The studbook of timber elephants of Myanmar with special reference to survivorship analysis - Khyne U Mar

Abstract

The purpose of the demographic analyses in this study was to calculate the basic life tables to determine the effects of the long-term captivity of Asian elephants (Elephas maximus), which are utilized extensively as draught animals, on survival, fecundity and viability. The studbook data were collected from the elephant log books and the annual reports of the Extraction Department, Myanma Timber Enterprise of the Union of Myanmar. We had access to a near-total of the records (n (9600) of elephants captured or born after the year 1875, including 3 070 calving records. It was documented that 32.5 percent of calves born in captivity failed to reach the age of five years. Life table analysis revealed that most mortality occurred before the age of five. Survivorship analysis of adults and sub-adults (more than five years) showed that wild caught elephants and female elephants had significantly higher survival rates (P <0.001) than captive born and male elephants, respectively. A similar analysis was conducted for calves (under five years) and comparisons were made between dam origins and sex. It was revealed that calves born from wild caught (WC) dams had higher survival rates than those born from captive born (CB) dams (P <0.001), while survivorship and sex showed no correlation.

Introduction

The Asian elephant (Elephas maximus) once ranged from the Euphrates-Tigris river systems in the west across Asia, from south of the Himalayas to the Yangtse-Kiang River in the east (Oliver, 1978; McKay, 1973; Goa, 1981; Dobias 1987; Lair, 1988; Sukumar, 1989 and Daniel, 1992). Having been extirpated from approximately 85 percent of its historical range, it now exists in 13 Asian countries: Bangladesh, Bhutan, Cambodia, China, India, Indonesia, Lao PDR, Malaysia, Myanmar (Burma), Nepal, Sri Lanka, Thailand and Viet Nam (Oliver, 1978; Lair, 1988; Sukumar, 1989 and Daniel, 1992). Although the elephant's present range still extends from the Indian sub-continent in the west to the rim of the Indo-Chinese peninsula, the total wild habitat available in Asia amounts to only about 500 000 km² (or about the size of Thailand) and is declining at an average rate of 4 000 km² per annum (Santiapillai, 2000). It is therefore one of the world's most seriously endangered species of large mammals (Oliver, 1978; Sukumar, 1989). Indiscriminate hunting and forest clearance are the principal causes of the decline in the number of elephants in Asia (Santiapillai, 2000). Indo-China's tropical rain forests, which are the home of Asian elephants, were seriously damaged during 30 years of constant warfare, particularly by the use of chemical defoliants, napalm and massive bombing during the US/Vietnamese conflict (Keele and Dimie-Ediger, 1999). The current population of Asian elephants is estimated at 35 000-50 000 individuals (Oliver, 1978; Sukumar, 1989 and Santiapillai and Jackson, 1990) inclusive of 15 000 elephants in captivity, mainly in India, Thailand, Myanmar and Sri Lanka (Hanks, 1979; Blower, 1985; Santiapillai and Jackson, 1990; de Alwis and Santiapillai, 1992; and Sukumar and Santiapillai, 1993).

Myanmar is home to the second largest Asian elephant population after India, with approximate 6 000 elephants in the wild and 6 000 elephants in captivity. The single largest remaining population of captive Asian elephants is found in the timber camps of the Union of Myanmar (Yin, 1967; Gale, 1971; Blower, 1985 and Krishnamurthy and Wemmer, 1995). The elephants play a crucial role in the forestry sector of the Union of Myanmar and the benefits from using them are far-reaching (Blower, 1985; Sukumar and Santiapillai, 1993 and Santiapillai and Ramono, 1992). Gale (1971) praised the timber elephants of Burma as Nature's greatest gift to Burma. Santiapillai and Ramono (1992) and Santiapillai (2000) likened the Asian elephant to an amphibious, weatherproof, multipurpose, four-legged machine that is the jungle's perfect cross-country vehicle. There is a growing recognition of this merit, particularly in the forest industry where the animal can extract timber with much less incidental damage to the environment than rapid but highly destructive machines.

