orthopterans and cockroaches were also important prey consumed by M. agilis at Ilha Grande (high I x values).
Those three prey types are usually among the most important items in the diets of M. agilis in other areas (Vrcibradic & Rocha, 1995a, 1996;Vrcibradic, 2001;Rocha et al., 2002) and in the diets of other Brazilian Mabuya species (Vitt & Blackburn, 1991;Vitt, 1995;Vrcibradic & Rocha, 1995b, 1996;Vitt et al., 1997;Pinto, 1999).Thus, it seems that the insular habitat and the absence of potential competitors (i.e., other similar-sized diurnal lizards) do not have a strong influence on the general food habits of M. agilis at Praia do Sul.
An interesting result was the absence of isopterans in the diet of M. agilis at Praia do Sul, although they are a frequent prey of M. agilis in some mainland restinga areas (Vrcibradic & Rocha, 1995a, 1996;Vrcibradic, 2001).However, the M. agilis population from Abrolhos Archipelago does not seem to consume isopterans either (Rocha et al., 2002).Because Mabuya spp.usually have a varied diet (e.g., Castanzo & Bauer, 1998;Vrcibradic, 2001), the absence of isopterans in the diets of some populations may simply reflect low availability of such insects in those areas.Alternatively, since the lizard sample was collected over a relatively short period of time (January to February) and isopterans can be a seasonally available prey in some areas, any interpretation must be done with caution, especially because the evaluation of the availability of isopterans to other lizard species is limited by the absence of local sympatric lizards.
Our data suggest that prey size is affected by lizard body and head size at Praia do Sul.In both cases the relationships were near significance, suggesting a biological trend.Because sample size was relatively small, simply considering that lizard size does not affect prey size would imply a significant risk of incurring in a type II error.
We conclude that M. agilis from Praia do Sul has a varied diet characterized by a relatively wide variety of arthropods and by a lack of plant material and isopterans, the latter being a usual component of the diet in other M. agilis populations.Apart from this fact, the diet did not differ significantly from those of other conspecific populations, suggesting that the effects of insularity and of the absence of other small sympatric lizards may have a negligible effect on the feeding habits of the Praia do Sul population.

TABLE 1 Volume (in mm³), number, frequency of occurrence and Index of Importance (I X ) for each prey category for Mabuya agilis (N = 21) in Praia do Sul, Ilha Grande, RJ. Volumetric values given to the right of the transverse bar represent the values recalculated after excluding the "miscellany" category; those values were the ones used to calculate
I X .