Abstract
. 1991. Population and habitat characteristics of two sympatric species of Ctenomys (Rodentia: Octodontidae). Acta theriol. 36: 87 -94.
Abstract
Demography, spatial distribution, body size and habitat characteristics of two sympatric species of Ctenomys have been studied seasonally at Necochea (Buenos Aires Province, Argentina). Densities were 11.3 individuals/ha for Ctenomys talarum and 5.2 indiv./ha for Ctenomys australis. Breeding was seasonal for both species. Sex ratios were unbalanced in favour of females in C. australis while there were not significant differences between sexes in C. talarum. The spatial distribution for both populations was clumped and related to habitat characteristics. C. talarum was found in areas with higher plant biomass and harder soils, while C. australis was restricted to the areas with lower plant biomass and less compact soils. Spatial separation of these two species may be caused by different commitment to life underground resulting from differences in body size and energetics. Two possible explanations for the spatial distribution of C. australis and C. talarum are given.
Introduction
Fossorial rodents of the genus Ctenomys, commonly termed tuco-tucos, arc among the most geographically variable mammals, being the most important genus according to number of species of subterranean rodents (Reig el al. 1990 ).
Introduction
The evolutionary process in a subterranean ccotope, which is characterized by its simplicity, stability and prcdecibility, leads to an adaptive convergence. The major evolutionary determinants of this convergence are specialization, competition and isolation acting between and within species (Nevo 1979 (Nevo , 1982 .
Introduction
As in other subterranean rodents, tuco-tucos show disjunts areas of distribution. Only one case of sympatry has been recorded, and that is in Monte Hermoso (Buenos Aires Province) between Ctenomys australis and Ctenomys talarum (Contreras and Reig 1965) . In the present study, we found that the distribution of these two species also overlaps further to the north at Necochea (Buenos Aires Province) where previously only C. australis was recorded.
Introduction
In the zone where C. australis and C. talarum are sympatric we hypothesize the development of life history traits and habitat selection mechanism that would prevent strong competitive interactions.
Introduction
In this study we provide completely unknown information on demography, habitat characteristics, reproduction and body size in two sympatric species of Ctenomys with the aim of understanding how they coexist.
Study area
The study site is a 4 -10 km wide coastal dune fringe which grades into the inland grassland, presently cultivated. The dune fringe is generally diagonal (Frenguelli 1928) and exhibits a vegetation cover of about 20%, composed mainly of Poa sp., Panicum racemosum and Calystelgia soldanella. The sand dunes reach altitudes ranging from 30 to 50 m, above sea level, and extend 200 to 2000 m (Frenguelli 1928 ).
Study area
In the interdune areas, humid conditions prevailed allowing the development of a more dense vegetation cover (up to 80%). Dominant vegetation was: Adesmia sp., Sllpa tricotoma, Stipa neesiana and Bromus unioloides. In these interdune depressions, thin soil is overlaying calcrete layers, locally known as "tosca". The alternation of dunes and interdunes areas, each one with different topography and vegetation patterns, can be considered as typical patchy-like environment.
Methods
From November 1987 to October 1988, 69 Ctenomys australis (C. a.) and 59 Ctenomys talarum (C. /.) were captured during four seasonal trappings. The captures took place in four different areas. These areas were staked in a grid pattern with the stakes spaced 20 m apart, one coordinate was indicated on the stakes by numbers, the other by letters. Exact trap site could thus be recorded. Snap traps Oneida-Victor No. 0 and plastics traps were placed at the entrances of the burrows to catch all animals present in the areas.
Methods
When a tuco-tuco was captured, the trap position was carefully estimated to the nearest meter on both coordinates and written on a map. This schedule allowed us to determine density and spatial distribution of both species.
Methods
All animals were autopsied to obtain information on reproductive condition, sex ratio, number of embryos and relative age. Females were classified as immatures (uterus threadlike, close vagina and no follicular activity) and mature (open or plugged vagina and ripe follicles). Pregnancy or post-partum pregnancy was also noted. The latter was determined from the period of overlap of visible pregnancy and lactation or placental scars. Males were also classified as immatures (seminal vesicles attached to the defferent ducts and lack of spermatozoa in epididymus). All these criteria were based on those from Pearson et at. (1968) and Malizia and Busch (in press). Relative age of animals was determined on the basis of dry weights of eye lens (Lord 1959) .
