The Density of Alouatta seniculus in the Eastern Llanos of Colombia

564 PRIMATES, 22(4): 564--569, October 1981 The Density of Alouatta seniculus in the Eastern Llanos of Colombia THOMAS R. DEFLER Programa Nacional Colombiano de Primatologia, INDERENA and The American Peace Corps ABSTRACT. The density of Alouatta seniculus groups was calculated and group components recorded in each of two areas of more than 100 ha, separated by 150 km in the llanos of eastern Colombia. Densities in each area were remarkably similar at 4 groups/km 2 and group structure comparable, although space utilization was somewhat different. INTRODUCTION Knowledge of primate densities is remarkably poor for most species and where some data are known, comparable data in different habitats are generally unavailable. The importance of such information for theoretical considerations as well as for basic management decisions is obvious. In some cases the density data available may reflect abnormal situations where primate populations have concentrated because of local protection and/or destruction of surrounding habitat. In the case of very wide-spread and adaptable species, density comparisons from different habitats may serve to suggest habitat preferences and may shed light on competitive interactions. METHODS During a study of Cebus albifrons (DEFLER 1979a, b, 1980) in the llanos of eastern Colombia from October 1977 to February 1978, the author mapped locations of sightings and counted all Alouatta seniculus groups within the study area of about 100 ha of forest surrounding a 100 rn high hill near the Orinoco River. Likewise, during a comparative study of Cebus apella 150 km to the west of the first study site, which took place during 1978, 1979 and 1980, all Alouatta senicuhts groups were likewise counted and located on a map of the study forest which contained about 150 ha. Adult males were defined as those males usually larger than the adult females and possessing prominent throats and large, muscular heads. A category of subadult male was used and these were males, usually about the size of adult females and with much less conspicuously developed head and neck. Adult females were the largest females of the group, often having smaller individuals associated with them. Infants were the smallest individuals of the group. They were intimately associated with an adult female, often being carried by her and seen to nurse and touch the adult female. Juvenile monkeys were those animals smaller than the adult female but more independent than the infant category and larger than them. Juveniles were never carried by an adult. Both studies took place in the 548,000 ha National Park of

Density of Alouatta seniculus 565 5~ 0 60 i.,, 1 I I km,qg COLOMBIA Fig. 1. Map of E1 Tuparro National Park. 1 : The first study site; 2: the second study site. E1 Tuparro, which is administered by INDERENA (the institute for Natural and Renewable Resources) of the Colombian Department of Agriculture (Fig. 1). RESULTS Results for each area are shown in Tables 1 and 2. DISCUSSION NEVILLE (1972, 1976) published data on the population structure and density of A. seniculus on a protected ranch in Venezuela and he found a density of 10 groups/km 2 for this site, which is located in the llanos of Guarico State. This density is 2.5 times that found by this author (Table 1) in the protected and virgin llanos of E1 Tuparro National Park. Also the average home range values calculated by NEVmLE of 3.21 ha are much smaller than those seen by this author of 13.3 and 23.75 ha in Areas 1 and 2, respectively. A study by RUDRAN (1980) of the same population as NEVILLE'S found essentially the same results five years later. Other less detailed density data for this species has been published by KLEIN and KLEZN (1976) for closed canopy forest north of the Guayabero River in La Macarena National Park, Colombia. Those data of 0.8-2.0 groups/km 2 are more comparable with the present study. Recent observations by this author in the Rio Miriti-Paranfi region (left bank affluent of the Caquet~t River) in the Colombian Amazon are suggestive of lower densities ofa. senicutus than are seen in the Colombian llanos (DvVLER, pers. obs.). It may be that this species is best adapt-

566 T.R. DErLER Table 1. Group structure and home range for observed Alouatta seniculus groups. Group structure for Size of home Group structure for Size of home Area 1 range Area 2 range West group insuf, data Group 1 26 ha 1 adult male 1 adult male 1 adult female 2 adult females l infant 1 subadult male 3 1 juvenile female 1 infant female North group 20 ha 1 infant male (born 3 adult males 3 adult females about 5-14-79) 2 juveniles 6-7 1 infant Group 2 25 ha 9,2 adult males 2 adult females East group 10 ha 1st infant seen 5-7-79 1 adult male 2nd infant seen 7-28-79 adult female 1 juvenile infant disappears by 8-13-79 3 4-6 South group 10 ha Group 3 23 ha 2 adult males 2 adult males 3 adult females 3 adult females 2 juveniles 1 infant 1 infant 6 8 Group 4 21 ha 3 adult males mean -- 5.75/group mean -- 13.3 ha 2 adult females 23 individuals/kin ~ infant first seen 10-19-79 5-6 Group 5 2 adult males 4 adult females 1 juvenile male 1 juvenile female 1 infant female 9 West group 2 adult males 3 adult females 2 juveniles 1 infant 8 mean = 6.8/group 27 individuals/kin e insuf, data at least 21 ha mean = 23.75 ha ed to the more seasonal type of dry forest found to the north of the closed canopy selva, as has been suggested by THORINGTON, RUDRAN and MACK (1980). Also, perhaps lessened competition is a factor, since the Venezuelan population shares its habitat with one other primate species; the E1 Tuparro howlers share their forest with 2-3 other species of primate (depending on the area); the KLEINS' study area A. senicu[us is found sympatric with four other species; while in the Miriti-Paranfi region the species is sympatric with seven other primates. Interestingly the average number of individuals/kin ~ is quite comparable to the numbers