The working elephants of Myanmar

The Union of Myanmar, with an area of 676 553 sq km, is one of the largest of the mainland Southeast Asian countries, with a rapidly growing population of 47.3 million. Myanmar has a well-established tradition of tree harvesting, based on the Myanmar Selective Felling System, which ensures the sustainable production of the country's timber resources. Timber extraction is dependent to a large extent on the draft power of working elephants (Mar, 1996 and Mar and Win, 1997).

Elephants are totally protected in Myanmar (Yin, 1967 and Blower, 1985). The Wildlife Protection (Amendment) Act 1956 forbids hunting, capture, possession, sale, or purchase of live or dead elephants or their products without proper permission. The term ‘government owned elephants' denotes those elephants employed at the Myanma Timber Enterprise (MTE) and the Forest Department, which are under the control of the Ministry of Forestry. According to the Elephant Regulation Act of 1951, all domesticated elephants - government owned and private owned elephants - should be registered by the Forest Department at the age of three months with the primary intention being to prevent the illegal trade and the illegal capture of elephants (Working People's Settlement Board, 1982). Myanmar has been a signatory to CITES since 1997.

The timber elephants used by colonial and post- independence governments have been derived from two different sources. The majority of elephants have always been born in the wild and are referred to as wild captured (WC), while a smaller number have been captive born (CB) elephants (Evans, 1910; Gale, 1971; Mar and Win, 1997). Traditionally, sub-adult elephants (around four to five years of age) are captured from the wild as these elephants are easily tamed in a short period. All calves born in captivity, reaching the age of four years, WC calves of at least 1.40 m (4.6 ft) (measured at shoulder height) and recently captured sub-adult elephants are systematically weaned and tamed/trained during the cool season of Myanmar that is between November and January (Gale, 1971). After the completion of taming/training, each individual elephant is given a registration number and a log book (known as Form J) in which the complete biodata of each animal, i.e. sex, name, age at time of acquisition, or date of birth (if captive born), age at taming/training, date of mating and calving, temperament of the animal, veterinary inspections and treatments, reproductive history, prescribed work load and nature of work, musth condition, etc. are recorded. The traditional elephant log books are equivalent to the ‘studbooks' kept in Western zoos. Trained elephants between 5 and 17 years are used as baggage elephants and classified as trained calves (TC). The training of these elephants is continued until they get used to the verbal commands, logging/baggage harnesses and fettering chains. Elephants over the age of 17 are classified as full-grown (FG) elephants and put into the work force until they reach the retirement age of 55. After retirement, elephants spend most of their time roaming and foraging and one mahout is assigned to each retired elephant to take care of its well-being. Some bull elephants sire calves after retirement. Two mahouts generally handle each individual elephant in the work force. Any bull in musth condition and some elephants with aggressive/unreliable temperaments are assigned an extra man armed with a spear (spear-man) (Myanma Timber Enterprise, 1998).

The elephants work a five to eight hours/day, five days/week, seven working months/year. Working elephants have 12 to 16 hours of foraging time at night during working months (June to January), enabling them to socialize not only with the camp elephants but also with wild elephants, because most timber camps are situated in the vicinity of forests where wild elephants roam. The elephants have more free-ranging time during non-working periods (February to May), which coincide with summer and the highest annual temperature of approximately 45 degrees Celsius. The working elephants are maintained as mixed herds consisting of adult males and females and calves of various ages, thus mimicking the social structure of wild elephant herds. Cows with suckling calves are allowed to stay out of work until the calf reaches one year old (Myanma Timber Enterprise, 1998).

Demographic analysis

Although Myanmar is home to the second largest population of Asian elephants after India, the demography of the captive elephant population has never been studied before in any detail. A life history analysis of captive elephants would provide in-depth knowledge of the factors influencing natality and mortality and thus provide an important tool for the management of this and other captive populations.

