Some inferences on the biology of tiger and leopard in Similipal Tiger Reserve

1. What they did

Most of what was known about wild tigers came from a few places, such as Kanha, Chitwan and the Sundarbans. The authors, the Field Director and Research Officer of Similipal Tiger Reserve in Orissa, India, used four years of pugmark (footprint) census records to see what the tracks could say about the biology of the reserve’s tigers and leopards.

• The census. Tigers and leopards were counted every winter from 1989 to 1992, in December and January, over six days of walking census routes. Each season more than 3700 soft soil pads, called Pad/Pug Impression Pads (PIPs), were laid along routes the tigers used, so that footprints would register clearly.
• Reading the tracks. Over these years the team refined ways to tell tiger from leopard tracks, front from hind and left from right pugmarks, and males from females, building on the census method devised by Saroj Raj Choudhury.
• Measuring. Each hind pugmark was traced, and a quadrangle drawn around the tracing gave its length (PML) and width (PMB). An animal was taken as male when length minus width was less than 1.5 cm for tigers or less than 1.0 cm for leopards.
• Analysing. For each season they mapped every animal’s sex, size and movement area, removed overlapping records so the totals gave the minimum population, and then pooled the four years by pugmark size class. They kept the results simple so that all the staff who took part could follow them.

2. What they found

Tigers

• The population seemed stable at about 94 animals a year (90 to 95), counting males, females and cubs, with on average 49 to 50 females.
• On average there were 2.2 ± 0.2 females for every male.
• Tigers with a hind pugmark of 12.0 to 12.9 cm were the most common: 27% of all records and 36% of adults.
• 86 cubs were recorded over four years, in three age classes from the first to the third year. The youngest cubs seen with their mothers had pugmarks of 7.0 to 7.9 cm (23% of cubs); the 8.0 to 8.9 cm class held the most cubs (35, or 41%).
• Because tracks overlapped, usually only one cub could be matched to each mother. On average 22 cubs were counted with 18 mothers, or 1.23 ± 0.19 cubs per mother. About 36% of females had cubs.
• About 72% of mothers with cubs had pugmarks of 12.0 to 13.9 cm.
• Numbers by pugmark size rose and fell in a wave, with peaks at 8.0 to 8.9 cm and 12.0 to 12.9 cm, a dip at 10.0 to 10.9 cm, and a decline above 12.9 cm.

Leopards

• Very few young leopards were identified. All leopards had pugmarks of 5.0 to 9.9 cm.
• The 7.0 to 7.9 cm class was the most common (42% of records); the largest class, 9.0 to 9.9 cm, held 13%.
• Among adults, the 8.0 to 8.9 cm class stayed the most stable from year to year.
• On average there were 1.8 ± 0.3 females for every male.

3. What they concluded

• The tiger population in Similipal appeared fairly stable over the four years.
• The dip in tigers at about 10 cm pugmark length marks young animals that have started to leave their mothers but have not yet set up their own territory: the wandering phase of a tiger’s life, here seen in numbers for the first time. These young animals may leave the usual movement routes.
• The fall in numbers above 12.9 cm suggests older tigers past their peak breeding years moving away from those routes. The census routes seem to overlie the territories of prime breeding adults, and the unsearched remote forest may be a retreat for both young dispersers and displaced old adults.
• Cubs are mainly born in August. Since the mothers counted had cubs from about 4 to 28 months old, the authors estimate that about 12% of adult females give birth each year.
• Litters of four cubs have been seen, but the census found only one cub with most mothers and two with a few. This shows a limit of the pad-based census, which needs further improvement.
• Leopards below 7.0 cm and above 8.9 cm may be animals not yet settled and animals pushed out of territories. Because tigers have more females per male than leopards, the authors infer that polygamy is more pronounced in tigers, with more females overlapping each male’s territory.
• The census technique needs improving to estimate leopard cubs.

4. How tracking was or was not used

The whole study rests on tracks. Census teams walked set routes for six days each winter and recorded tiger and leopard pugmarks on prepared soft soil pads (PIPs). Tracks were identified to species, front or hind foot, left or right, and sex, and hind pugmarks were traced with a “Tiger Tracer” and measured. Individuals were separated by their measurements together with where and when the tracks were found, and pugmark size was used as a guide to age. No trailing is reported.

5. Tracker or observer certification

Not reported. The census staff who recorded and traced the tracks are not described, and no test of their skill is given. The sexing and species methods come from the authors’ own earlier papers (1990 and 1991).

6. Other technologies, skills or innovations

• Pad/Pug Impression Pads (PIPs): more than 3700 soft soil pads laid each season to make tracks register along movement routes.
• Tracing pugmarks with a “Tiger Tracer”, transferring them to a census data sheet, and drawing a quadrangle around each to measure length and width.
• Moving the census to December and January, when fewer routes need checking because ground cover is still intact and the weather allows day-long walking.

7. Implications for conservation (from the paper)

• Census routes may miss young tigers that have left their mothers and old adults displaced from territories, as these may live in remote, unsearched forest.
• The census method needs refining to account for all cubs with their mothers in Similipal, and to estimate leopard cubs.