The status and other ecological aspects of the elephant population in Ruhuna National Park, Sri Lanka

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1 J. South Asian nat. Hist., ISSN April, Vol. 1, No. 2, pp ; 6 tabs., 7 figs. Wildlife Heritage Trust of Sri Lanka, 95 Cotta Road, Colombo 8, Sri Lanka. The status and other ecological aspects of the elephant population in Ruhuna National Park, Sri Lanka Mangala de Silva1, B.V.R. Jayaratne2 and Padma K. de Silva1 Abstract The elephant population of the Ruhuna National Park was studied through daily observations from 0600 to 1800 hrs of the park officials for two years in an area of 140 km2. The study area consisted mainly of scrub and grassland with scattered tall trees. The elephants observed in this area per month varied from 36 to 108 with an average of 75. However, there is no resident population as such in the area, the majority seen in the area being a part of a population which has a range extending to other parts of the park. The composition of the studied population was 47.4% adult females, 24.7% adult males (2.3% tuskers), 19.1% juveniles and 8.8% calves. More elephants were observed during and immediately after the major rainy season of October to January than during the drier months. The groups were encountered more often than the solitary individuals. The observed solitary animals contained 85.4% adult males, of which 12.3% were tuskers, and 14.6% adult females. The average group size was 6.2 while the largest group observed contained 36 individuals. The group size was larger during the wet season than during the dry season. During the day, the elephants were most often seen in the evening after 1600 hours and least often seen from 1030 to 1500 hours. The frequencies of observation at different times of the day of solitary males and the groups with and without calves were somewhat different. The monthly mean adult male:female sex ratio varied from 0.37 (1:2.7) to 0.74 (1:1.4) with an average of 0.52 (1:1.9). The monthly proportion of the adult males in mixed groups varied from 0.27 to 0.70 with a mean of This proportion was above the mean value during the rainy period and months immediately following it and also at the peak drought period. The number of adult males in groups varied from 1-4 with an average of Calves and juveniles were observed during all months. The monthly number of calves per adult female varied from 0.05 to 0.41 with a mean value of In general, this ratio was above the mean value during the rainy period and the period immediately following it. K e y w o rd s : elephant, Elephas, status, population, ecology, Sri Lanka, tropical. Introduction The Asian elephant, Elephas maximus Linnaeus, 1758, ecologically as well as economically, is perhaps the most important megaherbivore in the southern and Southeast Asia. The entire wild population of E. maximus is estimated to number between 30,000 and 55,000 and, throughout its range of occurrence which spans 13 countries (Santiapillai & Jackson, 1990), it is considered a highly endangered species, more so in some countries than in others. Of the three subspecies which are generally recognized, namely, E. m. maximus, E. m. indicus and E. m. sumatranus, the Sri Lankan population belongs to the nominate subspecies. The department of Zoology, University of Peradeniya, Peradeniya. 2Department of Wildlife Conservation, Ruhuna National Park, Palatupana, Tissamaharama, Sri Lanka.

2 d e S ilv a, J a y a r a t n e & d e S ilva Asian elephant is primarily a forest species (Sukumar, 1989). Sri Lankan elephant was common and distributed over a wide area throughout the island about a century ago, when the forests were relatively un-degraded and covered 70% of the land area (Geiser & Sommer, 1982). The forest cover was reduced to about 20% of the land area by 1992 (Legg & Jewell, 1992) and continues to be degraded and fragmented. Fragmentation of forests will lead to fragmentation of faunal populations, while forest degradation will lead to habitat degradation and habitat loss. These effects are particularly detrimental to large-mammal populations such as those of elephants. The elephant is one of the most studied large mammals in Sri Lanka, but surprisingly, authorities differ widely even in estimating the number of wild elephants in the country (McKay, 1973; Olivier, 1978; Hoffmann, 1978; Hendavitharana et al., 1994). The ecology of the Sri Lankan elephant has been studied previously by Nettasinghe (1973) in the Thamankaduwa area (northeast), McKay (1973) and Ishwaran (1979) in the Galoya and Lahugala national parks area (east), Eisenberg & Lockhart (1972) in the Wilpattu National Park (north-west), Kurt (1969) and Santiapillai et al. (1984) in the Ruhuna National Park (south-east), and Ishwaran (1993) in the Wasgomuwa National Park area (north-east). Olivier (1978) worked on the ecology of the Asian elephant in the Galoya National Park area as well as in the Malayan rain forest. Vancuylenberg (1974) studied the feeding behaviour of the elephant in south-eastern Sri Lanka while Jainudeen et al. (1972) studied "musth" in the domestic elephant of Sri Lanka. The present study in Ruhuna National Park differs from the previous studies in being based on daily observations extending over two years. Previous studies have provided information on a wet season-dry season basis (Santiapillai et al., 1984; Ishwaran, 1993) or on a monthly basis (Eisenberg & Lockhart, 1972; McKay, 1993; Nettasinghe, 1973; Ishwaran, 1979). Materials and methods Study area. Ruhuna National Park (RNP) extends over an area of about 1250 km2 and comprises five blocks. The Block I, on which the present study was based, covers an area of only 140 km2. However, it contains a variety of habitats in which the wildlife can easily be studied because of the good network of roads and tracks traversing the area (Fig. 1). Its vegetation consists mainly of scrub and grassland. The forest, present along the banks of the Menik River, and the saline flats and sand dunes along the sea coast form the other major habitats. There is a high population density of herbivores, e.g. elephants, buffaloes, sambar, spotted deer and small herbivores, in Block I because of the presence of a wide area of grasslands and several waterholes (de Silva, 1992). The climate, vegetation, and large mammal fauna of RNP have been treated elsewhere recently and are therefore dealt only very briefly here. Details of the physiognomy, climate and vegetation of Block I can be found in Mueller-Dombois (1968), Balasubramaniam et al. (1980), Chambers et al. (1983) and de Silva et al. (1994). The park is situated in the southeastern part of Sri Lanka and has a semiarid climate because of the very seasonal and limited amount of rain it receives. The mean rainfall for the twenty year period was mm (range to mm). The main rainy period is from October to January (Fig. 2) 186 J. South Asian nat. Hist.

