Extended Application of a Marked-Nest Census Method to Examine Seasonal Changes in Habitat Use by Chimpanzees

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1 International Journal of Primatology, Vol. 22, No. 6, December 2001( c 2001) Extended Application of a Marked-Nest Census Method to Examine Seasonal Changes in Habitat Use by Chimpanzees Takeshi Furuichi, 1,3 Chie Hashimoto, 2 and Yasuko Tashiro 2 Received July 27, 2000; accepted October 3, 2000 The Kalinzu Forest Reserve in Uganda comprises various types of vegetation, including mixed mature forest, Parinari-dominated mature and secondary forest, and Musanga-dominated secondary forest. We used a marked-nest census method to examine seasonal changes in chimpanzees use of the different vegetation types. We made 10 parallel line-transects in the study area; they were 5-km long and 500-m apart. During the first 3 4 walks along the transects, we marked all existing nests. We then conducted 10 main censuses of all transects at 15-day intervals, over a total period of about 5 months. In each main census, we recorded all unmarked nests visible from the transects and marked them. When we saw a nest, we searched for neighboring nests of the same age class 30 m of each other, in order to estimate the size and position of nest groups. To improve the accuracy of the estimation of nest density in each census period, we excluded nests that consisted only of brown leaves and corrected the number of nests observed by allowing for the proportion of newly-built nests that would still have green leaves at the next main census. We estimated the population density of chimpanzees in the study area both by the number of individual nests and by the number of nest groups; the two methods gave similar results. We found differences in number of chimpanzees that used different vegetation types in different fruiting seasons, and differences in nest group size related to the different fruiting seasons. KEY WORDS: chimpanzee; seasonality; habitat use; marked-nest census; Kalinzu Forest; Uganda. 1 Laboratory of Biology, Meiji-Gakuin University, Totsuka, Yokohama, , Japan. 2 Primate Research Institute, Kyoto University, Inuyama, Aichi, , Japan. 3 To whom correspondence should be addressed; furuichi@gen.meijigakuin.ac.jp /01/ $19.50/0 C 2001 Plenum Publishing Corporation

2 914 Furuichi, Hashimoto, and Tashiro INTRODUCTION The Kalinzu and the Maramagambo Forests together constitute the largest remaining block of forest in Uganda. Studies of primates in the Kalinzu Forest, including Pan troglodytes, Cercopithecus lhoesti, C. mitis, C. ascanius, Colobus guereza, and Papio anubis, have been undertaken since Hashimoto (1995) reported that the chimpanzee density in the Kalinzu Forest is as high as those at other study sites in Uganda (Kibale Forest: Ghiglieri, 1984; Budongo: Plumptre and Reynolds, 1996), which are in a higher range of chimpanzee densities reported from various sites in Africa. There are various types of vegetation in the Kalinzu Forest, in part as a result of different levels of exploitation by humans (Hashimoto et al., 1999). These range from a secondary forest that was heavily harvested mechanically, to the least disturbed mature forest, where there has been only limited illegal pit-saw logging. As such, Kalinzu Forest presents a valuable opportunity to examine the relationships between human exploitation and ecology of primates. We aimed to establish a method to examine how chimpanzees use the different vegetation types in different seasons. Researchers have used nest-count methods mainly to estimate the overall density of great apes in given areas (Furuichi et al., 1997; Ghiglieri, 1984; Hashimoto, 1995; Plumptre and Reynolds, 1996; Tutin and Fernandez, 1984; White, 1994). They usually involve counting the number of individual nests or nest groups that are visible from transects, and then using parameters such as mean life-span of nests, estimated number of nests built per day per chimpanzee, and proportion of nest-building individuals to the total number of group members to estimate the density. Of these, the mean life-span of nests is very long and quite variable between seasons, and between field sites, which reduces the certainty of density estimation using nest counts. Although Tutin et al. (1995) maintained that the life-span of nests should be estimated individually for each field site, the value given in their study has often been used by other researchers, because it takes a long time to obtain a good site-specific estimate of this parameter. To circumvent this problem, Plumptre and Reynolds (1996) developed a marked-nest census method in a study of chimpanzees in the Budongo Forest, and Hashimoto (1995) developed a similar method in the Kalinzu Forest. They made repeated censuses on the same transects, and counted the number of nests built between two successive censuses. As long as the interval between the two censuses is short enough, we can assume that all nests will remain until the next census, so there is no need to take the life-span of nests into consideration. The marked-nest census method raises another possibility. Because it deals with nests that are built between two successive censuses, the method

