# The difficulty of estimating population densities of nocturnal forest mammals from transect counts of animals

> Walking a forest trail at night with a headtorch, how many animals do you miss? This short paper explains why many nocturnal forest mammals go unseen even right beside the observer, and why that makes population densities from night transects unreliable. It sets out what survey reports should include instead.

Summary by Kersey Lawrence, Original Wisdom. Updated 9 November 2026. Page: https://www.originalwisdom.com/tracking-literature/1998-duckworth-difficulty-estimating-population-densities/

## Citation and access

- **Authors:** J. W. Duckworth
- **Year published:** 1998
- **Journal or source:** Journal of Zoology
- **Full citation:** Duckworth, J. W. 1998. The difficulty of estimating population densities of nocturnal forest mammals from transect counts of animals. Journal of Zoology, London 246(4): 466–468.
- **DOI:** 10.1017/s0952836998281210
- **Access status:** Publisher link only
- **Where to find it:** https://doi.org/10.1017/s0952836998281210

## Tags

- **Species:** Mammals > Bushbaby; Mammals > Pygmy slow loris; Mammals > Small-toothed palm civet; Mammals > Sunda slow loris
- **Region:** Africa > Ethiopia; Asia > Laos; Asia > Malaysia
- **Behaviour and ecology:** Behaviour
- **Methods and technology:** Detectability; Direct observation; Distance sampling; Population estimation
- **Data type:** Protocol or guidelines; Review
- **Observer qualification:** Not needed
- **Discipline:** Behaviour and ecology
- **Year published:** 1998

## Detailed keywords

- **Species:** Studied: nocturnal small to medium-sized forest mammals in general. Examples, grouped by how easy they are to detect: (1) partly nocturnal primates with weak eye-shine that alarm call at the observer: baboons (Papio spp.), colobus monkeys (Colobus spp.), brown lemur (Eulemur fulvus); (2) quiet mammals with bright eye-shine that call rarely: lorises (Nycticebus spp.), slow loris (Nycticebus coucang), pygmy loris (Nycticebus pygmaeus), lemurs (Cheirogaleus spp. and Avahi spp.), common palm civet (Paradoxurus hermaphroditus), colugo (Cynocephalus variegatus); (3) noisy mammals with bright eye-shine: kinkajou (Potos flavus), small-toothed palm civet (Arctogalidia trivirgata), bushbabies (Galago spp.); (4) quiet mammals with no eye-shine: sloths (Choloepus spp.), neotropical porcupines (Coendou spp.). Also mentioned: red spider monkey (Ateles geoffroyi), sportive lemurs (Lepilemur spp.), canids, deer, mouse-deer and canopy flying squirrels (no scientific names given for the last four).
- **Location and habitat:** Tropical forests in general, especially South East Asia. Examples from the author’s own surveys: low acacia bushland and adjacent dense forest at a site in Ethiopia (Nechisar National Park, from the reference list); a site in Laos; Borneo (Similajau National Park, Sarawak, from the reference list). Also cited: Barro Colorado Island (Panama, country not named in the paper), south-eastern Peru, Gabon.
- **Topics:** night spotlight (headtorch) transect surveys; eye-shine; detection by sound (calls, rustling, dropping fruit); detectability on the transect midline; assumptions of distance sampling; sample size (60 to 80 contacts); contact frequencies (encounter rates) versus population densities; placing survey lines on roads and paths; repeated walking of the same trail; statistical independence; walking speed; group size; visibility; hunting pressure and torch-shyness; activity patterns, lunar cycles and plant phenology; rapid assessment surveys; why signs (footprints, dung, scratch marks) are often not identifiable to species in the tropics; dung counts show sloths are common.
- **Statistics or models:** Discussion of distance sampling theory (Buckland et al. 1993, the “DISTANCE” approach) and its assumptions; weighting encounter rates by section length (as in Laurance 1990). No new statistical analysis.
- **Data type:** Review and recommendations (a short “Communication from the Mammal Society”), drawing on the literature and the author’s earlier field surveys in Ethiopia, Laos and Borneo. No dates of data collection given. The text cites “Duckworth, 1992” for road walking, while the reference list has Duckworth 1992a, 1992b and Duckworth et al. 1992; it is unclear which is meant.

## Synopsis

### 1. What they did

Many small to medium-sized nocturnal forest mammals, especially in the tropics, can only be surveyed by seeing or hearing each animal. The author explains why: signs such as footprints, dung and scratch marks are often not identifiable to species, because similar species often live together; villagers’ identifications are rarely clear enough for research; and the origin of animals sold in markets can rarely be pinned down.