3 E c o l o g y o f t h e e l e p h a n t (S r i L a n k a ) Figure 1. Ruhuna National Park, Block I (note the road system traversing most parts of the park and the scattered waterholes, which are more common in the coastal zone). and brings more than 500 mm of rain. It remains much drier from June to September, during which period strong dry winds constantly blow over this region aggravating the drought. The rainfall during the period February to May varies much from year to year. Methods. The study is based on the sightings of elephants in RNP-Block I by the park staff and on authors' personal observations during the period of February 1991 to January The game guards accompanying the visitors to the park, and other park officials were requested to keep a record of their sightings of elephants. They were asked to categorize the animals, where possible, to nine categories following Eisenberg & Lockhart (1972) and Santiapillai et al. (1984). These categories were the adult tusker, adult non-tusker male, adult female, subadult tusker, subadult non-tusker male, subadult female, juvenile tusker, juvenile and calf. Where the sex of a particular adult or subadult could not be ascertained it was classified as an 'unsexed' adult or 'unsexed' subadult, respectively. When the categories could not be ascertained in a group it was taken as an 'unclassified' group. Information on the time and place of sighting, number, sex, category, activity at the time of sighting (whether feeding, drinking, moving, etc.) were thus obtained. Game guards were personally interviewed when clarification was necessary. It was subsequently found that the classification of individuals into adult and subadult categories by game guards was not very accurate and therefore the subadult category was disregarded retaining only the calf, juvenile and adult categories. Thus, the adult categories in the following description include the correspond Vol. 1., No

4 d e S ilv a, J a y a r a t n e & d e S ilva ing subadult categories. Each game guard usually takes visitors three or four times to various areas of the park. The coastal zone (Fig. 1) is more often covered than the inner part as wildlife is more abundant and more easily seen in the grasslands of the coastal area than in the forested inner area (de Silva, 1992). The observations were made from vehicles and the elephants thus observed were mostly in the grasslands and waterholes or in the forest-grassland interphase. The number of visitors to the park tends to be higher in the mornings, and higher still in the evenings, than during midday. Thus, the number of guards in the park, and therefore the number of observations, were higher during the mornings and evenings than during the midday hours. In order to overcome this bias when estimating frequencies, the following correction was applied. The number of guards within the park at different hours of the day was noted on ten days on different months. The number of elephants observed at different hours was divided by the average number of guards present in the park during that hour and the number of animals observed per guard was used in estimating the frequencies. In estimating the number of individuals, repeat observations by different game guards at the same time and same place were eliminated, although the observations at the same place but at different times of the day were taken for estimating frequencies at different times. The observations were from 0600 to 1800 hrs. The park was closed to visitors from 1 September to 15 October and therefore no observations were made during this period in both years. Although elephants can move many kilometres in a single day, it was observed that the groups and even the solitary individuals often remain in a particular area for several days, especially when sufficient food and water were available, before moving on to another area. Such observations were made by Eisenberg & Lockhart (1972) as well in the Wilpattu National Park. Phillips (1984) also state that each herd, under normal circumstances, wanders about in a given area in the forest and will move away to another area when the feeding becomes poor. Therefore, in analyzing the observations, on the first run, weekly observations were taken and the apparent repetitive observations were eliminated. For instance, observations of solitary individuals in a particular area for a week were taken as referring to a single individual unless two such observations were made at different points at about the same time or different individuals were individually identified. The total number, the presence and number of tuskers, calves and juveniles in a group helped to eliminate the repeat observations of such groups. The times at which observations were made at different places also helped to determine the number of groups when two or more groups of similar composition were observed on the same day. It was noted that more than one herd may aggregate at a waterhole. The groups at the waterhole may have been counted as separate groups in other observations as well. Therefore, the composition of large groups was carefully compared with those of the smaller groups observed within the week. It was also observed that the solitary males joined and sometimes left the herds. This was particularly noted with individually identifiable tuskers. After eliminating the apparent repetitions in the weekly observations, in the 188 J. South Asian nat. Hist.