3 Examination of Seasonal Changes in Habitat Use by Chimpanzees 915 may be used to estimate the number of chimpanzees that used a certain area within a fixed period. We used the marked-nest census method, with some necessary modification, to investigate seasonal changes in the use of different types of vegetation. Another problem to be decided in nest census methods is whether estimates of density should be based on the number of individual nests or on the number of nest groups. Tutin and Fernandez (1984) recommended estimates based on the number of nest groups, because nests show clumped distribution. Many studies of nest counts estimated the density based on the number of individual nests (Furuichi et al., 1997; Ghiglieri, 1984; Hashimoto, 1995; Plumptre and Reynolds, 1996), while others based their estimation on the number of nest groups (Hashimoto, 1995; Tutin and Fernandez, 1984; White, 1994). In order to solve this problem, we compared results from the two methods applied to the same sample data. METHODS The Kalinzu Forest Reserve is located in western Uganda (30 07 E; 0 17 S) at m above sea level (Hashimoto, 1995; Howard, 1991). It is classified as a medium altitude moist evergreen forest, together with the Kibale Forest (Howard, 1991). There are two rainy seasons mid-march to the end of May; mid-september to the end of December and two dry seasons beginning of January to mid-march; beginning of June to mid- September. Annual rainfall from June 1997 to May 1998 was 1584 mm (Hashimoto et al., 1999). We established 10 parallel main line-transects, each 5 km long and 500 m apart, running from east to west (Fig. 1). We set the west ends of transects T1 and T2 at the boundary between the Kalinzu Forest and the Maramagambo Forest. The east ends of transects T3 T10 are along trails to the north and south of a sawmill. We built an additional rectangular transect, M, of 1.5 km around the sawmill to obtain supplementary data for the Musangadominated secondary forest. Each of the 10 main transects has ten 500-m sections. We established m sections on Transect M. Each of the 103 sections is classified into one of following 4 categories based on a vegetation survey of the trees (Fig. 1, Hashimoto et al., 1999). Mixed mature forest is without a predominant species. Parinari-dominated mature forest is relatively undisturbed forest dominated by Parinari excelsa. Parinari-dominated secondary forest has a similar tree species composition, but was more disturbed by harvesting. Musanga-dominated secondary forest was extensively disturbed by mechanical harvesting in the past, and has a canopy dominated by Musanga leo-errerae.

4 916 Furuichi, Hashimoto, and Tashiro Fig. 1. Line-transects and vegetation type of each 500-m section. We conducted the study from June 1997 to March By October 15, we had walked all of the 11 transects 3 4 times and had marked all of the nests that were visible from the transects using number tags. From October 16, we made repeated main censuses at intervals of about 15 days until March 12. The census of the 11 transects in the latter half of October is the Oct-2 census, that in the first half of November is the Nov-1 census, that in the latter half of November is the Nov-2 census, and so forth. Based on a census of fallen fruits on the same transects, we consider the censuses from Oct-2 to Jan-2 to be in the high-fruiting season, and those from Feb-1 to Mar-1 to be in the low-fruiting season (Furuichi et al., 2001). During the censuses, one of the authors and a local assistant walked in search of nests, at a speed of about 1 km/h. We recorded only nests that were visible from the transects to estimate density based on the number of individual nests. When we located some nests from the transects, however, we searched the area for other neighboring nests that were not visible from the transects in order to determine the size number of nests and geometrical center of nest groups. A nest group is a collection of nests that seemed to have been built in a cluster on the same day, and includes all nests of the same age class that are 30 m from the nearest other nest. We distinguished 6 age classes of nests. Nests in which all leaves were green and fresh are fresh nests. Among fresh nests, ones with feces or urine of

