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

> Can a footprint tell you how old and how heavy a young rhinoceros is? In Zimbabwe’s Matusadona National Park, seven young black rhinoceroses were regularly weighed and their back footprints measured over nearly three years. Print length predicted both age and weight well, offering a way to monitor young rhinoceroses without seeing them and to judge drug doses before darting.

Summary by Kersey Lawrence, Original Wisdom. Updated 9 November 2026. Page: https://www.originalwisdom.com/tracking-literature/2007-purchase-spoor-determine-age-weight/

## Citation and access

- **Authors:** Duncan Purchase
- **Year published:** 2007
- **Journal or source:** South African Journal of Wildlife Research
- **Full citation:** Purchase, D. 2007. Using spoor to determine the age and weight of subadult black rhinoceroses (Diceros bicornis L.). South African Journal of Wildlife Research 37(1): 96–100.
- **DOI:** 10.3957/0379-4369-37.1.96
- **Access status:** Hosted (freely available elsewhere)
- **PDF:** https://www.originalwisdom.com/wp-content/uploads/2026/11/2007_Purchase_using-spoor-to-determine-the-age-and-weight-of-subadult-black-rhinoceroses.pdf

## Tags

- **Species:** Mammals > Black rhinoceros
- **Region:** Africa > Zimbabwe
- **Tracking:** Spoor age estimation; Track measurements; Tracking; Tracks
- **Methods and technology:** Linear regression
- **Data type:** New field data
- **Observer qualification:** Not reported
- **Discipline:** Wildlife tracking
- **Year published:** 2007

## Detailed keywords

- **Species:** Studied: black rhinoceros (Diceros bicornis), seven subadults (three males and four females) of known age, born in a semi-captive environment.
- **Location and habitat:** Matusadona National Park, Zimbabwe. All prints were measured on the same swept patch of ground (about 10 × 3 m) between the night enclosures (bomas) and a cattle scale: soft, fine sand over hard ground.
- **Topics:** spoor (footprint) measurement; estimating age from spoor; estimating body weight from spoor; recruitment and mortality monitoring; indirect population monitoring; drug dosage for immobilization (darting) with etorphine (M99); growth of subadults; effect of substrate and wet versus dry soil on print size; structured release programme. Inconsistencies in the paper: the Introduction says measurements were taken from 7 to 42 months of age, while the Methods give individual age spans up to 44 months and say the spoor data cover ages 11 to 44 months; the age versus spoor plots (Fig. 2b and 2c) show spoor data only from about 13 to 37 months. The regression results are given as r² in percent (for example 95.4%).
- **Statistics or models:** Four simple linear regressions with spoor length or width (cm) as predictor and age (months) or weight (kg) as the response: Age = −38.0 + 2.83 × spoor length (r² = 95.4%, n = 263); Age = −38.3 + 3.46 × spoor width (r² = 94.2%, n = 266); Weight = −563 + 56.8 × spoor length (r² = 95%, n = 260); Weight = −554 + 68.1 × spoor width (r² = 90.9%, n = 263); all P < 0.01. Measurements were averaged per age month across the same seven animals, so the data are not independent (acknowledged by the author). Sexes were pooled.
- **Data type:** New field data, May 1998 to March 2001. An exploratory short communication.

## Synopsis

### 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.

### 8. Implications for tracking (Original Wisdom's observations)

_These are Original Wisdom's observations, not the authors'._

- The study shows how much a known measuring point matters: the author measured only the right back print, from set points on the toes and heel. Trackers using these equations would need to measure the same print in the same way.

- The prints were made in soft sand over hard ground, and the author notes that wet and dry soil already changed how sharp the edges were. Applying these figures to prints in other substrates may give different results.

## Cite this summary

Lawrence, K. 2026. Summary of: Using spoor to determine the age and weight of subadult black rhinoceroses (Diceros bicornis L.) (Purchase, 2007). Original Wisdom, Tracking Literature. https://www.originalwisdom.com/tracking-literature/2007-purchase-spoor-determine-age-weight/