Density of Alouatta seniculus 567 Table 2. Ratios for adult males to adult females and adult females to immatures, and average home range for each area studied. No. of Average size of Area groups Adult males: adult females Adult females: immatures home range 1 4 7:8 (1:1) 8:8 (1:1) 13.3 ha 2 6 12:16 (1:1.5) 16:12.51~ (1:0.78) 23.75 ha 1) Immatures calculated on mean observed in group throughout study. calculated by KLEIN and KLEIN (1976), i.e., 23/km'-' in Area 1 and 25-29/km e in Area 2, although only about 1/4 the number calculated by NEVmLE (1972, 1976) and RUDRAN (1980). Also, the mean of the groups studied by NEVILLE and RUDRAN was much larger in Venezuela at 8.46 and 8.9, respectively, while in the present study (with a much smaller N) the means were 5.75 for Area 1 and 6.8 for Area 2. KLEIN and KLEIN (1976) found a mean of almost five. Alouatta palliata is probably the most studied New World primate in the field and has yielded group densities comparable to the present study as summarized by HELTNE and THORINGTON (1976): 1.5-1.8 groups/kin 2 (CARPENTER, 1964); 1.9 groups/krn 2 (COLLIAS SOUTHWlCK, 1951) ; 2.8 groups/km 2 (CARPENTER, 1964) ; 4.0-5.6 groups/kin ~ (CHlvERS, 1969) ; and 2.0-2.5 groups/km 2 (FREESE, 1976). A tremendous density of 55 groups/km 2 recorded by BALDWIN and BALDWIN (1976) was analyzed by them as an example of a refuge situation where the population had become abnormally concentrated because of surrounding habitat destruction. Such high densities point out the great flexibility of this genus in surviving changing conditions. To date, studies of A. senicu/us have shown an adult male to female ratio of only slightly more females to males and this study is no exception (Table 2). The studies of NEVILLE (1972, 1976) and of RUDRAN (1980) showed that there was a gradual increase in the populations studied and that this increase probably continues. HELTNE, TURNER and SCOTT (1976) discuss the use of the ratio of adult females to the sum of juveniles and infants (J+ 1 =~ "immatures') as a useful measure of the "health" of that population, since many adult females without young should indicate a population which may be in difficulty and shrinking. Likewise, higher numbers of young should indicate that numbers are being replaced or that the population is expanding. Since this study took place in a virgin ecosystem, it might be postulated a priori that the population is probably stable (obviously not necessarily true if factors are present which are depressing the population). The ratio of adult females to immature in Area 1 is unity and there were slightly fewer immatures than adult females in Area 2 (0.78) in this study. The same ratio (F: immatures) in the NEVILLE study is 1:1.69 and in the RUDRAN study 1:1.92, probably reflecting the increase which they believed to be occurring in that population. HELTNE, TURNER and SCOTT (1976) present two theoretical models for A. palliata which hypothesize four years of active female reproduction and eight years of active female reproduction. The first model, called the "7-year model," assuming three years of female preadult life, suggests a F: immature ratio of 1:1.5 for a stable population. The "ll-year model," assuming the same three years of female preadult life, suggests a F: immature ratio of about 1:0.75 on the average for a stable population. The above models also assume one infant per year per female, a sex ratio of 1:1 at birth and a 50 ~ preadult mortality for both sexes. The author also suggest that a conservative estimate of F: immature ratio for a stable population ofa.palliata would be in the neighborhood of 1:1. The models support the hypothesis that the populations of the present study are stable, if sirailar assumptions are made for A. seniculus as were made for A. palliata.

