Statural Growth in Elephants

Observers have long recognized differences in the body sizes of elephants from various regions in Africa and in Asia. Apart from the very obvious size difference between the African forest elephant (Loxodonta africana cyclotis) and the savanna or bush elephant (Loxodonta africana africana), the latter also comes in varying sizes across the continent, the smallest ones approaching the forest elephant in stature. Data compiled by Phyllis Lee and Cynthia Moss (1995) for the African savanna elephant show that the asymptotic heights of females vary from as short as 232 cm at Amboseli, comparable to or even somewhat smaller than the Asian elephant (Elephas maximus) in Sri Lanka (Ceylon) and India, to as much as 300 cm at Hwange in Zimbabwe. In Asia, there also is a range of sizes, with the elephants in Sumatra and in Borneo being the smallest, although precise growth data are not available. The oft-repeated statement that the African savanna elephant is larger than the Asian elephant is only true for the males of the two species. Interestingly, the female Asian elephant in Sri Lanka and the mainland is heavier than its African savanna counterpart in most regions, even if the latter is on average taller.

Statural growth in an animal is the product of its genetic makeup as expressed under the influence of resources available in its environment. The environmental influence is possibly more important in determining the dynamics of growth, especially from birth well into adulthood. We know that Asian elephants kept in Western zoos, in which they are provided a high-nutrient diet but little physical activity, grow much faster than do their wild or tame counterparts in the range states. Fred Kurt (1995) observed that the zoo elephants not only grow faster, but also are 33%-78% heavier than captive animals of similar height in Asian timber camps. It is conceivable that levels of nutrition available for wild elephants would also determine their growth rate on both a seasonal or interannual basis and over their life span. Thus, wet season growth can be expected to be greater than dry season growth in highly seasonal habitats. Elephants in tropical rain forest may also be relatively smaller in size because forage is widely dispersed and of poorer quality compared to forage in dry forest and savanna.

Lee and Moss explained the variation in adult body size across savanna elephant populations as reflecting genetic differences and perhaps the past history of hunting, which was selective against larger-size animals. Explanations for adult body size differences among elephant populations can also be sought from considerations of life history evolution (chapter 7). In fluctuating environments, the seasonal regimes of resource availability or interannual variations would favor larger body size, which can endure fasting during the dry season or a drought year. The larger body size of elephants in the climatically fluctuating and highly seasonal semiarid habitats compared to those in the more stable and aseasonal tropical moist forest is in line with this expectation.

Size difference between the sexes is generally considered to be the outcome of selection favoring a larger body in males of polygynous species (chapters 3 and 4). While male and female calves seem to be similar sized at birth in both Asian and African elephants, differences in growth rate between the sexes become obvious by age 2-5 years, although one study at Etosha found a divergence in growth curves between the sexes only after 10 years (Lindeque and van Jaarsveld 1993). At about 10 years, when the elephants may be at the threshold of puberty, the male is distinctly taller and heavier than the female. There are hints of a secondary growth spurt in male African (Laws et al. 1975) and Asian (Sukumar et al. 1988) elephants during the postpubertal years, but this is by no means established as it has not been seen in any other elephant population for which growth has been studied. Female elephants attain their maximum height by age 20-25 years, but continue to gain weight for another 5-10 years. Male elephants reach their asymptotic height by 25-30 years, although growth in body weight may continue well beyond 40 years. The tusks of male elephants keep growing throughout their life span.

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