The study of life history strategies originated in the late 1940s and the early 1950s from the combination of animal demography and evolutionary theory (Sibley and Calow, 1986 and Caswell, 1989). That analysis was based on age-specific rates of mortality and reproduction, and ecologists were well aware that those rates varied in interesting ways both within and among species (Caughley, 1966; Caswell, 1989 and Caughley and Gunn, 1996). The most important task in the development of a captive propagation plan is the compilation of basic data required for population analysis and management (Ballou and Foose, 1996). The best source of such data is a studbook, which is a chronology of a captive population listing vital information on animal identities, sexes, parentage, birth and death dates and ages. To construct age- or sex-specific life tables, cohorts of individuals are followed from birth under one or more sets of conditions and their age-specific survivorship and fecundity are recorded throughout their lives (Ballou and Foose 1996 and Gutierrez, 1996). The tabulation of birth rates for females of different ages in a population is called a fecundity schedule or table, which lists the mean fecundity of animals in each age class and is a measure of the number of live offspring produced over an interval of age (Caughley, 1977 and Gutierrez, 1966). Fecundity tables are used to estimate the net reproductive rate per generation (R₀), the number of females born (per female) and the intrinsic rate of population increase or per capita increase (r) that is the measure of the contribution of the different ages to the ancestry of the future generations (Deevey, 1947; Ballou and Foose, 1996; Gutierrez, 1996 and Collett et al., 1997). If the net reproductive rate R₀, is less than 1, the mothers in the study population are not producing well enough for offspring to replace themselves (Caughley, 1977 and Gutierrez, 1996). For a stable population, population growth is zero or R₀ is 1, which means that each animal exactly replaces itself in the population in every generation. R₀ is used to calculate the animal lifetime reproductive objective in captive population management (Ballou and Foose, 1996; Caughley, 1977 and Gutierrez, 1996).

The life tables include the following components:

f_x = the frequency/number of cohorts still surviving at age x out of the total number born

d_x = the probability of dying in each age interval x, x+1, calculated as f_x/Sf_x (age-specific mortality)

l_x = the probability at birth of surviving to age x, is calculated as 1-Sd_x (age-specific survival)

q_x = the proportion of animals alive at age x that die before age x+1 or simply mortality rate, which was calculated as d_x divided by l_x (age-specific mortality rate)

p_x = the survival rate, calculated as 1- q_x (age-specific survival rate).

Generation time (T), which is the average age at which a parent produces young (age at first calving in this study, in which T is calculated as Sxl_x m_x divided by R₀, where x is age in years) (Caughley, 1977). Knowing R₀ and T allow us to estimate the net per capita rate of increase for a population (r), which is calculated as lnR₀ divided by T, where 1n is the base of the natural logarithms. The net per capita rate of increase for a population (r) can be interpreted as birth rate minus death rate or r = B - D. The negative value of r indicates that birth rates are lower than death rates and the population is declining. A value greater than 0 (zero) would indicate a stable population (Gutierrez, 1996).

Objectives of the study

The demographic analyses in this study were designed to demonstrate the effect of the long-term captivity of Asian elephants of Myanmar on survival, fecundity and viability.

In this study, we provide information on:

1) the basic life tables and population dynamics for captive elephants of Myanmar;

2) the age- and sex-specific fecundity and mortality rates of the working elephants of Myanmar to compare between captive born and wild caught elephants;

3) the net reproductive rate per generation (R₀), or the number of females born per female, and the intrinsic rate of population increase or per capita increase (r) in the working elephants of Myanmar to determine whether the captive domesticated elephants of Myanmar were declining or increasing through time.

Materials and methods

The life history biodata used in this study came from the working or logging elephants owned by the state-owned Myanma Timber Enterprise. The studbook data were collected from the elephant log books and the annual reports of the Extraction Department of the Myanma Timber Enterprise. These were later transferred to a computer spread sheet, using version 7.5 of MS Excel with the following fields:

1) Registration number;

2) Name;

3) Origin (wild caught or captive born);

4) Date of birth;

5) Place of birth;

6) Method of capture;

7) Year of capture;

8) Place of capture;

9) Year or age of taming;

10) Total no. of calves, followed by date of calving (month/day/year), sex of calf, dam name, dam registration number and birth origins of mother (wild caught or captive born);

11) Date of death;

12) Cause of death.

