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 age-specific 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.
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