5 E c o l o g y o f t h e e l e p h a n t (S r i L a n k a ) F A J A 0 D F A J A O D Figure 2. The structure of the elephant population in RNP-Block I in different months. Monthly rainfall during the study period is also given. second run, the monthly observations were again examined and the apparent repetitions were eliminated. Results The total number of reported sightings was 1611 and the number of animals noted during these observations was The number of sightings in different months, after eliminating repeat observations, is given in Fig. 2. This analysis suggests that the number of elephants monthly frequenting RNP-Block I varied from 36 to 108 with an average of 75. Therefore, the crude density of elephants in the area works out as 0.54 km 2. The average crude density of adult elephants (including subadults) was 0.37 km*2. The composition of the population is given in Table 1. The proportion of Table 1. Composition of the elephant population in RNP-Block I. Proportion (%) Per 100 adult females Adult female Adult male (non-tusker) Adult male (tusker) Juvenile (non-tusker) Juvenile (tusker) Calf Vol. 1., No

6 d e S ilv a, J a y a r a t n e & d e S ilva Figure 3. The frequency of occurrence of different size groups during the study period. Figure 4. The variations over the months of the proportion of males in groups and the number of calves per female. juvenile tuskers, estimated according to the number of observations, with respect to the adults is too high. Fifteen tuskers, of which 12 were adults (which included two recently translocated ones) and three juveniles were observed in Block I. Of the 15 tuskers, two adult tuskers were very rarely seen in the study area. Therefore, the proportion of adult tuskers to juvenile tuskers in Block I should be 10:3. Since the percentage of all the tuskers in the population is 3.7 (Table 1), the percentages of adult and juvenile tuskers are better given as 2.8 and 0.9 respectively. Similarly, the proportion of non-tusker males would have 190 J. South Asian nat. Hist.

7 E c o l o g y o f t h e e l e p h a n t (S r i L a n k a ) Table 2. The frequencies of groups of 3 to 6 individuals containing different number of adult females (AF) (n=number of groups). Group Size n 1 AF 2 AF 3 AF 4 AF 5 AF 6 AF been higher and that of non-tusker juveniles lower than the estimates given in Table 1. In general, more elephants were observed during and immediately after the major rainy season of October to January than during the drier months (Fig. 2). However, during August 1991 more elephants were observed than during June and July of the same year. The groups were encountered more often than the solitary individuals. The frequencies of observation of different group sizes are given in Fig. 3. The frequency of solitary individuals was 34.6%. The next most frequent categories were herds containing 3 and 4 individuals, which were observed 11.9 and 11.3%, respectively The average group size was 6.2 while the largest group observed contained 36 individuals. Another group of 35 increased to 36 individuals the next day, a tusker, which was earlier observed as a solitary individual, having joined the group. Such large groups were observed for only 2 to 3 days near waterholes, and were clearly the temporary aggregations of several smaller herds. The observed solitary animals included 85.5% adult males (of the males 12.2% were tuskers) and 14.5% adult females. No solitary juveniles were observed, although on one occasion, a solitary juvenile was observed to move through the forest calling repeatedly which was answered by an adult, probably its mother. The herd containing this juvenile was later observed. Pairs were observed 65 times, of which 15 contained the mother and calf, 7 contained mother and juvenile and 15 contained an adult female and an adult male. The herds of three individuals were observed 76 times of which nine contained two adult females and one calf, six contained two adult females and one juvenile, and 29 contained two adult females and one adult male. Of the observed 72 herds of four individuals, four contained two adult females and two calves, three contained two adult females and two juveniles. The number of herds containing various numbers of adult females is given in Table 2. The monthly proportion of the adult males in mixed groups varied from 0.27 to 0.70 with a mean of In 1991 /92, this proportion was above the mean value during the rainy period and the months following it (i.e. October 1991 to March 1992) (Fig. 4). The ratio was also higher during July and August The number ofvadult males in groups varied from 1-4 (Table 3) with a mean value of Only two all male groups were observed. Each of these contained three adult males. However, two adult males together were observed four times during the study period. Vol. 1., No

8 d e S ilv a, J a y a r a t n e & d e S ilva Table 3. The frequencies of groups of different sizes containing different number of adult males (n=number of mixed groups with one or more adult males). Group size n Number of adult males Table 4. The frequencies (%) of solitary individuals, groups with and without calves seen at different times during the day. Time Solitary Groups With calves Without calves n=438 n=402 n= The frequencies of elephants observed at different times during the day are given in Fig. 5. During the daylight hours the elephants were most often seen in the evening after 1600 hours. There was also a minor peak in the sightings around 0830 hours. They were least seen from 1030 to 1500 hours. The frequencies of solitary males and the groups with and without calves seen at different times of the day differed somewhat (Table 4). All categories were seen most frequently in the late afternoon and also quite frequently in the morning. However, solitary males were seen more than the groups between 0900 hrs and 1500 hr s. The groups containing calves were less often seen than the groups containing no calves during this period but they were more often seen after 1500 hrs than the other categories. The monthly adult maleifemale sex ratio varied from 0.37 (1:2.7) (August 1991 and July 1992) to 0.74 (1:1.4) (December 1991) with an average of 0.52 (1:1.9) (Fig. 6). The mean values for the 12-monthly periods of February 1991 to Janu- 192 J. South Asian nat. Hist.