We had access to a near total of records (n (9 600) of elephants captured or born after the year 1875. Approximately 900 elephants, which were born between 1950 and 1960, were in captivity continuously for approximately 50 years and it is possible to trace their third generation offspring, making this database one of the most comprehensive documents for any captive elephant population in the world. Those elephants caught from the wild were aged by comparing the size, height, and body condition of captive born elephants with known age. Some records (n (600) did not have complete data on date of birth or death and those elephants that were impossible to trace because of escape or as a result of being taken/stolen by insurgents, sale etc., were excluded from the analysis as it was meaningless to include them.

The practice of ‘censoring' was used in the data analysis. Censoring was used when we did not want to know the time of death for all of the individuals. This idea was based principally on some individuals/animals that were lost/hidden from the study (i.e. impossible to trace), but contributed something to our knowledge of the survivor function but nothing to our knowledge of the age of death (Crawley, 1993).

The original (pooled) data set included (9 600 records. The data were stored in two major files, consisting of elephants of age five years and over and calves less than five years. The number of elephants used in this study was 6 246 (M = 2 689 and F = 3 557), between 5 and 45 years. Among 6 246 elephants, 1 421 elephants (M = 691 and F = 730) died before they reached the censored age of 45 years and the life history analysis was constructed on the basis of these elephants. The term"birth origins” was used in elephants (5-45 years) in order to differentiate WC and CB elephants.

All calves (n = 3 070) born from CB and WC working elephants between 08/12/1942 and 17/03/1999 were included in this study. In pooled data, calves born in captivity were categorized as captive born (CB) whether they were born from CB or WC dams, because their dates of birth were known and each individual elephant had a registration number. In order to access the effect of mother on longevity of calves, the birth origins of dams were specially categorized as"dam origins” in calving records. The survival analysis for calves was based on mortality records (n = 660, CB = 232, WC = 428, M = 358, F = 296, unknown sex = 6).

The calculations of life table statistics of captive working elephants of Myanmar in this paper were based on the principles described by Caughley, 1966 and 1977, Pielou, 1977; Avis et al., 1995; Ballou and Foose, 1996 and Gutiérrez, 1996. Statistical analysis was made using the Statistical Package for Social Studies (SPSS) Release 7.5.1 (SPSS Inc., Illinois).

Results

1. Life table and survivorship curves

All age classes of elephants were used to construct the life table (Table 1). In most studies, survivorship curves plotted the cohort's age between 0 and time t. We decided to calculate two different survivorship analyses for elephants less than five years and over five years. In Myanmar, elephants with an estimated age of 5 years are captured from the wild to become working elephants, as at this age they are easy to tame. After taming, systematic recording of their life histories is made in the log books assigned to all elephants. So, it was virtually impossible to get data for wild caught elephants before their captivity. The comparisons of survivorship between birth origins and sexes revealed that the survival rate of WC elephants (5-45 years) was higher than CB cohorts (Fig. 1) while female elephants had a higher survival rate than male elephants (Fig. 2).

The survival analysis was separately conducted for calves (<5 years) and a comparison was made between dam origins and sex to understand the effect of the mother on the longevity of the calves. It was revealed that the survival rate was declining with age with the sharpest decline occurring at the first half year of life (0.5 year), as shown in Fig. 3. Calves born from CB dams had significantly higher survival rates than WC dams, while survivorship rates for males and females showed no difference (Fig. 4).

2. Seasonality of mortality in working elephants

The death dates of calves (<5 years) and working elephants (5-45 years) were charted and compared monthly (Fig. 5). The mortality pattern of elephants (5-45 years) revealed that the highest death rate occurred in April and May, the hottest period of Myanmar, while the death rate for calves (<5 years) peaked in January, which coincides with the taming period, followed by a second wave of deaths in April and May.

3. Seasonality of births

When the total births (n = 3 070) were charted month-wise (Table 5), the peak number of births occurred in January and the lowest number of births occurred in August, which coincided with the monsoon season.