Methods
Environmental variables (aboveground and belowground plant biomass, plant cover, percent of clay-loam fraction, soil moisture, depth and compaction) were measured from samples (n = 54) taken near burrows for the purpose of characterizing habitat preferences of both species. Aboveground and belowground plant biomass were estimated by dry weight of extracted vegetation samples of 30 cm diameter and 30 cm belowground depth. Plant cover was measured on 0.25 m 2 square using an ocular estimate method (Daubenmire 1959) . Soil compaction was measured using a soil penetrometer and soil moisture was determined by differential weightings before and after drying the soil samples. Clay-loam fractions were separated using a sieve set.
Methods
An NTSYS Principal Component Analisis (PCA) program, through a correlation matrix to generate a reduced set of orthogonal vectors from original variables was performed. Results arc given as x ± SE. Student's /-tests and x 2 were used to analyze differences between means and frequencies, respectively.
Results

Density and spatial distribution
During the period of the study (November 1987 -October 1988) the average densities (without considering lactating individuals captured in the mother's burrow) were 11.32 ± 2.56 and 5.15 ± 0.76 individuals/ha for Ctenomys talarum and Ctenomys australis, respectively. C. t. had a peak density in autumn (June) whereas for C. a. highest numbers of individuals were recorded in spring (December) ( Table 1) . Poisson distribution analysis indicated that both species had a clumped distribution < 0.05). Distance between individuals was smaller (/; < 0.01) in C. t. than in C. a.; distance to nearest neighbour was 14.56 m ± 1.32 (n = 55) and 20.79 m ± 1.49 (n = 40), respectively. Interspecific distance to nearest neighbour was 17.12 m ± 1.28 (n = 43).
Density and spatial distribution
Habitat characteristics Spatial distribution of populations was related to habitat characteristics as expressed by different soils and vegetation preferences. The PCA analysis used for the characterization of habitats of both species showed the following results:
Density and spatial distribution
Plant cover, soil compaction and plant biomass (aboveground and bclowground) load positively on PC 1, while soil depth loads negatively on this vector. The PC 1 explained 58.9% of the total variance. In Table 2 , correlation coefficients for each of the first three PC vectors and the original measurements arc given. Soil moisture and percent of clay-loam fraction load positively on PC 2, which explained 22.3% of total variance (81.2% cumulative). Only bclowground biomass loaded on PC 3; this vector explained 8.3% of the variance (89.5% cumulative).
Density and spatial distribution
The significance of these differences in environmental variables to discrimination among the habitat of both species, is evident from the bivariatc PC plots (Fig. 1) . One group was formed by soil and vegetation samples taken near C. t. burrows, and it is characterized by the highest plant biomass, plant cover and shallowest and hardest soils. All these traits arc related to intcrdune areas. A second group was formed by samples taken near C. a. burrows, having a lower plant biomass, lower plant covcr and deeper and less compact soils, which characterized dune zones. Although it seems to exist a sharp delimitation of both habitats occupied by C. t. and C. a., in zones where different soil and vegetation arc intermingled this pattern is not clearly evident. Table 3 shows mean values and statistical significances for environmental variables measured for both species.
Density and spatial distribution
• Although males of both species of tuco-tucos showed a year round reproductive competence, breeding was seasonal. Fig. 2 Prevalence of pregnancy in C. a. peaked at the end of winter, when 100% (n = 10) of the females were pregnant. On the other hand, the highest activity for C. t. was recorded in spring, when 67% (n = 6) of the females were pregnant (Fig. 2) . C. a. had a lower number of embryos/females than C. t. (0.1 < p < 0.2). Mean number was 3.18 ± 0.95 and 4.17 ± 1.94 embryos/females for C. a. and C. t., respectively.
Density and spatial distribution
In spring 78% (n = 9) of the C. a. pregnant females were also lactating suggesting that this species has a post-partum pregnancy and two litters/year. On the other hand, there is no evidence of a post-partum pregnancy in C. t., since no female was recorded pregnant and lactating at the same time.
Sex ratio
Adult sex ratio, expressed as male/female + male, was unbalanced in favour of females in C. a. (sex ratio = 0.32; p < 0.05; n = 47). In C. t., the sex ratio did not depart significantly from expected 0.5 (0.51; p > 0.5; n = 47). In both species subadult sex ratios were not different from the expccted 0.5 (p > 0.25). Table 4 . C. a. is 3 times heavier and 1.5 longer than C. t. Males of both species are heavier than females, but not longer. Further research to find out the causes of habitat differentiation arc planned.

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