9 E c o l o g y o f t h e e l e p h a n t ( S r i L a n k a ) Time(months) Figure 6. The variation of the maleifemale sex ratio during different months. ary 1992 and February 1992 to January 1993 were 0.53 and 0.51, respectively. In both years, a tendency was observed of the sex ratio to increase during the major rainy period and to remain relatively high during the months following the rainy period. Calves and juveniles were observed during all months but in different proportions (Fig. 2). The number of calves per adult female (C/F ratio) varied Vol. 1., No

10 d e S ilv a, J a y a r a t n e & d e S ilv a monthly from 0.05 to 0.41 with a mean value of This indicates that calving intensity (or the survival of calves, or both) is not the same in all months. However, it is difficult to see a seasonal regularity in the pattern of calving. The C/F ratio was above the mean value during the periods of March-May, July-August and October-December in 1991, January-March and December 1992 and January If the observations were considered separately for the two 12-month periods (i.e. February 1991 to January 1992 and February 1992 to January 1993), then it is seen that the mean value of the C/F ratio of the first period was much higher (0.27) than that of the second period (0.15). Thus, the periods of April- May 1991 and November 1991-January 1992 of the first period, and February, March and August 1992 and December 1992-January 1993 of the second period are above the respective mean values. There appears, therefore, to be some increased calving (or higher survival of calves, or both) during and months immediately following the major rainy period (however, the C/F ratio was low in February 1991 and relatively high in August 1992). The mean value of the monthly proportion of the adult males in groups was the same (0.47) for both the above mentioned 12-months periods. Thus, it appears that there is no difference in the proportion of males associating with the females in both periods. This ratio was relatively high during the periods of November-February and July-August. Table 5. Some important characteristics of various elephant populations in Sri Lanka (for comparative purposes, data of a South Indian population are also given). (GNP-Galoya National Park; LNP-Lahugala N.P.; RNP-Ruhuna N.P.; WNP-Wilpattu N.P.; WsNP- Wasgomuwa N.P.; TMN-Thamankaduwa area). Area Number Crude density (km 1) Adult male: female Mean group size Largest group C/F ratio % Study Authori calves period GNP LNP >13.9 > RNP > TMN WNP WsNP S.India McKay (1973); 2. Ishwaran (1981);3. Kurt (1974); 4. Santiapillai et al; 5. Present work; 6. Hendavitharana et al. (1994); 7. Nettasinghe (1973); 8. Eisenberg & Lockhart (1972); 9. Ishwaran (1993); 10. Sukumar (1989). 194 J. South Asian nat. Hist.

11 E c o l o g y o f t h e e l e p h a n t (S r i L a n k a ) Discussion Elephant densities have been estimated in different parts (mainly in various protected areas), of Sri Lanka (Table 5). According to these data, the crude density of elephants varies very much in different parts of the island. A relatively high density of elephants has been estimated for RNP-Block I. It was noted in the present study that most of the solitary animals and the groups observed there were not really confined to RNP-Block I, although some solitary animals and herds stayed within the area for the major part of the year. The study area appears to be a part of a larger range of an elephant population, which extends perhaps throughout the park complex of about 1500 km2 (Fig. 7) and outside of it as well. Thus, because of the movements of elephants in and out of Block I, it is perhaps not appropriate to estimate the crude density of elephants in the RNP- Block I alone. Eisenberg (1980) in a study by direct observation gives the home range size of solitary males as km'2 and that of groups as 25 (wet season) to 64 km2 (dry season) within the Wilpattu National Park in the north west of Sri Lanka. However, Sukumar (1989), in his study by direct observation of a South Indian elephant population, found that solitary adult males had a home range of km2, which involved a linear distance of at least km, and that herds had a range of km2, which involved a linear distance of at least km. Olivier (1978) in his radio telemetry studies of the elephants in Malaya found the minimum home range of the male to be about km2 and that of females-i-young to be about km2. In a recent radiotelemetric study in Chilla Wildlife Sanctuary, India, a male was found to have a maximum home range of 16 km2, while a female in a group (size varying from 2-61) was found to have a range of 24+ km2 (Joshua, 1991). The size of home range of elephants appears to depend on several factors including the availability of food and water and the habitat diversity (Eisenberg & Lockhart, 1972; Olivier, 1978; Sukumar, 1989). Although particular solitary individuals and groups are evidently resident in certain areas, given the opportunity, males will mingle with different groups and there will be genetic flow through many of the groups resulting in populations with very large ranges. It is therefore more appropriate to consider (but there are no data to do so), the population and the crude density of the entire complex of adjoining parks and reserves of the RNP complex. Similarly, it is not appropriate to extend the crude density determined for a particular area to the other areas. Most of the Sri Lankan wildlife reserves are separated from each other by non-forest areas (Fig. 7), which consists mainly of cultivation, human settlements and developed areas. There is a great deal of human-elephant conflict in areas adjoining almost all the wildlife reserves as the elephants in their seasonal movements move into cultivations and settlements in search of food. The sex ratios of different populations have also been found to differ greatly (Table 5). In fact, sub-populations within an area are shown to have different sex ratios (McKay, 1973; Ishwaran, 1993). There is evidence that the sex ratio does not vary from the expected ratio of 1:1 up to the subadult stage (Sukumar, 1989; Ishwaran, 1993). (However, Sukumar (1989) reported of a captured population of 88 animals which had 18 calves, of which only 5 were males (a calf sex ratio of Vol. 1., No