4. Live births and stillbirths

A total of 738 calves died before the age of five years, inclusive of 96 stillbirths. That is, 32.5 percent of calves born in captivity failed to reach the age of 5. The number of calves that died between age 0.5 and 5 years is displayed in Table 6. The overall data of calves born in captivity (n = 3 070) show that Myanmar cow elephants gave birth to 2 971 (96.72 percent) live offspring at birth with 96 (3.13 percent) delivered as stillbirths/abortions. It is apparent that the highest number of stillbirths was produced by dams between 25-35 years (Table 3).

5. Fecundity analysis

The total number of calving records used in this study was 3 070, inclusive of 17 pairs of twins; the number of calves born to primiparous mothers (those dams that dropped their first offspring) was 1 276 (CB = 408 and WC = 868). The prime reproductive age is the age of a cow, where the maximum number of calvings occurs. Among the total records of 3 070 calvings, peak calving (n = 562) was noticed in the age group 25-30 years followed by cows in the 25-30 years age group with 539 calvings (Table 4). The age classes in which maximum number of calving occurred in WC and CB dams were 25-30 vs. 20-25 years (332 vs. 272 calvings) respectively. The number of calves born to primiparous mothers in the whole calving data set was 1 276. The earliest age at which a female gave birth was at 6.48 years (to a cow named Mya Yee) (registration no. = 2 577). This cow was herself born in captivity and thus her exact age is known. It was the first documented case of a working Asian elephant being sexually mature by the age @5 years in Myanmar. This recorded youngest known age of a primiparous cow was, however, exceptional as no other captive cow in Myanmar had calved before the age of seven years. There were 12 cows (CB = 7 and WC = 5) that gave birth before the age of ten years. The oldest authenticated age at which a CB cow dropped its first calf in this data set was 45.50 years. The maximum number of calves that a cow could give birth to in her life time was ten. The mean age ±standard deviations (SD) of the primiparous CB and WC cows were 19.69 ±6.22 and 29.77 ±9.99 years respectively. Age at first calving in CB dams was significantly earlier than WC cows (P < 0.001). The highest number of calvings occurred at 18 years (n = 43 calves) and 24 years (n = 52 calves) in primiparous CB and WC cows, respectively. The comparison of age-specific calving potential, which was the number of calves that a cohort cow produced at a particular age (calves/cohort cow) between CB and WC cows demonstrated that CB cows had lower calving potential than WC cows (P < 0.001) (Fig. 6).

Discussion

The life table that contains information on birth, reproduction and mortality rates is the central focus of attention in population biology (Caughley, 1977; Begon et al., 1990 and Collett et al., 1997).

The behaviour studies of Asian elephants in Sri Lanka documented that in the wild, in a matriarchal elephant society, the oldest cows were dominant, leading the herd and maintaining discipline among the young and sharing the responsibilities of taking care of juveniles (Eltringham, 1982 and Poole et al., 1997). In this study, we expected that WC cows would be better mothers as they grew up in a natural environment, which was basically governed by matriarch cows. However, the survivorship analysis revealed that calves born from CB dams possessed significantly higher survival rates than those born from WC dams. The survivorship of calves seems to be related with (i) parity or rank order of a calf born from an individual mother (ii) mothering ability of allocated dam and its age, origin and experience of the birthing process and (iii) period of bonding or the duration that calves stayed with their blood and allocated mothers before weaning. More studies are needed to verify these factors.

The earliest age of calving among Burmese timber elephants recorded by Burne (1942) was 16 years. The youngest age of calving in our data set was 6.8 years. The mean age ±standard deviation (SD) of the primiparous CB cows with a known date of birth was 19.69 ±6.22 years. Taking into account the gestation period of Asian elephants, which lasts 20.61 ±0.49 months (Poole et al., 1997), the majority of CB cows will become sexually mature at »18 years. Regarding age at sexual maturity in Asian elephants in India, Sukumar (1989) stated that the age of puberty of cow elephants was very plastic and he assumed that the mean age at first calving might be as late as 18-20 years. Quoting reports of Evans (1910), Flower (1943) and Robinson (1934), Mikota et al. (1994) suggested that the age at which a female Asian elephant attained sexual maturity ranged from 6 to 12 years, but most of the data were based on exhibiting elephants in the North American zoos. According to Schmidt (1986 and 1993), the earliest known onset of sexually mature age for an Asian cow elephant was 6 years. Comparatively, the age of puberty in African elephants was estimated at 12-14 years (Smith and Buss, 1973 and Lee, 1991). The same authors commented that cows less than ten years may be capable of ovulation but less likely to become pregnant. Lee (1991) stated that the range of fecundity was dependent on the local environmental conditions (rainfall, food supply, etc.), the presence of a suckling calf and its sex, and the age of the cow, and that those 15 to 50 years old were most likely to conceive while young and fertility was reduced by age. This agreed well with our findings that the calving potential of CB cows declined after the reproductive prime age at 21. Work-related stress and the continuous burden of rearing calves could be the main cause of decreasing fertility in later life.

