Interpreting Visual Signs of the Indian Crocodile

1. What they did

Lala A. K. Singh, of Project Tiger in Similipal Tiger Reserve, India, wrote this short field note for the Crocodile Specialist Group Newsletter. It describes the tracks and signs a field worker may find for India’s crocodilians, the gharial and the mugger: impressions of the head, body, tail and feet (pugmarks), droppings, and feeding signs. It explains how each can be used to identify the species and to get an idea of the sizes of crocodilians in an area.

This copy also includes photographs (a mugger high walk, gharial tail scutes and tail track, a gharial nesting site, an otter track and a freshwater turtle track) and the “Similipal Tiger Reserve: 2000 Crocodile Monitoring” protocol from the author’s website. The protocol lists what to look for, how to estimate body length, and a form for recording the field work.

2. What they found

Feeding signs

• Half-eaten fish, fish heads and dead fish with jaw marks are often found, but except in rare cases they are hard to use for judging size.
• This holds even where gharial and mugger live together, although the gharial has a long, saw-like snout with finer teeth. Jaw marks are only occasionally very clear, when a fish has escaped a gharial’s grip.

Droppings

• Gharial droppings are very seldom found, because gharials mostly defecate in water.
• Mugger droppings are very common on land. When they keep their shape, they give an idea of the animal’s size or age class (juvenile, young adult, large adult).

Head impressions

• Head prints in sand or mud can sometimes give an accurate measure of total body length.
• Body length is roughly 5 times head length in the gharial and about 6.5 times in the mugger and the saltwater crocodile.

Hind footprints

• Clear footprints are usually of the hind feet, and they are easier to read than front footprints.
• The hind foot has four toes: three clawed and one soft and fleshy. On good ground the print may show scale (scute) impressions, which separate it from an otter print, as otters leave hair, not scutes.
• Measured from the heel to the start of the claws, the hind footprint of a mugger is about one twelfth to one fourteenth of total body length. Gharial footprints are relatively small.
• For the mugger, footprint length gives a more reliable size estimate than tail scute length.

Tail drags and tail scutes

• Tail drags are the most common crocodilian tracks. They show curved parallel lines, and the space between two lines shows the size of one scute on the underside of the tail.
• Measure at least two or three large scutes. For the gharial, total body length (mm) = 53.6 + 62.7 x mean scute length (mm); a thumb rule for field workers is 70 times the mean scute length.
• For the mugger and saltwater crocodile the relationship has not been established, but it is about 65 times. The author links this to the number of tail scutes: about 24 in the gharial, 16 to 18 (sometimes only 14) in the mugger, so each gharial scute is smaller.

Telling tracks apart

• The body drag is a sure sign for identifying the species where gharial, mugger, otters and turtles could be confused. To a new observer, a dragged log can look like a crocodilian body drag, so look for tail marks and footprints.
• A young crocodilian’s track, a turtle track and an otter track can look alike to an untrained eye. Crocodilian tracks show scute impressions from the body, tail and feet. A turtle may leave a tail drag (particularly males), but their footprints are close together and form two parallel lines either side of the body drag. An otter’s tail drag can have a brushed (“brooming”) look from the hair, and their footprints curve inwards.
• Gharials have weak legs and cannot lift their body in a “high walk”, so they always leave a body drag with the tail drag. A tail drag without a body drag therefore belongs to a mugger, which the author calls a sure way to tell the two apart where they live together. When a tail drag comes with a body drag, other signs are needed.

The 2000 monitoring protocol

• Crocodiles left undisturbed bask in the morning and afternoon, and on winter days may bask all day, on islands and sloping banks near deep water.
• Muggers avoid very hot seasons (and another season; a word is missing in the original) by digging tunnels that open into water, with narrow mouths about 30 to 60 cm high, up to 5 m deep.
• Body length is estimated by eye for crocodiles seen, or from tracks: about 14 times hind footprint length, or about 65 times the maximum distance between two tail-drag lines.
• The recording form asks for date, river system, administrative area, place, body length, the number of crocodiles seen directly, the number identified from tracks, and whether the tail track or footprint was used.

3. What they concluded

• Tracks and signs of the head, body, tail and feet, droppings and feeding signs can tell field workers which crocodilians are present and give an idea of their sizes.
• The author calls for these and similar visual signs to be developed species by species for other crocodilians.

4. How tracking was or was not used

Tracking is the whole subject of the paper. It describes tracks and signs identification for gharial and mugger (head impressions, footprints, tail and body drags, droppings and feeding signs), size estimation from track measurements, the high walk gait as a way to tell species apart, and how to separate crocodilian tracks from otter, turtle and log drag marks. The protocol asks field workers to record how many crocodiles they identified from tracks and which tracks they used.

6. Other technologies, skills or innovations

• A simple field recording form that counts crocodiles identified from tracks separately from those seen directly.
• The article and protocol were also shared on the author’s website, with photographs of the tracks.