12 d e S ilv a, J a y a r a t n e & d e S ilv a 1. Vavunikulam Sanctuary 2. Madhu road Sanctuary 3. Giant's reservoir Sanctuary 4. Padaviya reservoir Sanctuary 5. Anuradhapura Sanctuary 6. Trincomalee naval headworks Sanctuary 7. Thrikonamadu Nature Reserve 8. Minneriya-Giritale Nature Reserve 9. Victoria-Randenigala-Rantembe Sanctuary 10. Galoya valley NE Sanctuary 11. Galoya valley SW Sanctuary 12. Peak Wilderness Sanctuary 13. Sinharaja Fforest Reserve 14. Yala Strict Nature Reserve 15. Wirawila-Tissa Sanctuary Figure 7. The present distribution of forest cover and the location of major (those with areas of more than 30 km^) protected areas of Sri Lanka. Red, protected areas; green, closedcanopy forest. The national parks (NP) are as follows: B, Bundala NP; F, Flood Plains NP; G, Galoya NP; H, Horton Plains NP; M, Maduruoya NP; P, Wilpattu NP; R, Ruhuna NP; S, Somawathiya NP; U, Udawalawe NP; W, Wasgomuwa NP; Y, Yala East NP. 196 J. South Asian nat. Hist.

13 E c o l o g y o f t h e e l e p h a n t (S ri L a n k a ) 0.28)). The lower numbers of adult males in these populations are attributed to poaching and selective killing of solitary adult males as they often invade human habitations and plantations (McKay, 1973; Sukumar, 1989). However, poaching for ivory is not a very great problem in Sri Lanka as only about 10% of the males have tusks in Sri Lanka (Deraniyagala, 1955; Hendavitharana et al., 1994) in contrast to the South Indian populations in which tusks are present in more than 90% of the males (Sukumar, 1989). Indeed the male:female sex ratio in South Indian populations could be as skewed as 0.21 (1:4.8) or more (Sukumar, 1989). Santiapillai (1994) points out that, of the 1,163 animals which died or were captured (or were lost to the wild population) during the period 1951 to 1969 in Sri Lanka, at least 639 (55%) were killed in defence of crops; in addition, others would have been killed by hunters. McKay (1973) showed that of the 11 elephants that died in the Galoya area during the period , only one death could be definitely attributed to natural causes; of the seven animals that were known to have been killed by man, four were males. Of the 38 known elephants deaths that occurred in Thamankaduwa area during the period of June 1968 to June 1971, although four were apparently due to natural causes, 32 were caused by man; of the latter, 26 were males (Nettasinghe, 1973). Thus, the higher mortalities of males in Sri Lankan elephant populations could be attributed to killing of them as they invade human habitations and plantations. If this view is correct, then one would expect a more skewed adult sex ratio in the areas of heavy agricultural land use than in areas with more forest cover. The skewed sex ratios make the genetically effective population size (EPS) less than the actual population size. A sex ratio of 0.52, as found in RNP-Block I, will make the EPS only 0.90 of actual population size. In fact, to reduce the EPS to one-half and one-tenth of the actual population size, the sex ratio should be as skewed as about 1:6 or vice versa and about 1:38 or vice versa, respectively. This assumes that all adult males have an equal probability of mating with females. However, this is not probably true because of the social hierarchy existing in the species. Eisenberg & Lockhart (1972) point out that although males are capable of reproduction from 6 to 7 years of age within the normal herd structure, those under 18 years of age are probably prevented from mating due to their lower dominance position with respect to older females, as well as being dominated by the fully adult males (i.e., those over 18 years old) living in the population. Sukumar (1989) is of the opinion that the mean age of sexual maturity in both males and females is the same for a particular population (17-18 years) and that males may not be able to mate probably until older than 20 to 25 years. Thus the true EPS could be significantly less than appears from the adult male-female ratio. Since EPS is related to detrimental effects such as inbreeding depression and the genetic drift, selective male mortality will increase the chances of extinction of small populations. More elephants were observed in RNP-Block I during and following the main rainy season of October-December. During this period the waterholes become full and more importantly the scrub and grasslands, which dry up during the dry season, become lush and full of new growth. Thus, elephants may find a ready and succulent food source in Block I during the rainy season and the proceeding period. Other authors also observed more animals during the rainy season than during the dry season in RNP as well as in other areas. McKay (1973) Vol. 1., No