5 Examination of Seasonal Changes in Habitat Use by Chimpanzees 917 the previous night or more recent are fresh-1, and those without new feces or urine are fresh-2. Nests with both green and brown leaves recent. Recent-1 nests have brown or dried green leaves only on the upper part, recent-2 nests consist entirely of mixed green and brown leaves, and recent-3 nests have green leaves only at the bottom. Nests with only brown leaves are old nests. To examine the aging process, we monitored nests that we found as fresh-1 on the main transects (n = 75). We checked their age classes in subsequent censuses, until they disappeared, or until the last Mar-1 census. We also monitored fresh-1 nests on Transect M (n = 53), because there was no fresh-1 nest in the Musanga-dominated secondary forest on the main transects. Following a standard method for line-transect analysis (Buckland, 1985; Buckland et al., 1993; Burnham et al., 1980; Whitesides et al., 1988), We estimated the density of nest-building chimpanzees, ˆD by ˆD = n f 2L ˆ (0) 1 ˆpdt. In the estimation based on the number of nests, n is the number of nests of fresh or recent age classes that were detected from the transects. In the estimation based on the number of nest groups, n is the product of the number of nest groups detected from the transects and the mean nest group size. L is the total length of transects. f ˆ(0) is given by ˆ f (0) = 1/ 0 g(x) dx, wherein detection function g(x) shows the probability that a nest at perpendicular distance x is found by observers walking the transects (Buckland, 1985; Buckland et al., 1993). We determined the detection function by approximation to the frequency distribution of nests, or nest groups, at each distance. We used a computer program Distance 3.5, release 5 developed by Buckland and others to determine the detection functions. ˆp is the proportion of nests remaining until the next census in fresh or recent age classes. d is the number of nests built per day per individual. We set this value at 1.15 ± (SD) per study in the Budongo Forest (Plumptre, 2000; Plumptre and Reynolds, 1997). t is the length of the interval in days between two censuses. This was set to 15.0 ± 2.1 days for the estimation of number of chimpanzees per km 2 who used each vegetation in each period, and to ± 1.7 days for the estimation of density throughout the study period.

6 918 Furuichi, Hashimoto, and Tashiro RESULTS Proportion of Nests Remaining Until the Next Census Figure 2 shows the aging process of the nests that we found as fresh- 1 on the main transects. All of them became fresh-2 or older within one census interval (15 days), but 96% remained as recent-3 or newer. The basic method for the marked-nest count assumes that all existing nests were marked during the first several walks, and therefore assumes that all nests in the main census were made between the first walks and the main census. However, with repeated main censuses, we sometimes found unmarked old nests that were apparently made before the previous main census. To reduce the erroneous count of those overlooked nests, we excluded all old nests from the count, and corrected the count of nests by using an estimated proportion of newly-built nests that remained as recent-3 or newer until the next census. The proportion of the nests remaining as recent-3 or newer is approximated by a half-normal function: r(x) = exp ( ax bx 2 ), wherein x is the number of days from construction (Fig. 3). The proportion of nests that Fig. 2. Aging process of nests along the main transects.