The captive born (CB) reached puberty before the WC cows, which might be because of the higher level of nutrition and/or less stressful life in captivity. Schmidt (1993), based on his experience with Asian elephants in zoological parks, reported that captive elephants generally became sexually mature earlier than free-ranging elephants. Delayed puberty in WC cows might be because of the physiologically and psychologically stressful conditions after capture, but they regained their calving vigour between 31 and 44 years. This was striking evidence that captured elephants were able to cope with the stress of being in captivity without losing reproductive vigour, especially later in life.

The calving records of 3 070 elephants during the period of 1942 and 1999 showed that peak calving for working cow elephants occurred in January. Based on the birth records of 261 cow elephants, Sukumar (1989) reported that the Asian elephants of South India had a similar calving pattern with the peak number of births in January. This indicated that conceptions peaked during September/October (assuming a mean gestation period of 20-21 months), which was three to four months after the onset of the southwest monsoon (Sukumar et al., 1997). The total number of calves (n = 3 070) born between 1942 and 1999 demonstrated the mean calving rate in captive elephants as 53.9 calves per year. The value of the net reproduction rate Ro (0.50) in this study confirmed the fact that, overall, the cow elephants were not producing enough daughters to replace themselves, which means the population was declining. The mean per capita rate of increase (r) was 0.017 calves/year indicating an intercalving interval of 58.8 years. The resulting generation time (T) of 28.93 years was much longer than the mean age at first calving (26.54 (10.11 years) based on the pooled data.

The maximum age-specific fecundity rate (m_x = 0.027) was noticed at age 27 (23 calves from 4 355 cohort cow elephants). The maximum calving of CB and WC dams was 272 and 332, which took place at the age of 20-25 and 25-30 years, respectively. It was apparent that WC cows produced more offspring than CB cows (537 and 1 175) in the latter part of their lives, beyond these ages of peak production of calves. According to the Master Plan of the American Zoological Association (1997-2002), Ro, T, r of the Asian Elephants (M = 20, F = 115) in North America were 1.15, 26.27 and 0.005 respectively, during the period of 1980 and 1998, which indicates that the Asian elephants in American zoos are increasing in number, but the mean per capita rate of increase (r) is too low to maintain the captive herd in sustainable condition (Keele and Dimeo-Ediger, 1999). One of the main reasons was that over 30 percent of the animals conceived were stillborn, aborted, rejected by the mother, or killed by the mother and some neonates failed to survive the first 30 days following birth as a result of unknown causes (Kurt and Mar, 1996 and Keele and Dimeo-Ediger, 1999). According to Taylor and Poole (1998), 55 percent of stillbirths (11 out of 20) were recorded in member zoos of the European Endangered Species Programme (EEP) and the Species Survival Plan (SSP) of North America.

Agalactia (lack of or deficient milk formation) in working cow elephants was one of the major causes that lead to neonatal deaths. Agalactia was pronounced in multiparous cows. Similar reports on deficient lactation in Asian cow elephants were noticed in the exhibiting Asian elephants kept in North America (Mikota et al., 1994). The calving records of the Myanmar Asian Elephant Studbook documented that the shorter intercalving interval in working cow elephants was not a desirable characteristic as it provoked higher neonatal mortality in younger calves as a result of there being less opportunity to get enough milk during the first few months of age. The heavy burden of lactating two calves at the same time would lead to a longer interbirth interval in later pregnancies. Among a total of 738 mortality records in calves, death because of general weakness as the result of agalactia ranked the highest (27.1 percent), followed by deaths as a result of snake bite (14 percent), and accidents, such as falling, drowning, strangulation by their own chains etc. (8.2 percent).