14 d e S ilv a, J a y a r a t n e & d e S ilv a found that during the wet season there were about 1.7 times as many elephants as during the dry season in the Galoya area, and that this factor was more than six times in Lahugala area. However, in Wasgomuwa National Park area, Ishwaran (1993) observed about twice as many elephants during the dry season as during the wet season. Sukumar (1989) found that the crude density of elephants in his study area in South India during November-December (wet months) was 0.72 whereas that during March-April (dry months) was This shows that the densities during wet and dry seasons could have different patterns of variation from region to region. It is well documented that males, leave the herds as they mature, to lead a semi-solitary life, subsequently joining the herds periodically and temporarily for mating (Eisenberg & Lockhart, 1972; McKay, 1973). Santiapillai et al. (1984) estimated that in RNP-Block I the proportion of solitary animals to be 55% of the total population and of the solitary animals 91% to be adult males. This is somewhat more than the corresponding values of 34.5% and 85% respectively, estimated in the present study. The most cohesive unit in the elephant populations is the mother and her offspring and a herd contains individuals closely related to each other (i.e. mother, daughter, grand daughter etc.); but a limit to the herd size is usually set by the large food requirements of these megaherbivores (Eisenberg & Lockhart, 1972; McKay, 1973). Thus a typical elephant herd is characteristically small as has been reported by several workers; the larger groups are formed by the temporary aggregation of several herds (Eisenberg & Lockhart, 1972; McKay, 1973; Nettasinghe, 1973; Ishwaran, 1981; Santiapillai et al., 1984). The observations of the present study agree with this conclusion. It has also been shown that, within the same species, forest-dwelling animals usually form smaller herds than those living in open areas (Dasmann & Taber, 1956; Peek et al., 1974, Franklin et al., 1975). Thus, one would expect a larger herd size in areas with relatively more grassland habitats than in those with more forest habitats. Since most of the grasslands in Sri Lanka are formed owing to human disturbances, it is reasonable to expect a larger herd size in an area with more human disturbance. Ishwaran (1993) found the herd size in the Galoya area to be larger in disturbed areas than in undisturbed areas. A wildlife reserve such as the RNP, being an area with a low disturbance by man, should have a smaller group size, as has been observed in the present study. It is seen that elephant populations in regions with a high degree of cultivation (e.g. Thamankaduwa, Galoya, Lahugala areas) have higher mean herd sizes (Table 5). It has also been shown that the mean herd size is larger during the wet season than during the dry season. McKay (1973) found that the mean herd sizes during the wet season and dry season in Galoya area were 8.20 and 6.98, respectively, whereas those in Lahugala area were and The observation of the present study that the mean herd sizes in the months July-August (peak of dry season) and December-January (peak of wet season) were, respectively, 4.3 and 5.1 agrees with McKay's observations. However, Ishwaran (1993) reported a larger mean group size during the dry season (July-August) than during the wet season (December-January) in Wasgomuwa National Park area. During daytime, more elephants were observed after 1600 hours. In the morning there was a small peak in the observations between 0800 and 0900 hours. 198 J. South Asian nat. Hist.

15 E c o l o g y o f t h e e l e p h a n t (S r i L a n k a ) Such peaks, observed also by others, have been described as activity peaks on the assumption that during the noon hours most of the elephants rest in the forest (Eisenberg & Lockhart, 1972; McKay, 1973). Phillips (1984) states that the elephants sleep and rest for about five hours from about 1000 hours to 1500 hours, and also during the midnight hours, feeding being confined mainly to early morning, evening and night. During the present study however, the animals were found to be feeding (except when they were moving) in almost all instances when they were observed even within the forest. Thus, the so-called activity peaks may also be the times at which most elephants come out of the forest interior to the forest-grassland interphase or to grasslands for feeding or to the waterholes for drinking. It has been shown by several authors that the preferred food of elephants in the dry zone of Sri Lanka is grass (Eisenberg & Lockhart, 1972; McKay, 1973; Vancuylenberg, 1974, 1977; Ishwaran, 1984; Santiapillai et al., 1984). Even during the dry season, elephants feed on dry grass (Santiapillai et al., 1984). However, Sukumar (1989) considers browse to be more important for the elephants in South India. The teeth of Elephas are better adapted for coarse food such as grass than those of Loxodonta (Sikes, 1971) and therefore one would expect the Asian elephant to be more of a grazer than the African elephant. It may be that elephants avoid the hottest hours for coming out to the open grasslands, where the temperatures at these times are higher than that inside the forest. There is also little shade to be had in the open grasslands. Table 6 shows the frequency of elephants observed at different times of the day during the cool and cloudy months of December and January and the hot and "clear sky" months of July and August. A higher frequency of elephants was recorded during the hottest daytime period of hours of "cool" months than during the corresponding period of "hot" months. Furthermore, during the "hot" months the frequency of elephants seen in the late afternoon was higher than that during the "cool" months. In some studies (McKay, 1973; Nettasinghe, 1973), the solitary animals were observed throughout the diurnal period, whereas the herds were found only towards evening. However, during the present study, groups were observed throughout the day, although solitary animals, in comparison to herds, were seen relatively more frequently from 0900 to 1500 hours (Table 3). The C/F ratio of 0.19 observed in the present study must be an underestimate since the subadult females are also included in the adult category (see Methods). The proportion of subadults in RNP-Block I has been variously estimated as 10.34% (McKay, 1973), 10.1% (Kurt, 1974), 16.20% (Santiapillai et al., Table 6. The frequency (%) of elephants observed at different times of the day during the periods December-January and July-August. Time December-J anuary July-August n=380 n= Vol. 1., No

