Measures of dung bolus size for known-age African elephants (Loxodonta africana): implications for age estimation

1. What they did

Knowing the ages of elephants helps in understanding their populations, but most ageing methods need good visibility, special equipment or handling the animals. The authors tested whether the diameter of a dung bolus can be used instead, using the Amboseli elephants of Kenya, where most individuals’ birth dates are known.

From July 2002 to July 2003, they measured the long and short axes of intact boli from fresh dung of known individuals (28 males and 51 females), fitted growth curves of bolus diameter against age for each sex, checked how much boli varied within one pile and between piles from the same elephant, and compared their curves with those from elephants in Kasungu National Park, Malawi.

2. What they found

• Bolus diameter grew with age in the same pattern as shoulder height and foot length.
• Up to 12 years, males and females could not be told apart: boli grew quickly from 5 to 6 cm to about 11 cm.
• Female boli stopped growing quickly at about 20 years and levelled off at 30 to 35 years. Male boli kept growing until about 25 years and levelled off at about 46 to 56 years.
• The growth curves fitted well (males R² = 0.929; females R² = 0.844). The difference between the sexes was not statistically significant, probably because there were few males and the sexes are alike when young.
• Two boli from the same pile differed by only 0.33 cm on average, and boli from the same elephant within six months did not differ significantly.
• The Amboseli curves matched those from Kasungu, Malawi.

3. What they concluded

• Dung bolus diameter is a suitable way to estimate the age of individual savanna elephants, most accurately below 20 years for females and 25 years for males (boli up to about 14 to 15.4 cm).
• With dung density surveys, bolus diameters can give a population’s age structure, using broader age classes as boli get larger and grouping all elephants over 20 to 25 years together.
• Growth curves from one population can be used, with caution, for another, but more data are needed before using them for forest elephants.

4. How tracking was or was not used

The study is built on a sign: elephant dung. Fresh, intact dung boli of known individuals were found and measured in the field, and only boli not deformed by hitting the ground were used. The paper also discusses hind track length as another way to age elephants, noting that it needs ground that preserves tracks well. No trailing is described.

5. Tracker or observer certification

Not reported. Field assistants of the Amboseli Elephant Research Project helped identify the elephants and measure dung, but no tracking qualifications are given.

6. Other technologies, skills or innovations

• Long-term individual recognition and birth records of the Amboseli Elephant Research Project.

7. Implications for conservation (from the paper)

• Dung measurements offer a simple, low-cost way to monitor the age structure of elephant populations where they are hard to see, such as in forest or when they raid crops at night, which matters for judging the effects of poaching, culling or habitat change.