|Birth, Death, and Chance The Dynamics of Elephant Populations|
The dynamics of elephant populations hold great theoretical and practical interest. The elephant shares many demographic traits with humans—age of sexual maturity, long gestation, single offspring, low death date, and high longevity. It is not only the statistical details of elephant demography, but also the evolution of life history traits and population regulation in this large mammal that are of considerable interest.
From a practical viewpoint, an adequate understanding of demographic processes in elephant populations is essential to making sound conservation judgments. Whether it is comprehending how density influences demography or how populations respond to the pressures of poaching, a fundamental consideration in conservation is the trajectory of animal numbers.
Understanding the dynamics of populations requires not only good empirical data from the field, but also the backing of robust mathematical models. Often, very subjective assessments are made about the dynamics of a population. I have, for instance, commonly heard statements to the effect that, “Every elephant group has a calf and thus the population must be doing very well.” This could be correct, or it could be completely wrong. The mere presence of a calf in every group does not by itself say anything about the trends in a population. If the population is suffering a high death rate, it could be declining.
“Rule-of-thumb” assessments sometimes can be made about the dynamics of a population based on fragmentary data, but this requires additional knowl-edge about its demographic attributes. More important, mathematical models allow us to explore the conditions under which a species population is likely to be increasing, stable, or declining. Even if the population itself is increasing,
its structure may be getting distorted; for instance, the sex ratio may be skewing in favor of females, as in many parts of southern India, because of selective poaching of male elephants.
Field data on all aspects of demography are not easily available for elephant populations, especially for those living in forests. Even when these have been collected, they are usually snapshots of the population in time. A longlived species perforce calls for a long-term demographic profile before we can begin to understand its dynamics. Only at Amboseli in Kenya have detailed birth and death records of identified elephants been maintained for longer than one elephant generation. Useful, long-term annual records are also being maintained at Dzanga Bai (Central African Republic), Addo and Kruger (South Africa), eastern Sri Lanka, and the Nilgiris (India). None of these has yet been
fully analyzed and presented.
Demographic variables in elephant populations
The most fundamental consideration in tracking the dynamics of a population is the estimation of birth rates and death rates. In some instances, the rates of immigration and emigration may also be important. While the overall birth and death rates of a population provide a simple picture of its trends, our understanding of its dynamics is considerably enhanced if we can estimate agespecific fecundity and age-specific mortality. We need to know the age at which a female first gives birth (and not merely becomes sexually mature, although the two are related), the number of young born with each pregnancy, the interval between successive births at different stages in life, the age of last birth, and the death rate at different ages for both males and females. Observations of both captive and wild elephant populations show that the birth of twin calves constitutes less than about 1% of all births (one exception being Etosha, with a 4% rate) and thus can be practically ignored for analyzing their dynamics.
Age span of reproduction and birth rates
Puberty is generally taken as the onset of the first estrous cycle in the female and the production of viable sperm in the male. Sexual maturity is the age at first ovulation in the female and the presence of a dense mass of motile sperm in the male. In both sexes, there may be a difference of 2–4 years between puberty and sexual maturity among elephants. Although there is tremendous
variation across elephant populations in the age of sexual maturity, the early East African studies showed that females and males within a population generally attain maturity around the same age. This seems broadly true across African and Asian elephant populations, with possibly the cows maturing 1 or 2 years prior to the bulls.
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 underrepresented in a collection of remains discovered; thus, estimation of agespecific 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 . Elephants may die of natural predation; a host of viral, bacterial, parasitic, or noninfectious diseases; malnutrition; thirst; injuries; and accidents . 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 importance, as seen from their contribution to overall mortality rates, of some of these causes.