16 d e S ilv a, J a y a r a t n e & d e S ilv a 1984) and 5.88% (Hendavitharana et al., (1994). If 10% is taken as the frequency of subadults in the population then the proportion of subadult females could be assumed as 5%. When the necessary correction is applied the C/F ratio becomes The C/F ratio estimated for RNP-Block I area is around 0.20 in three studies (McKay, 1973; Hendavitharana et al., 1994; present study), much higher (0.50) in one study (Kurt, 1974) and much lower (0.13) in another (Santiapillai et al, 1984) (see Table 5). If 0.20 is taken as the C/F ratio, then the birth interval in the population would be about 5 years (assuming the mortality rate of calves to be zero), since the calf stage lasts about one year. If the mortality rate of calves is 10%, 15% or 20%, then the birth interval would be, respectively, 4.5,4.25 or 4.0 years. Sukumar (1989) estimates the birth interval of the south Indian population to be 4.7 years. The C/F ratios of 0.50 and 0.60 reported by Kurt (1974) and Ishwaran (1993) respectively appear to be too high since this implies 50% or 60% of the adult female population calving in the given period making the birth interval 2.0 or 1.7 years, respectively. However, Sukumar (1989) reports of a captured population of 88 elephants which had 30 adult females and 18 calves (a C/F ratio of 0.60). Ishwaran's (1993) data also show that one herd he studied had 46 adult females and 35 calves and another had 10 adult females and 13 calves (C/ F ratios of 0.71 and 1.30, respectively, if there is no sampling error). These indicate that there can be years with exceptionally high calving rates. Sukumar (1989) also quotes Douglas-Hamilton, who found the number of calves born annually to an identified group of 98 African elephant females varied from 8 to 34. If the birth interval is about five years, then a 'good year' with a high proportion of calves will be followed by at least one or two years of below-average calving. The calving pattern seen in the present study agrees with this view. Sukumar (1989) also found similar results in South Indian elephant populations. However, in order to ascertain such cyclical patterns, studies extending over at least five years, or preferably over years, are required. The changes in the mortality rate from year to year could also obviously distort the birth rates estimated from the number of calves observed in the population. A relatively higher C/F ratio was observed in the present study during the major rainy period and in months immediately following it. Although inconclusive, this may indicate an increased calving (or survival of higher proportion of calves, or both) during this period. Eisenberg & Lockhart (1972), McKay (1973) and Nettasinghe (1973), however, found no evidence of seasonality in the breeding of elephant in Sri Lanka. Although, Ishwaran (1981) found a late dry and an early wet season peak in the birth of calves in one year, this was not repeated during the next year. The wet season is the suitable time for parturition because of the easy availability of food and water, which are required in greater quantities by lactating mothers. But, this means breeding should take place anticipating such appropriate times 21 months ahead, since the gestation period of the elephant is said to vary from 18 to 23 months (Phillips, 1984) or 20 to 22 months (Sukumar, 1989). If there is a regular birth peak during the major rainy period and the months immediately following it (November to March), then more male elephants could be expected to be associated with the females during the period February to June. However, increased associations were seen in the present study in the November to February and July to August periods. In African elephants 200 J. South Asian nat. Hist.

17 E c o l o g y o f t h e e l e p h a n t (S ri L a n k a ) also there is controversy whether the calving pattern is positively related to annual rainfall (Douglas-Hamilton in Sukumar, 1989) or not (Hanks, 1972). Jainudeen et al. (1972) are of the opinion that the occurrence of musth in male Asiatic elephant, which according to them shows some seasonality in domestic elephants, is a vestige of a truly seasonal rut, which may have existed during some earlier part of the evolutionary history of the species. During the period of musth the aggressiveness of the male is increased markedly and this presumably can give the male a relatively high dominance state with respect to females and other males thus increasing its chances of successful mating. Kurt (1969) is of the opinion that musth is seasonal only in wild elephants and that it occurs in RNP commonly during the period of February to May. Katugaha (1993) reports that the largest number of elephants in RNP come to musth in the months of February and March. Interestingly, these periods agree with the period (February to June) predicted earlier for increased mating if the birth peak were to occur during the major rainy period and the months immediately following it (November to March). Acknowledgements We wish to thank the Director, Department of Wildlife Conservation and the Warden, Ruhuna National Park for granting us permission to carry out this work. Our special thanks are due to the wildlife guides of the park who made available to us their daily observations on elephants. We are grateful to John F. Eisenberg of the Florida Museum of Natural History for suggesting valuable improvements. Thanks are also due to T.S.B. Alagoda of Department of Zoology, University of Peradeniya, for drawing the figures. Literature cited Balasubramaniam, S., Santiapillai, C. & M.R. Chambers Seasonal shifts in the pattern of habitat utilization by the spotted deer (Axis axis Erxleben 1777) in the Ruhuna National Park, Sri Lanka. Spixiana, 3: Chambers, M.R., Balasubramaniam, S., Santiapillai, C. & N. Ishwaran Composition, biomass and primary production of some grasslands in the Ruhuna National park, Sri Lanka. Ceylon J. Sci. (Biol. Sci.), 16: Dasmann, R. & R. Taber Behaviour of Colombian black-tailed deer with reference to population ecology. J. Mammal., 37: Deraniyagala, P.E.P Some extinct elephants and their relatives, and the two living species. National Museum of Ceylon, Colombo. 153 pp., 47 pi. de Silva, M., Dissanayake, S. & C. Santiapillai Aspects of the population dynamics of the wild Asiatic water buffalo (Bubalus bubalis) in the Ruhuna National Park, Sri Lanka. J. South Asian nat. Hist., 1: de Silva, P.K Status of large mammals of Ruhuna National Park (Block I) of Sri Lanka, with special reference to elephant, water buffalo, spotted deer and wild pig. Interim Report of Project No /Sri Lanka, WWF International, Gland, Switzerland. 30 pp (mimeo). Eisenberg, J.F The density and biomass of tropical mammals, in Soule, M.E. & B.A. Wilcox (eds.), Conservation Biology. Sinauer Associates Inc., Massachusetts. 395 pp. Eisenberg, J.F. & M. Lockhart An ecological reconnaissance of Wilpattu National Park, Ceylon. Smithsonian Contr. Zool., 101: Franklin. W.L., Mosmann, A.S. & M. Dole Social organization and home range of Roosevelt elk. J. Mammal., 56: Vol. 1., No