7 Examination of Seasonal Changes in Habitat Use by Chimpanzees 919 Fig. 3. Proportion of nests that remained as recent-3 or newer on the main transects (a) and on Transect M (b). chimpanzees made between two censuses and that remained until the second census is given by ˆp = r(x) dx/ dx. 0 0 Thus, we estimated that the remaining proportion is ± (SD) for the main transects. Estimation of Chimpanzee Density Over the Whole Area Throughout the Study Period In the 10 repeated censuses of 10 main transects, we recorded 715 nests of recent-3 or newer that were visible from the transects. Per Buckland et al. (1993), we pooled all nests found in all repeated main censuses for each transect, so the sample size for the analysis is identical to the number of transects (10). Figure 4 shows the frequency distribution of nests at each perpendicular distance, and a detection function given by the program Distance. The density of nest-building individuals in the whole study area was estimated to be 1.45 ± 0.34 (SD)/km 2 (Table I). If we assume that 82.5%

8 920 Furuichi, Hashimoto, and Tashiro Fig. 4. Detection probability of nests at each perpendicular distance from the main transects. Figures on the bars show the observed number of nests at each distance. of the total population built nests, as in the Budongo Forest (Plumptre and Reynolds, 1996), the density of chimpanzees in the study area is estimated to be 1.76/km 2. For comparison, we also estimated the density over the whole study area based on the number of nest groups. In all censuses, we found 315 nest groups from the main transects, which consist of 1159 nests including ones invisible from the transects, giving a mean nest group size of However, there was a tendency for nest groups located farther from the transects to be larger. Such a tendency seemed to appear because nest groups with more members covered a larger area, so they were more likely to be detected from transects even when their centers were far from the transects. Therefore, the Distance program employed an expected mean group size of 2.71, which was estimated based on a regression of nest group size on detection probability (Buckland et al., 1993). We derived a detection function from the frequency distribution of nest groups at each perpendicular distance between the transects and the geometric center of nest group members. The density estimated by this method is 1.50 ± Thus, there is no significant difference between the estimated population density based on individual nests and that based on nest groups. The following analyses are based on the number of individual nests.

9 Examination of Seasonal Changes in Habitat Use by Chimpanzees 921 Table I. Estimated density of chimpanzees in the whole area and estimated number of chimpanzees that used each vegetation type Estimated density of nest-building Estimated Sample chimpanzees density of Vegetation n L(m) size n/l f ˆ (0) ˆp d t ±SD (1/km 2 ) chimpanzees Whole area , ± (based on the number of (0.226) (0.024) (0.007) (0.041) (0.011) (0.232) individual nests) Whole area 854 a 50, ± (based on the number of (0.224) (0.040) (0.007) (0.041) (0.011) (0.231) nest groups) Mixed mature , ± 1.20 ] 4.96 (0.287) (0.033) (0.007) (0.041) (0.011) (0.293) Parinari-dominated mature , ± (0.502) (0.040) (0.007) (0.04) (0.011) (0.506 Parinari-dominated secondary 57 12, ± 0.12 ] 0.51 (0.276) (0.087) (0.007) (0.041) (0.011) (0.293) Musanga-dominated secondary b 185 3, ± (0.326) (0.046) (0.042) (0.041) (0.011) (0.335) Note: Figures in parentheses are coefficients of variation (CV). We calculated CV in estimated density from those in the five parameters (Plumptre, 2000). a 2.71 (expected mean nest group size) 315 (number of nest groups). This number is larger than 715 because we included nests that were not visible from the transects in the estimation based on the number of nest groups. b Including data from Transect M. Analyses of density in the other vegetation types and in the whole area include data from the main transects only. Significant difference, p < 0.05.

