Ecological Differences in Tropical Sympatric Skinks (Mabuya macrorhyncha and Mabuya agilis) in Southeastern Brazil

Abstract

The scincids Mabuya macrorhyncha and M. agilis occur sympatrically and syntopically at the Barra de Marica restinga, in Rio de Janeiro State, Southeastern Brazil. The use of three different niche dimensions (space, food, and time) by the two species was compared and certain morphometric differences between them were analyzed. The two species differed considerably in microhabitat use both horizontally and vertically. Mabuya macrorhyncha was strongly associated with bromeliads, while M. agilis was found mainly on the leaf-litter and had a low degree of arboreality. Both species had a unimodal activity pattern with a peak between 0900 and 1200 h. The two species also differed in diet composition by volume and by number of prey ingested (although spiders and cockroaches are important items in the diets of both), but did not differ in mean prey size. We also observed morphological differences between the two species, which are presumably associated with their differential microhabitat preference: Mabuya macrorhyncha had greater mean values of hand plus finger length than M. agilis, which had greater values of head height and body height. The longer fingers and more flattened body plan of M. macrorhyncha probably allow, respectively, a greater degree of arboreality and more efficient movements between tough bromeliad leaves compared to the ground-dwelling M. agilis. We concluded that the two species of Mabuya at Barra de Marica differ mostly in microhabitat use (compared to other ecological aspects we studied) and that such differences in the horizontal and vertical use of the spatial niche are closely related to some morphological differences. The importance of competition in the determination of differences in the utilization of one or more niche dimensions between sympatric species has been, until quite recently, overestimated (e.g., Pianka, 1973; Schoener, 1977). More recent studies have indicated that competition in lizard communities may, in fact, be less common (Dunham, 1980, 1983; Tinkle, 1982; M'Closkey and Baia, 1987) or, at least, less important in community structuring (e.g., Barbault and Maury, 1981; Gonzalez-Romero et al., 1989) than it has been thought. In many studies of the structure of lizard communities, differences in resource utilization between species have been suggested to reflect mainly their specific ecological needs rather than competitive pressures (e.g., Barbault and Maury, 1981; Ortega et al., 1982; Barbault et al., 1985; GonzalezRomero et al., 1989; Bergallo and Rocha, 1994). Nevertheless, resource partitioning may obviously reduce potential and actual competition between sympatric species, although it is often very difficult to evaluate the role of interspecific competition, past or present, in the evolution of different resource utilization patterns (e.g., * Corresponding author. Schoener, 1974; Rozenweig, 1981; Vitt et al., 1981). Among lizards of the genus Mabuya (Scincidae), two or more species commonly occur in sympatry and/or syntopy in certain areas (Pianka, 1971; Huey and Pianka, 1977; Barbault, 1976). These skinks are known to explore a great variety of microhabitats (Vanzolini and ReboucasSpieker, 1973; Rebouqas-Spieker, 1974; Barbault, 1976; Huey and Pianka, 1977; Van Sluys, 1991; Vitt, 1991; Vitt and Blackburn, 1991) and food resources (Vanzolini and Rebouqas-Spieker, 1973; Huey and Pianka, 1977; Vitt and Blackburn, 1991), which presumably facilitates their coexistence in a patchy environment. Some cases are known in which seven or more congeneric species of skinks (Ctenotus) occur in sympatry (Pianka, 1969). At the Barra de Marica restinga in the Rio de Janeiro State, Southeastern Brazil, two skinks of the genus Mabuya (M. macrorhyncha and M. agilis) occur sympatrically and syntopically (Araujo, 1984). Both species have a mean SVL of 60-75 mm when adult and feed mainly on arthropods (pers. obs.). Probably, the two species adopt both active and sit-and-wait foraging strategies (sensu Schoener, 1971), like other species of Mabuya (Vanzolini et al., 1980; Vitt, 1991). Mabuya macrorhyncha and M. agilis are This content downloaded from 207.46.13.113 on Wed, 05 Oct 2016 04:44:57 UTC All use subject to http://about.jstor.org/terms TROPICAL SYMPATRIC SKINKS diurnal and coexist with seven other species of lizard in the area: Tropidurus torquatus and Liolaemus lutzae (Tropiduridae); Cnemidophorus ocellifer, Ameiva ameiva and Tupinambis teguixin (Teiidae); Hemidactylus mabouia and Gymnodactylus darwini (Gekkonidae) (Araujo, 1984, 1991). Here, we compare similarities in utilization of spatial, trophic and temporal niche dimensions between the two Mabuya species and analyze some morphological differences that may have ecological implications. MATERIAL AND METHODS Study Area. -The study was carried out at the Restinga da Barra de Marica (22057'S, 43?50'W), located approximately 38 km east of the city of Rio de Janeiro, Rio de Janeiro State, Brazil. Restingas are Quaternary salt-sandy substrates covered by herbaceous and shrubby vegetation, and are common along the Brazilian coast (Suguio and Tessler, 1984). The climate of the area is seasonal with a warm and rainy summer (December to March) and a dry season (May to September) (Franco et al., 1984; Rocha, 1992). Mean annual temperature varies between 22 and 24 C and mean annual rainfall varies from 1000 to 1350 mm (Nimer, 1972, 1979). Vegetation of the area is composed predominantly of species of Erythroxylaceae, Clusiaceae, Rubiaceae, Leguminosae, Myrtaceae and Bromeliaceae (Silva and Sonmer, 1984). Among the Bromeliaceae, Neoregelia cruenta is one of the most abundant species in the area and both Mabuya species are closely associated with it. Collecting Methods and Analysis.-Lizards were collected monthly from May 1989 to February 1992 using a rifle. Some of the days were dedicated only to field observations, without collection. Both collections and field observations were made from 0600 to 1800 h during the day. For each species, individuals collected were weighed (to the nearest 0.2 g) using a Pesola? spring balance and SVL measured with a caliper (to the nearest 0.1 mm). Time of collection and microhabitat of the lizard at its first sighting were also recorded. The microhabitat of each lizard was registered according to four categories: (a) on bromeliad; (b) on leaf litter; (c) on shrub; (d) on cacti. Height of the lizard above ground in the microhabitat (to the nearest cm) was recorded using a measuring tape. In the laboratory, lizards were dissected and the stomach contents identified to order. The length, width and depth of each prey item were measured with a caliper (to the nearest 0.1 mm) and the volume estimated by multiplying these three dimensions (Schoener, 1967). Differences between the two species in microhabitat utilization, activity patterns and diet composition by prey volume and number were tested using the Kolmogorov-Smirnov test (Siegel, 1956). Comparisons of vertical use of microhabitat (height above ground, in cm) betwe n the two species were made using a oneway analysis of variance (ANOVA). We also used a one-way analysis of variance to test the differences in mean prey size between the two lizard species. Mean prey size was expressed as the mean value of length x width of the five largest pre items in the stomach of each lizard. We made a regression analysis (Zar, 1984) between mean prey size and jaw width and SVL for each species. Differences in sample size presented in the prey size relationships and in tables and figures in the results are due to lizards that had the abdominal region (which impeded stomach contents analysis) or head (which impeded head measurements) destroyed during collection. Also, for activity and height above ground description we included data based only on field observations, in addition to those obtained during collections. The temporal, spatial and trophic niche breadths were estimated (the last by prey number and volume) for each species using Simpson's diversity index (1949):