568 T.R. DEFLER The habitat of Area 1 has been described by DEFLER (1979a). It differed from Area 2 in being divided into four discrete zones containing a canopy of 20-25 m with emergents to 30 m. Each of these zones was separated from the other by a much lower and more xeric vegetation characterized by the palm Syagrus orinocensis and many ground bromeliads. The Canopy of the xeric vegetation was usually only 10-15 m with few or no emergents and A. senieulus groups were never observed to use these parts of the forested study area, restricting their activities only to those zones of the forest that were more developed and mesic. In Area 1, except for long-distance calling, interactions between A. seniculus groups were never observed. In contrast, Area 2 was a gallery forest exhibiting in its central portions a continuous canopy of 20-25 in with frequent emergents to 35 m. Almost all parts of this forest were utilized by the A. seniculus groups and several close interactions between groups were observed. One interaction involved the chasing of Group 4 by the larger Group 5 away from the boundary between the two groups' home ranges. This chase resulted in one of the males of Group 4 falling out of the tree in his haste to escape. Because of the unaltered state of the areas in this study and the great distance between them, it can be hypothesized that the densities of 4 groups/km 2 may be characteristic of large areas of the llanos. It would be worthwhile to calculate the density for this species in unaltered gallery forests of the llanos further removed from these sites as well as in other major types of habitat, such as lowland rain forest. Acknowledgements. The author is grateful to INDERENA and the Colombian government as well as to the Peace Corps for making it possible to spend four years working in various parts of El Tuparro National Park. Particular thanks are extended to ERNESTO BARRIGA of the Peace Corps and to JORGE HERN~NDEZ of INDERENA for their support. REFERENCES BALDWIN,.]. D. & J. I. BALDWIN, 1976. Primate populations in Chiriqui, Panama. In: Neotropical Primates. Field Studies and Conservation, R. W. THORINGTON, JR. & P. G. HELTNE (eds.), National Academy of Sciences, Washington, D.C., pp. 20-31. CARPENTER, C. m., 1964. Behavior in red spider monkeys in Panama. In: Naturalistic Behavior of Nonhuman Primates, C. R. CARPENTER (ed.), Pennsylvania State Univ. Press, University Park, pp. 93-105. CHIVERS, D. J., 1969. On the daily behavior and spacing of the howling monkey groups. Folia Primatol., 10: 48-102. COLLIAS, N. E. C. H. SOUTHWICK, 1951. A field study of population density and social organization in howling monkeys. Proe. Amer. Philos. Soc., 96: 143-156. DEFLER, T. R., 1979a. On the ecology and behavior of Cebus alibij?ons in eastern Colombia. I. Ecology. Primates, 20: 475-490.,1979b. On the ecology and behavior of Cebus albifrons in eastern Colombia. II. Behavior. Primates, 20: 491-502., 1980. Notes on interactions between the tayra (Eira barbara) and the white-fronted capuchin (Cebus albifrons). J. Mammal., 61 : 156. FREESE, C., 1976. Censusing Alouatta palliata, Ateles geoffroyi and Cebus capucinus in the Costa Rican dry forest. In: Neotropical Primates. FieM Studies and Conservation, R. W. THORINGTON, JR. & P. G. HELTNE (eds.), National Academy of Sciences, Washington, D.C., pp. 4-9. HELTNE, P. G. & R. W. THOmNGTON, JR., 1976. Problems and potentials for primate biology and conservation in the New World. In: Neotropical Primates. FieM Studies and Conservation, R. W. THORINOTON, JR. & P. G. HELTNE (eds.), National Academy of Sciences, Washington, D.C., pp. 110-124.

Density of Alouatta seniculus 569 - -, D. C. TURNER ~I; N. C. SCOTT, JR., 1976. Comparison of census data on Alouattapalliata from Costa Rica and Panam~L In : Neotropical Primates. Field Studies and Conservation, R. W. THORmGTON, JR. & P. G. HELTNE (eds.), National Academy of Sciences, Washington, D.C., pp. 10-19. KLEIN, L. L. & D. J. KLEIN, 1976. Neotropical primates: Aspects of habitat usage, population density and regional distribution in La Macarena, Colombia. In: Neotropical Primates. Field Studies and Conservation, R. W. THORINGTON, JR. & P. G. HELTNE (eds.), National Academy of Sciences, Washington, D.C., pp. 70-78. NEVILLE, M. K., 1972. The population structure of red howler monkeys (Alouatta seniculus) in Trinidad and Venezuela. Folia Primatol., 17: 56-86. - -, 1976. The population and conservation of howler monkeys in Venezuela and Trinidad. In : Neotropical Primates. Field Studies and Conservation, R. W. THORINGTON, JR. & P. G. HELTNE (eds.), National Academy of Sciences, Washington, D.C., pp. 101-109. RUDRAN, R., 1980. The demography and social mobility of a red howler (Alouatta senieulus) population in Venezuela. In: Vertebrate Ecology in the Northern Neotropies, J. F. EISENBERG (ed.), Smithsonian Institution Press, Washington, D.C., pp. 107-126. THORINGTON, JR., R. W., R. RUDRAN 8r D. MACK, 1980. Sexual dimorphism of Alouatta senieulus and observations on capture techniques. In: Vertebrate Ecology in the Northern Neotropics, J. F. EISENBERG (ed.), Smithsonian Institution Press, Washington, D.C., pp. 97-106. --Received November 28, 1980; Accepted May 7, 1981 Author's Name and Present Address: THOMAS R. DEFLER, Apartado Akreo 4559, Bogotd, Colombia.