10 922 Furuichi, Hashimoto, and Tashiro Number of Chimpanzees in Each Vegetation Type Throughout the Study Period Each transect incorporated 3 or 4 vegetation types. In estimating the number of chimpanzees per km 2 that used each vegetation type, we took contiguous sections of the same vegetation type on the same transect to be one sample (Fig. 1). Therefore, the sample sizes are 11 for the mixed mature forest, 15 for the Parinari-dominated mature forest, 14 for the Parinaridominated secondary forest, and 4 for the Musanga-dominated secondary forest. We used the supplementary Transect M, which contains 3 sections of Musanga-dominated secondary forest, to estimate the number of chimpanzees in this vegetation type. Thus, the total sample size for the Musangadominated secondary forest is 5 (Table I). As many as 51% of nests in the Musanga-dominated secondary forest were in trees of Musanga leo-errerae and Carapa grandiflora; they decayed much faster than nests in trees of other species. Therefore, we estimated the proportion of nests remaining until the next census ( ˆp) for the Musangadominated secondary forest and other types of vegetation separately (Fig. 3). The estimated proportion for the Musanga-dominated secondary forest is ± (SD), via data from monitoring nests in Transect M. In the main transects, all of the fresh-1 nests were in the other 3 vegetation types, and there is no significant difference among them in the proportion of remaining. Therefore, we used the proportion estimated from the data on the main transects as a common value for the 3 types. We estimated the f ˆ(0) for each vegetation type separately because visibility in the forest varied among them (Table I). The estimated density of chimpanzees is very high in the mixed mature and the Musanga-dominated secondary forests, and low in the Parinaridominated mature and Parinari-dominated secondary forests (Table I). We compared the densities via t-tests without assumption of equal variances (Buckland et al., 1993, p. 89; Plumptre, 2000, appendix I). There are significant differences between the mixed mature and Parinari-dominated mature forests (t cal = 2.8, df = 12.2, p < 0.05), between mixed mature and Parinaridominated secondary forests (t cal = 3.1, df = 10.9, p < 0.05), and between Parinari-dominated secondary and Musanga-dominated secondary forests (t cal = 2.7, df = 4.5, p < 0.05). Seasonal Change of Number of Chimpanzees Who Used Each Vegetation Via the data from the repeated censuses, we estimated the number of chimpanzees that used each type of vegetation in each period. For example,

11 Examination of Seasonal Changes in Habitat Use by Chimpanzees 923 nests found in the Oct-2 census represent use of each vegetation in the midst of October, and those found in the Nov-1 census represent use from the end of October to the beginning of November. Because of the small sample size, we could not obtain good estimates of f ˆ(0) for each vegetation for each period. However, because the Kalinzu Forest is an evergreen forest, visibility in the forest did not change greatly between seasons. We examined the change of f ˆ(0) in each vegetation type by pooling data for the rainy and dry seasons, but there is no significant tendency for decrease in f ˆ(0), i.e., increased visibility, in the dry season. Therefore, for each vegetation type, we used f ˆ(0) estimated from all main censuses (Table I) as a common value throughout the study. Table II shows the seasonal change in number of chimpanzees that used each vegetation type. Because of large errors due to the small sample size in each period, it was difficult to make comparisons among periods. However, there is a marked difference between the high-fruiting season (Oct-2 to Jan-2) and the low-fruiting season (Feb-1 to Mar-1). The number of chimpanzees in the mixed mature forest decreased in the low-fruiting season (t cal = 2.2, df = 12.6, p < 0.05), when it increased in the Musangadominated secondary forest, although the increase is not statistically significant. Seasonal Changes of Nest Group Size in Each Vegetation Type Using data from the repeated main censuses, we also examined seasonal changes in nest group size, which might roughly reflect the size of foraging parties. Nest groups whose geometric center was further from the transects tended to be larger in size (Fig. 5; ANOVA, F = 8.6, df = 5, p < 0.001). In the estimation over the whole area throughout the study period, the Distance program solved this bias via a regression of nest group size on detection probability. However, the number of nest groups for each period in each vegetation type was too small for such analyses. Post hoc analyses on nest group size via Tukey s HSD showed that nest groups are divided into two uniform subgroups; ones 50 m and ones >50 m. Therefore, in this analysis, we excluded nest groups at >50 m. Due to small sample size, we could not detect meaningful changes in nest group size in each vegetation type in each period. However, the nest group size for the whole study area shows significant difference between the high- and low-fruiting seasons (Table II). The standard deviation of nest group size decreased from 5.5 in the high-fruiting season (n = 240) to 1.7 in the low-fruiting season (n = 70)(F = 11.0, df 1 = 239, df 2 = 69, p < 0.001).

