Causes of death and mortality rates

The causes of death and the mortality rates are notoriously difficult to estimate for most wild mammalian species, including the elephant. Carcasses in the field are usually putrefied, making it difficult to identify the cause of death, especially if a pathogen is involved. Juveniles in the population are usually under-represented in a collection of remains discovered; thus, estimation of age-specific mortality rates for the younger age classes, vital for understanding population dynamics, usually becomes guesswork. With the elephant, the mixture of natural and human-caused deaths further complicates the study of mortality. The ratio of natural to human-caused deaths among elephants obviously varies with time across populations and the sexes, depending mainly on the pressures of ivory poaching, although deaths related to agricultural conflict may also be regionally significant (chapter 8).

Elephants may die of natural predation; a host of viral, bacterial, parasitic, or noninfectious diseases; malnutrition; thirst; injuries; and accidents (fig. 7.4). It is beyond the scope of this discussion to provide a detailed treatment of causes of elephant diseases or metabolic disorders. However, I highlight the possible

Figure 7.4

Causes of mortality in wild female and male Asian elephants in a large sample of deaths (n = 1,189 for females and 1,531 for males) in southern India during 19762000.

Figure 7.4

Causes of mortality in wild female and male Asian elephants in a large sample of deaths (n = 1,189 for females and 1,531 for males) in southern India during 19762000.

importance, as seen from their contribution to overall mortality rates, of some of these causes.

Adult elephants are immune even to the largest natural predators (lions and tigers), but young elephants are certainly vulnerable. Malan Lindeque attributed a significant 32% of all deaths of elephants less than 3 years old during 1971-1987 in Etosha National Park to predation, mainly by lions. Otherwise, the death of calves is mainly due to premature birth, malnutrition if the mother's condition suffers during a drought, infections, and accidents.

Parasitic infections of elephants seem common, but these are poorly understood. James Barnett reported that, during culling operations from 1967 to 1972 in Zimbabwe, the commonly found parasites were "oestrid fly larvae in the stomachs of young elephants, flukes (flatworms) in the small intestine, hookworms in the bile duct and a number of nematodes (roundworms) mainly in the caecum and large intestine" (1991, p. 103). Elephants may normally be able to tolerate the presence of such parasites, but suffer from clinical disease only when they are stressed, such as through malnutrition.

Nematodes and helminths are also common in Asian elephants. Protozoan parasites such as trypanosomes are seen in Asian elephants, but clinical symptoms are restricted to captive animals or certain wild populations, such as in northeast India.

Among the several bacterial diseases known in elephants, only anthrax, which is usually endemic, seems also to reach epidemic proportions. Anthrax epidemics are known both for wild Asian elephant populations in northeastern India and for African elephant populations in countries such as Namibia. The prevalence of anthrax epidemics in Etosha, an extremely arid habitat, runs contrary to the belief that these epidemics only occur in moist regions.

Pneumonialike infections, possibly by bacterial and viral agents, also contribute to a significant number of deaths. The herpes virus is known to infect both African and Asian elephants and in recent years has been identified as the cause of death in several Asian calves in captivity.

A large number of noninfectious diseases have been described for various organs and organ systems among elephants. Those involving cardiovascular, pulmonary, gastrointestinal, urogenital, and reproductive systems are known to result directly in the premature death of elephants, although their prevalence in wild populations is poorly documented.

Sylvia Sikes related several signs of ill health in elephants in East African grassland habitat to arteriosclerosis or progressive thickening and blockage of arteries. This condition was much more common in habitats in which elephants had a diet dominated by nutritionally poor grass than in the riverine habitats in which they also browsed. She differentiated two major types of sclerosis. Medial sclerosis (deposition of lime on the artery wall), as a consequence of abnormal calcium metabolism or excessive vitamin D, was attributed to increased exposure to sunlight as well as an incorrect calcium-phosphorus balance in the diet. Another condition, atheroma (excessive fat deposits on the inner artery wall), also seemed related to a poor diet. The clinical significance of these conditions has been questioned. It is possible that the fermentation products of a predominantly grass diet include a high concentration of saturated fatty acids.

Disorders of the reproductive system may be significant as causes not only of direct mortality in mature female elephants, but also more indirectly through inducing miscarriage, stillbirths, and premature births. Dystocia may be associated with some of the birth disorders. There are hints of increased risk of death among cows during their first pregnancy.

Drought-related mortality, common in African semiarid regions, may be due to both dehydration and starvation from a diet of poor quality. Injuries and accidents also are responsible for a substantial proportion of deaths among elephants. Calves or even adults may fall down steep slopes or into pits, get stuck in rocky clefts or trapped in soft mud, be trampled by a stampeding herd, drown in swiftly flowing water, or be bitten by poisonous snakes. Male elephants, of course, may be fatally injured in sparring bouts or serious fights; more commonly, they break a tusk, which eventually results in infection of the pulp cavity. Cow elephants may likewise be seriously injured by the tusks of an aggressive bull.