The high death rate at the age of four to five years might be a result of taming-related causes. Taming was traditionally conducted at the approximate age of four years or in the case of some sub-adults (four to seven years olds), immediately after capture. Male calves generally resisted taming and breaking procedures and often sustained more injuries than female calves during taming processes. It was generally agreed that the older the calf at the time of taming or the stronger/bigger body configuration, the longer it took time to finish the taming processes. Taming procedures would not last more than two to four weeks for captive born calves. Further research needs to be carried out to identify the relationship between stress, age and temperament of calves and the method/duration of taming.

The survivorship curves of CB and WC elephants showed that WC cohorts had higher survival chances than CB elephants until the age of 39. WC elephants lost their survival vigour after this age (l_x = 0.80 at age 39 vs. l_x = 0.76 at age 45) while CB elephants showed stable lx (l_x = 0.78 at age 39 and. l_x = 0.77 at age 45) (Table 1). Different patterns of survivorship were seen in elephants less than five years, notably WC calves retained their survival vigour until they reached the age of five.

Continuous decline of survival by age reflected the expanding workload per elephant by year, which in turn reflected the increase in yearly timber production by Myanmar to meet the ever- growing demand of timber products in the world's markets. At the same time, loss of habitat together with reduced quality forage led to nutritional imbalance or insufficient food intake, which were the major causes of mortality in working elephants.

Conclusions

The life table analysis of working elephants of Myanmar documented that:

1) The population trend of the government-owned captive working elephants in Myanmar was not sustainable (R₀ = 0.50) as, overall, cow elephants were not producing enough calves to replace themselves. This might primarily be a result of the high mortality rate (32.5 percent) of calves under five years and the low calving rate that was insufficient to replace the deaths.

2) Survivorship analysis showed that wild caught and female elephants had significantly higher survival rates than captive born and male elephants (0-45 years), respectively.

3) Survivorship analysis for calves (less than five years) showed that calves born from CB dams had significantly higher survival rates than those born from WC dams, but the survival rate and sex of calves were found to show no correlation.

Acknowledgements

This project was funded by the International Foundation for Science (IFS) of Sweden and the Oregon Zoo Foundation Conservation Fund (United States of America). Additional support in terms of fellowships was granted to the author by the Smithsonian Institution (United States of America), International Timber Trade Organization (Japan), Three Oaks Foundation (Canada), Whitley Award Foundation (United Kingdom), Prospect Burma (United Kingdom) and Charles Wallace Trusts (United Kingdom) during the period between 1994-2000. Mr Richard Gayer (United Kingdom) and Dr Marcus Rowcliffe (Institute of Zoology) generously provided comments that greatly improved this paper. Finally, sincere thanks go to the Myanma Timber Enterprise, Ministry of Forestry, the Government of the Union of Myanmar for allowing the author to join its work force as Manager (Research) from April 1994 to September 1999.

Table 1. Life table analysis of working elephants from pooled data

Table 2. Percent mortality of calves (<5yr) and overall elephants (5-45yr) by months

Table 3. Age-specific calving potential in CB and WC cows, showing number and percent of calves born alive and dead at the time of parturition

Table 4. Prime reproductive age and dam age at the first calving

Table 5. Births by month

Table 6. Calf mortality by sex* and by dam origins

* Two calves were of unknown sex.

Fig. 1. Survivorship curve for WC and CB elephants (5-45) yr)

Fig. 2. Survivorship curve for male and female elephants (5-45 yr)

Fig. 3. Survivorship curve for calves (<5yr) born from WC and CB dams

Fig. 4. Survivorship curve for male and female calves

Fig. 5. Seasonal mortality of calves (<5 yr) and elephants (5-45 yr)

Fig. 6. Calving potential of WC and CB dams

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