12 924 Furuichi, Hashimoto, and Tashiro Table II. Seasonal changes in the number of nest-building chimpanzees and the size of nest groups Census period Vegetation type Oct-2 Nov-1 Nov-2 Dec-1 Dec-2 Jan-1 Jan-2 Feb-1 Feb-2 Mar-1 Highfruiting season Lowfruiting season Difference between seasons Number of Mixed mature p < 0.05 nest-building (5.5) (1.0) (0.8) (2.7) (1.0) (1.6) (2.5) (0.9) (0.8) (0.5) (1.6) (0.5) chimpanzees Parinari-dominated n.s. that used each mature (0.1) (0.3) (1.3) (0.4) (0.9) (0.4) (0.2) (0.4) (0.1) (0.2) (0.4) (0.2) vegetation type Parinari-dominated n.s. secondary (0.1) (0.3) (0.2) (0.1) (0.3) (0.3) (0.7) (0.5) (0.1) (0.3) Musanga-dominated n.s secondary (1.3) (1.2) (1.3) (1.8) (2.3) (3.3) (1.4) (3.7) (2.0) (3.5) (1.1) (2.8) Nest group size Whole study p < area (6.6) (2.6) (4.2) (9.0) (3.4) (4.3) (5.8) (1.7) (2.1) (1.1) (5.5) (1.7) Note: Figures in parentheses are standard errors. Data from Transect M are included only in the Musanga-dominated secondary forest density estimates.

13 Examination of Seasonal Changes in Habitat Use by Chimpanzees 925 Fig. 5. Size of nest groups at each perpendicular distance from the main transects. Figures on the bars show the number of nest groups. Mean size of the nest groups also decreased from 3.8 in the high-fruiting season to 2.0 in the low-fruiting season (t cal = 4.4, df = 307.6, p < 0.001). DISCUSSION Our purpose was to extend the application of nest census as a survey method. Although nest census has been used to estimate the density of chimpanzees in a given area, we could examine seasonal changes of habitat use by the marked-nest count method proposed by Plumptre and Reynolds (1996) and Hashimoto (1995), with some necessary modifications. The first problem was whether we should estimate the density based on the number of individual nests or on the number of nest groups. Tutin and Fernandez (1984) defined a nest group as nests of the same age class that were recorded along a 20 m stretch of a transect. They counted only nests that were visible from transects. However, Buckland et al. (1993) recommended that all members in the group should be counted in the estimation of density based on the number of groups. Therefore, we assumed that all

14 926 Furuichi, Hashimoto, and Tashiro nests of the same age class 30 m of the nearest other nest constitute a nest group, irrespective of the visibility from the transects. To correct for a bias by lower detectability of small nest groups that are far from the transect, we used an expected mean group size that was estimated based on a regression of nest group size on detection probability (Buckland et al., 1993). As a result, the two methods did not show significant difference either in the estimated density or in the standard deviation. So estimation based on individual nests appeared to be an acceptable method when the time required to determine the size and geometric center of each nest group is not available. In the marked-nest census method, Plumptre and Reynolds (1996) and Hashimoto (1995) assumed that all nests made before the first walks had been marked already, and that no newly-built nest would disappear between the first walks and the main census. These assumptions are acceptable if the first walks are repeated several times. With repeated main censuses, we sometimes found old nests that were apparently made before the previous main census because the main census was carried out only once for each transect in each period. Because we dealt with small samples due to the short interval between main censuses, including previously overlooked nests in the analysis seemed to bias considerably the results on seasonal changes in nest density. To moderate this problem, we excluded all old nests from the observation, and corrected the count of observed nests via an expected proportion that newly-built nests remained as recent-3 or newer until the next census. By these modifications of the marked-nest census method, we were able to increase the reliability of estimates of the number of chimpanzees that used each vegetation type in each period. A major problem with this method is sample size. Because the number of nests that were built between two censuses was small, we needed to make the census distance as great as possible. However, censuses of 50-km transects at 15-day intervals was the maximum capability for one team of observers. A length of 50 km is large enough to get a reliable estimation of the number of chimpanzees in each vegetation type throughout the study periods. However, it is too small to estimate the number of chimpanzees in each vegetation type in each period. Therefore, we used the detection functions that were obtained from data for the whole study period in the estimation for each period. Such a treatment is justifiable, because the visibility did not increase in the dry season in the evergreen Kalinzu Forest. Several prior studies revealed changes in ranging or feeding behaviors of chimpanzees in response to the changes of fruit availability (Chapman et al., 1995; Tutin et al., 1997; Wrangham et al., 1991,1996). However, those studies were based on direct observation of chimpanzees, and therefore are limited to a few field sites. Our study, using the marked-nest census method,

