White Tiger Cub Mortality and Longevity

WhiteTigerCubs

Analysis of the breeding records of approx 1700 Bengal tigers in Indian zoos has shown that white tigers have the same cub mortality and longevity as orange tigers.

 

There are some “statistics” floating around about white tigers that I have not seen backed up with any proof – such as the mythical 80% cub mortality, and a belief that white tigers die younger than orange. The average age for orange tigers in captivity is usually quoted as 16 years, while white tigers were thought to live to only 12 or 14 years on average.

I always want proof for any statements made, so I assembled a dataset of captive Bengal tiger breeding in Indian zoos from 1950’s to 2015, and analyzed it to find out what truth (or not) there was in these figures, and to see if the changes in breeding practices over the decades had had any effect.

I found cub mortality rate to be practically the same for white, orange, and heterozygous tigers, approx 35%, and the average age to be 12-13 years for all captive-born tigers.

As heterozygous orange tigers are the result of the white tiger breeding efforts, I separated known-heterozygous tigers into their own group (Hz) – otherwise they may have skewed the results for the orange tigers.

Details of the dataset, what it includes, and how it was built can be found here: The White Tiger Dataset  –  and tables of the results by decade are at the end of this article.

 

Cub Mortality

Cub mortality was calculated as the number of cubs who died at less than 1 year of age, divided by the total number of cubs born – per decade of breeding, and for the total period.

The average across the entire period, for each of the 3 groups, was practically the same:

  • White tigers:   36%
  • Hz tigers:        37.5%
  • Orange tigers: 34%

These figures are also comparable to the AZA-quoted rate for captive tigers in USA, 35%:

“Historically, the neonatal mortality rate for captive tiger cubs within the first year was about 40%; it has improved to about 35% (data from the AZA Tiger SSP)”

[Management and Conservation of Captive Tigers, M Bush et al]

Cub mortality can be caused by different things – first-time mothers sometimes neglect their litter, or cause death by accidental rough handling. Stressful conditions can cause a mother to reject her young, and differences between zoos may have an impact. Lack of appropriate nutrition during pregnancy can result in unhealthy cubs. Disease can strike down even healthy cubs. Inbreeding depression results in high (or total) cub mortality.

This analysis grouped all cub deaths together regardless of cause, but for white tiger breeding, the probability of inbreeding affecting cub mortality was of special interest.

The graph shows cub mortality rates per decade of breeding – and it reveals an interesting trend. While the rate for orange tigers remained relatively constant, the White and Hz rates peaked in the 1970’s, dropped sharply in the 1980s, then continued their downward trend to the “normal” level in the 1990’s.

This matches the historical evidence that a small period of intensive inbreeding was followed by improved breeding practices and the resulting healthier population.

CubMortalityGraph

Cub Mortality Rates of Captive-born Tigers in Indian Zoos 1950’s to 2015

1970’s: Short Period of Intensive Inbreeding

During the 1970s, all white tigers born in India were descended from only 2 wild-caught founders – Mohan and Begum. The increasing level of inbreeding resulted in inbreeding depression, as shown by the high cub mortality. This was duly noted back then, and the practice ceased by the 1980’s.

1980’s: A New Line of White and More Outcrossing

During the 1980’s, 2 more pivotal changes occurred: – the discovery of a new line of white tigers, the Orissa line, and the introduction of new orange founders into the white tiger breeding pool. During this decade, the number of founders jumped from 2 up to 10 for white tigers, and up to 14 for Hz tigers.

Continuing through the 1990’s, the number of zoos breeding white tigers increased, outcrossing continued, and the cub mortality rate dropped further, down to the “normal” level for orange tigers.

Longevity

The average age (at death) was calculated for adult tigers that reached reproductive maturity, 4 years of age (ie any tigers dying at less than 4 years were excluded). The average age for each of the groups was practically the same:

  • White tigers:   13.2 years
  • Hz tigers:         13.3 years
  • Orange tigers: 12.2 years
  • Wild-caught:   14.3 years

The tables at the end of the article show the age groups per decade of birth for each color type.

The greatest age recorded for a captive-born tiger was 24 years – 1 white and 1 orange tiger reached this age.

The results for wild-born tigers kept in captivity are shown for comparison. Wild-caught tigers appeared to reach a higher average age, and this age seemed to decrease from 17 in the 1950’s down to 14 BUT there is more uncertainty with the wild-born figures because:

  • The birth date of wild-caught tigers is an estimate, and accuracy may have changed over the decades.
  • The wild tigers were captured from a pool that already had the weaker members weeded out, so the survivors do not represent the full spectrum of ages etc.
  • Capture decisions have changed over time, wild tigers are no longer captured at will, but only the problem or rescued tigers are now brought into captivity.

 

Conclusion

White tigers in Indian zoos have the same cub mortality rate and longevity as orange tigers, ie approx 35% and 12-13 years.

There is no indication that the average cub mortality rate is or has ever been 80%.

The cub mortality rate of both white and heterozygous tigers has improved over time, matching the changes in breeding practices.

 

Data Tables

The figures quoted should be taken as indications only, for comparison, not as absolute values. Information on the dataset used can be found here:  The White Tiger Dataset

The tables show the births by decade for white tigers, heterozygous, orange, and wild-caught tigers kept in captivity. The number of tigers that are known to be still alive (at 2015) is shown, then the number of tigers for which no death date is recorded are in the “unk” unknown column (many of these are likely still alive). The % of tigers with unknown death date is in the “A+U” column.

The tigers with known death date are then grouped by age at death. The <1 year group represents the cub mortality group.

The age group %’s are calculated on the total number born. Note that meaningful data is not available for the higher ages from 2000 onward as many tigers born since then are still alive. For these, the total is given to 1999 instead of to 2015.

The final column gives the average age reached by adult tigers. Tigers less than reproductive age (4 years) were excluded from this calculation.

Tigers with unknown date of death (ie still alive or unknown) were also excluded – with the exception of 7 records from 1990-99 of aged tigers that were still alive as at 2015. They were included in the calculations at their current ages, being: Orange = 15 16 18 18 20 White=15 18

Table-ww-ow

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table-oo

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