Mortality rates in elephants are relatively low, in line with their expected decline with increased body size among mammalian herbivores. I have mentioned the difficulties in estimating age- or sex-specific mortality rates in mammals. Several approaches have been taken to come up with such estimates in elephant populations. The "life table" approach treats a sample of naturally dead elephants (typically aged from their dentition) as a cohort or group of individuals born at the same time and dying at different ages to compute age-specific death rates. An alternative is to age a sample of living elephants or age a onetime cull and look at the shrinkage of successively older age classes to compute death rates.

Several tautologies can arise in the life table approach, as pointed out by Graeme Caughley. An underlying assumption is that the population has a stable age distribution, and that its growth rate r is zero. If r is not zero but its value is known, a correction can be introduced; however, this may make it redundant to estimate age-specific death rates because its purpose could have been to estimate r in the first place!

Nevertheless, a life table, even if imperfect, allows us to understand how the risk of death varies with age for a population. I have constructed survivorship curves from the life table carefully computed by Malan Lindeque from the age structure of culled elephants at Etosha and from demographic records of captive elephants that V. Krishnamurthy and I have maintained in southern India (fig. 7.5).

The survivorship curve for elephants follows the typical "type I" pattern associated with many large mammals—a moderate risk of death during the juvenile stage, a very low death rate from there onward through most of adult life, and an increased risk in old age. The data from Etosha indicated a 25% mortality rate during the first year of life and about 10% during the second

Survivirship Curve Southern Sea Otter

Figure 7.5

Survivorship lx curves for female elephants, plotted as the proportion of individuals surviving from birth to a given age. The survivorship values for African elephants at Etosha are based on a culled sample in 1985 (from Lindeque 1988), and for captive Asian elephants in southern Indian timber camps they are based on actual ages at death (from Sukumar et al. 1997.)

Figure 7.5

Survivorship lx curves for female elephants, plotted as the proportion of individuals surviving from birth to a given age. The survivorship values for African elephants at Etosha are based on a culled sample in 1985 (from Lindeque 1988), and for captive Asian elephants in southern Indian timber camps they are based on actual ages at death (from Sukumar et al. 1997.)

year, with these figures declining to about 4%-5% per year from the third year to 50 years of age, and thereafter progressively increasing to 10% per year by 60 years of age. Mortality rates seem the most variable during the first year. At Amboseli, they have gone from a few percent during a good year to as much as about 75% during a drought year (a 33% mean annual rate has been indicated for 1973-1978, when the population was in general decline). In the highly productive Lake Manyara, a mortality rate of 10% per year was computed for calves less than 1 year old during 1968-1970. Mortality rates in subadult and adult females can be expected to vary much less; mean annual rates of 1%-5% are reported for various African savanna populations.

As with the fecundity variables, we can expect lower variance in death rates of elephant populations in moist forests compared to those in semiarid habitats. While we have yet to see natural mortality data for elephants from tropical forests, my observations over two decades in the deciduous forests of the Nilgiris indicate that interannual variations in mortality are certainly low relative to African savanna habitats. During the period 1980-2000, only during one year (1996) was there any appreciable increase in number of female elephants dying naturally, possibly a response to stress induced by the early withdrawal of the winter monsoon the previous year. In the adjoining Biligiri-rangans, I estimated female mortality rates of 5%-15% for calves less than 1

year old and about 3% per year for subadults and adults between 5 and 40 years old. There was also a clear indication that young males died at about twice the rate of young females from birth to age 5 years.

In most elephant populations, we can expect male elephants to die at a higher rate than the females, at least from the age of postpubertal dispersal from the natal family. This follows from the higher risk of injury during fights with other males and also from the higher metabolic costs believed to be imposed by the larger body size in the male of a polygynous mammal. The slightly female-biased sex ratios, progressing with increasing age, in many elephant populations supports this expectation. However, there are populations in which sex ratios do not deviate from equality, suggesting that longer-term mortality rates may not vary naturally between males and females.

Timothy Corfield's detailed analysis of mass elephant deaths in Tsavo during the great drought of 1970-1971, possibly the worst in recent memory for a major elephant population/habitat, brought out an interesting difference in the resilience of male versus female elephants. He not only found a dramatic overall increase in mortality rates (it is now believed that a fourth of Tsavo's 40,000 elephants died) and a prolonged phase of juvenile mortality, but also found a much higher increase in adult female mortality compared to adult male mortality. Clearly, there is much to learn about determinants of mortality across age and sex in elephants.

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  • allen
    What are the causes of survivoship curve to have higher mortality?
    8 months ago

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