15 Examination of Seasonal Changes in Habitat Use by Chimpanzees 927 may open further opportunities for the ecological studies of chimpanzees in various new field sites (Furuichi et al., 2001). ACKNOWLEDGMENTS The Uganda National Council for Science and Technology, the Uganda Forestry Department, and the Uganda Wildlife Authority gave us permission and valuable assistance for the study in the Kalinzu Forest. Mr. M. G. Moses and staff of the Nkombe Forest Station, and Nkombe Sawmill Co. Ltd. helped our study. Dr. T. Kano, Kyoto University, gave us the opportunity to work in the Kalinzu Forest. Prof. T. Nishida, and members of the Primate Research Institute and Laboratory of Human Evolution, Kyoto University gave us valuable advice. We are very grateful to them all. This study was supported by grants under the Monbusho International Scientific Research Program awarded to T. Kano (No ) and to T. Furuichi (No ). REFERENCES Buckland, S. T. (1985). Perpendicular distance models for line transect sampling. Biometrics 41: Buckland, S. T., Anderson, D.R., Burnham, K. P., and Laake, J. L. (1993). Distance Sampling: Estimating Abundance of Biological Populations, Chapman and Hall, London; Reprinted in 1999 by RUWPA, University of St. Andrews, Scotland. Burnham, K. P., Anderson, D. R., and Laake, J. L. (1980). Estimation of density from line transect sampling of biological populations. Wildlife Monogr. 72: Chapman, C. A., Wrangham, R. W., and Chapman, L. J. (1995). Ecological constraints on group size: An analysis of spider monkey and chimpanzee subgroups. Behav. Ecol. Sociobiol. 36: Furuichi, T., Hashimoto, C., and Tashiro, Y. (2001). Fruit availability and habitat use by chimpanzees in the Kalinzu Forest, Uganda: Examination of fallback foods. Int. J. Primatol. 21(6): Furuichi, T., Inagaki, H., and Angoue-Ovono, S. (1997). Population density of chimpanzees and gorillas in the Petit Loango Reserve, Gabon: Employing a new method to distinguish between nests of the two species. Int. J. Primatol. 18: Ghiglieri, M. P. (1984). The Chimpanzees of Kibale Forest. Columbia University Press, New York. Hashimoto, C. (1995). Population census of the chimpanzees in the Kalinzu Forest, Uganda: Comparison between methods with nest counts. Primates 36: Hashimoto, C., Furuichi, T., Tashiro, Y., and Kimura, D. (1999). Vegetation of the Kalinzu Forest, Uganda: Ordination of forest types using principal component analysis. African Study Monogr. 20: Howard, P. C. (1991). Nature Conservation in Uganda s Tropical Forest Reserves. IUCN, Gland, Switzerland. Plumptre, A. J. (2000). Monitoring mammal populations with line transect techniques in African forests. J. Applied Ecol 37: Plumptre, A. J., and Reynolds, V. (1996). Censusing chimpanzees in the Budongo Forest, Uganda. Int. J. Primatol. 17:

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