Using spoor to determine the age and weight of subadult black rhinoceroses (Diceros bicornis L.)

1. What they did

Black rhinoceroses are endangered and need close monitoring, but dense habitat and their behaviour make them hard to see. To estimate how many young are joining a population (recruitment) and how many die, managers need to know the age of subadults, ideally without seeing them. Vets darting rhinoceroses also have to prepare the immobilizing drug before they see the animal, and the right dose depends on body weight.

During a structured release programme in Matusadona National Park, Zimbabwe, between May 1998 and March 2001, the author had a chance to test whether spoor size could predict age and weight. Seven subadult black rhinoceroses (three males and four females) of known age were kept in enclosures (bomas) at night and roamed freely by day with a general hand and an armed National Parks Scout.

• Weighing. Each morning the rhinoceroses were lured with horse cubes onto a flatbed scale built for weighing bull cattle.
• Measuring spoor. The ground leading to the scale was swept before each animal walked across, so a clean print was left. Only the right back print was measured, because the back foot tends to land on top of the front print. Width was measured from the outer edge of one side toe to the outer edge of the other; length from the outer edge of the front toe to the furthest point at the back of the foot. Measurements were rounded to the nearest 5 mm.
• Analysis. Averages for each month of age were used in four linear regressions predicting age and weight from spoor length and width.

2. What they found

• All four relationships were significant despite a lot of variation in the spoor measurements.
• Spoor length was the best predictor of age (r² = 95.4%), with spoor width close behind (r² = 94.2%).
• Spoor length was also the best predictor of weight (r² = 95%), followed by spoor width (r² = 90.9%).
• Over the ages measured, average weight rose from about 200 kg at 7 months to about 1000 kg at 43 months.
• As the young rhinoceroses neared adulthood, their spoor measurements varied more, and spoor became a less reliable guide to age and weight. The author suggests the foot reaches its full size while the animal is still getting older and heavier.

3. What they concluded

• Spoor measurements could be used to estimate the age of subadult black rhinoceroses for monitoring, without disturbing them by trying to see them, and with less subjectivity than judging age by eye.
• Spoor could also give an estimate of weight before darting, helping the vet choose a safe and efficient dose. Field staff tracking the rhinoceros ahead of the immobilization could measure the spoor and relay it to the vet.
• The variation around each measurement is a concern: one spoor size gives a range of possible weights, which needs to be considered when setting the dose. A larger study should test whether the variation is too large for immobilization work.
• Prints were measured on only one substrate. Soft sand may make prints slightly larger, a hard surface with a thin layer of powder smaller, and mud may distort them. The same animals should be measured on a variety of substrates.
• With more independent data from several projects, a special measuring ruler could be made, marked with the predicted age or weight for each spoor size.

4. How tracking was or was not used

The study measured footprints (spoor) of known animals on a prepared patch of swept sand, rather than finding or following tracks in the wild. The author proposes that trackers in the field could measure the right back print of a subadult to estimate their age and weight.

5. Tracker or observer certification

Not reported. The paper does not say who took the measurements or how consistent they were between measurers.

6. Other technologies, skills or innovations

• A flatbed cattle scale, with the animals lured on with horse cubes, gave regular weights of habituated subadults.
• Sweeping the ground before each crossing ensured a clear, fresh print for every measurement.
• The proposed field ruler would turn a measured print straight into an estimated age or weight.

7. Implications for conservation (from the paper)

• Indirect monitoring from spoor could help estimate recruitment and mortality in black rhinoceros populations, which is especially valuable where habitat is dense, animals are not used to people, and money and skilled staff are limited.
• Estimating weight before darting could reduce problems during immobilization and make operations more cost-efficient, since etorphine (M99) is expensive.