Such animals are usually surveyed by an observer walking slowly with a headtorch, searching all levels of the vegetation for reflecting eyes (eye-shine) or body shapes, and listening for calls, rustling and dropping fruit. Results may be given as survey time divided by the number of contacts, and are sometimes turned into population densities. The author asks whether the assumptions of distance sampling, a widely used way of estimating density from transect counts (Buckland et al. 1993), hold for these mammals. The author draws on the literature and on night surveys of their own in Ethiopia, Laos and Borneo.

### 2. What they found

Animals on the line are missed

Distance sampling assumes that every animal on the transect line itself is detected. If not, densities are underestimated. The author shows that many nocturnal forest mammals are missed for reasons that have nothing to do with their distance from the line. The author sorts them into four groups by how easy they are to detect:

- Partly nocturnal primates with weak eye-shine that alarm call (baboons, colobus monkeys, brown lemur): very hard to see even from directly below; found almost only by their calls.

- Quiet mammals with bright eye-shine that rarely call (lorises, some lemurs, common palm civet, colugo): many are hidden by vegetation. Some are only seen after the observer has passed and looks back; when the observer stops, more animals turn up several minutes later; and a second observer walking alongside always finds animals the first one missed. A slow loris watched in a leafless tree for 55 minutes was invisible for several minutes at a time simply because they were not looking towards the torch.

- Noisy mammals with bright eye-shine (kinkajou, small-toothed palm civet, bushbabies): the two carnivores are usually found by sound. In Ethiopia, bushbabies in low acacia bushland were usually found by eye-shine, but those in nearby dense forest were heard first. At a site in Laos, 4 pygmy lorises and 9 small-toothed palm civets were found; only one civet was found by eye-shine, which suggests many pygmy lorises were passed by unseen.

- Quiet mammals with no eye-shine (sloths, neotropical porcupines): sightings are rare, yet dung counts show sloths are among the commonest forest mammals.

Detection, even on the line, also changes with activity and calling through the night and the seasons, the moon, the fruiting and leafing of plants, and hunting pressure, which makes animals shy of torches.

Sample sizes

Density estimates need at least 60 to 80 contacts. Over much of South East Asia, mammals as a whole are rarely found more than once an hour, and even common species rarely more than once every 6 to 9 hours. Thousands of hours per site would be needed.

Roads and repeated walking

- Survey lines should be placed at random, which is unlikely when roads are walked. But roads give good visibility and quiet walking, while walking inside the forest gives few sightings because the observer sees little and flushes animals with noise.

- Because roads are scarce, the same line is often walked many times. A line walked three nights in a row often gives mostly different species, and different individuals, each night, because encounters depend so much on chance. This is acceptable for contact rates but is at odds with the near-complete coverage that density calculations assume.

- The author suggests dividing the survey area into sections, walking each whole section the same number of times, and combining the encounter rates of each section weighted by its length.

### 3. What they concluded

- For most nocturnal forest mammals, transect sightings cannot be used to calculate population densities, because major assumptions of distance sampling are not met.

- For many purposes, a rough idea of a species’ status is what is most needed, and comparable data from more areas may be more useful for conservation than density estimates.

- Until methods deal with animals missed on the line, results are best given as contact frequencies, with this context:

- Contacts per unit of time rather than distance, because a stationary observer keeps finding animals. Walking speed also changes which species are recorded: in Borneo, walking faster gave more mouse-deer and fewer canopy flying squirrels.

- Calculations and statistical tests based on the number of contacts (a group counts as one contact), with the total number of animals also given.

- Visibility in the habitat and typical sighting distances.

- The main way each species was detected.

- Hunting pressure in the area.

- Whether observations were made from roads, paths or at random across the habitat, with results given separately for each.

### 6. Other technologies, skills or innovations

- Headtorch spotlighting for eye-shine, combined with listening for calls, rustling, displaced vegetation and dropping fruit.

- The paper draws on observer skill: stopping to scan, looking back after passing, and the value of a second observer.

### 7. Implications for conservation (from the paper)

- Quick assessments of animal status are increasingly important for conservation, and a repeatable field method for nocturnal mammals is needed to compare sites.

- Population estimates may not be the best use of limited survey time; comparable sighting data from more areas often help more in setting conservation priorities for species and areas.

- Raw data and methods should be published, so that others can try alternative calculations.

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

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

- The author sets signs aside because, in the author’s experience in tropical forests, footprints, dung and scratch marks are often not identifiable to species. Yet the paper also notes that dung counts reveal sloths as among the commonest forest mammals, although they are rarely seen. This points to what tracks and signs can add for animals that sightings miss, where observers can reliably identify them.

## Cite this summary

Lawrence, K. 2026. Summary of: The difficulty of estimating population densities of nocturnal forest mammals from transect counts of animals (Duckworth, 1998). Original Wisdom, Tracking Literature. https://www.originalwisdom.com/tracking-literature/1998-duckworth-difficulty-estimating-population-densities/