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Chapter 5 – Habitat Use, Diet Choice, and Nutritional Status in Female and Male Amboseli Elephants

In Chapter 5, Habitat Use, Diet Choice, and Nutritional Status in Female and Male Amboseli Elephants, Keith Lindsay describes the feeding ecology of males and females, whose strikingly distinctive physiologies and reproductive strategies define very different nutritional requirements and means to meet them.

Studies of habitat and diet choice of sexually dimorphic mammals - that is, males and females having very different size, shape and behaviour - have concluded that the two sexes may function ecologically as distinct species. Elephants are markedly dimorphic: for example, adult males can be two to three times heavier than similar-aged females.

In a chapter box, Size and Energetics of Elephants, Phyllis Lee outlines the implications of elephant body weight and shape for metabolism, energetics, and ecology. Energy use becomes more efficient as weight increases (so-called 'allometric scaling'). Despite elephants' large food requirements, given the efficiency boost of their XXXL body size, they can tolerate low-quality plant material and better survive droughts when food is scare. But there are extra energetic 'costs' - for males, during their extended period of growth and seasonal musth episodes, and for females during pregnancy and lactation. Droughts sap the reserves of females and diminish nutrients for calves under two years old who are dependent on maternal milk.

The field study period included both high and low rainfall years with corresponding highs and lows of available forage. Observations were made of elephants' choices of plant communities, species and parts at different times of year, and between good and bad years. Intake rates of forage dry matter and nutrients (energy and protein) were estimated by counting plucking rates and estimating trunkful sizes. From such observations, it was possible to compare male and female nutrient intake in good and bad years with calculated requirements for maintaining body condition and rearing offspring.

Facts:

• Elephants feed less while moving and socializing, particularly in the wet season when food is abundant.
• Males may fail to meet minimum nutritional requirements in the dry season if they join cow-calf groups, because they spend less time feeding and more time socialising.
• Males on their own or in bull groups are better nourished in both good and bad years from feeding in swamps and on woody plants away from females.
• Female elephants in Amboseli satisfy their energy and protein needs for both basic maintenance and lactation during wet seasons, even in drought years.
• In the dry season, nutritional intake may drop below lactation requirements and even below maintenance requirements by the late dry season.
• Feeding times increase as forage quality declines in the dry season.
• Bushland and open woodland habitat types are preferred during wet seasons when good quality grass is abundant.
• Swamps and swamp-edge woodlands are used more in dry months when coarser forage remains available.
• Thus, diet preferences shift from mainly grass in the wet season to woody browse during the dry season, a pattern common to elephants in all parts of Africa.

How do the two very different sexes avoid interference competition for resources? Largely through habitat segregation - using different parts of the ecosystem at different times - which is based on differing foraging strategies.

Social dynamics, physiological needs, and food availability shape foraging behavior and nutritional outcomes, all of which have important consequences for elephant survival and reproduction.

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