18 d e S ilv a, J a y a r a t n e & d e S ilv a Geiser, U. & M. Sommer Up-to-date information on Sri Lanka's forest cover. Loris, 16: Hanks, J Reproduction of elephant, Loxodonta africana, in the Luangwa valley, Zambia. J. Reprod. Fertil., 30: Hendavitharana, W., Dissanayake, S., de Silva, M. & C. Santiapillai The survey of elephants in Sri Lanka. Gajah, 12:1-30. Hoffmann, T.W Distribution of elephants in Sri Lanka. Loris, 14: Ishwaran, N Ecological studies on populations of elephants in the Gal Oya area in relation to distribution, habitat preference, competition and food availability. Unpubl. M.Sc. thesis, Univ. Sri Lanka, Peradeniya. 350 pp. Ishwaran, N Comparative studies on elephant populations in Gal Oya, Sri Lanka. Biol. Conserv., 21: Ishwaran, N Ecology of the Asian elephant in lowland dry zone habitats of the Mahaweli River Basin, Sri Lanka. J. Trop. Ecol. 9: Joshua, J Habitat use by a male and female Asiatic elephant in Summer in Chilla Wildlife Sanctuary. Fifth annual research seminar, Wildlife Inst. India, Dehra Dun, p. 6. Katugaha, H.I.E Some observations of elephant in the Ruhuna National Park, Sri Lanka. Gajah, 10: Kurt, F Some observations on Ceylon elephant. Loris, 11: Kurt, F Remarks on the social structure and ecology of the Ceylon elephant in the Yala National Park, in Geist, V. & F. Walthe (eds.), The behaviour of ungulates and its relation to management.iucn Publ., New Series, Morges. 24: Legg, C. & N. Jewell A new forest map of Sri Lanka. Forest Department, Colombo. McKay, G.M Behaviour and ecology of the Asiatic elephant in southeastern Ceylon. Smithsonian Contrib. Zool., 125: Miieller-Dombois, D Ecogeographic analysis of a climate map of Ceylon with particular reference to vegetation. Ceylon Forester, 8: N ettasinghe, A.P.W The inter-relationship of livestock and elephants at Thamankaduwa Farm, with special reference to feeding and environment. Unpubl. M.Sc. Thesis, Univ. Peradeniya, Sri Lanka. 110 pp. Olivier, R.C.D On the ecology of the Asian elephant. Unpubl. Ph.D. dissertation, Univ. Cambridge. 454 pp. Peek, J.M., LeResche, R.E. & D.R. Stevens Dynamics of moose aggregations in Alaska, Minnesota and Montana. J. Mammal., 55: Phillips, W.W.A Manual of the mammals of Sri Lanka. 2nd (revised) ed. Wildlife & Nature Protection Society of Sri Lanka, Colombo. 389+xxxv pp. Santiapillai, C Elephant mortality in Sri Lanka. Gajah, 12: Santiapillai, C., Chambers, M.R. & N. Ishwaran Aspects of the ecology of the Asian elephant Elephas maximus L. in the Ruhuna National Park, Sri Lanka. Biol. Conserv., Santiapillai, S. & P. Jackson The Asian elephant: an action plan for its conservation. IUCN, Gland. 79 pp. Sukumar, R The Asian elephant: ecology and management. Cambridge Univ. Press, Cambridge.251 pp. Vancuylenberg, B. W.B The feeding behaviour of the Asiatic elephant in southeastern Sri Lanka. Unpubl. M.Sc. dissertation, Univ. Sri Lanka, Peradeniya. 407 pp. Vancuylenberg, B.W.B Feeding behaviour of the Asiatic elephant in southeast Sri Lanka in relation to conservation. Biol. Conserv., 12: J. South Asian nat. Hist.