To answer these questions and others, it is helpful to have a dataset of captive tiger breeding records that can be analyzed.
The Bengal tiger population of Indian Zoos was chosen for this dataset because of the public availability, variety, and long time period covered by the records – and, of course, they include both white and orange tigers.
The resulting dataset contains approx 1700 records of individual tigers born from 1950’s to 2015, in a studbook format, with additional information. White tigers, orange tigers, known-heterozygous tigers, and wild-caught tigers are included.
Primary sources of the Dataset
1. Studbook of White Tigers in India, by A K Roychoudhury et al, 1989
Appears to contain complete records from 1950’s up to 1988 for tigers involved with the white tiger breeding program in India. It states whether the tiger is white or heterozygous.
2. International Studbook of Bengal Tigers 2012
Contains records for both orange and white tigers. It is dated 2012, but comparison with the CZA Inventory showed that the Inventory contains more records of births and deaths since 1995 than are recorded in the studbook.
3. Central Zoo Authority of India (CZA) Inventory Records 1995-2015
Contain totals of opening stock, births, deaths, acquisitions, disposals and closing stock per zoo, for each year from 1995 to early 2015, with separate data for Bengal Tigers and Bengal Tigers-White (although a few zoos did not differentiate). One prominent zoo in the studbook had no Inventory records available.
Press Releases / Newspaper Archives
Mainly from 2000 onwards – these were found to contain a wealth of information from the zoos, regarding tiger death dates, transfers, cub births and deaths. This information was used to help fill the gaps in the Studbook and Inventory records.
Method Used to Build the Dataset
Records from all 4 sources were grouped by zoo, combined, and correlated with each other (a lengthy process!).
- Heterozygosity for records after 1988 was ascertained from the color/heterozygosity of the parents and/or the presence of white descendants.
- In mixed color litters, where no other information was available, white births were assumed to be in the proportion genetically expected. (This assumption applied to only a small number of litters and increased the number of white cub deaths slightly.)
- Transfer records were included in the correlation process, but separated out before analysis, thus ensuring that each tiger was counted only once.
- For tigers > 1year, age was calculated to the nearest year. For cubs < 1 year, age was calculated to the nearest month, then any < 12 months were kept in the <1 year category.
- Date of death was recorded where known. From 2000 onwards there were many tigers with no date of death available – tigers that were known to be still alive at 2015 were classed as such, others were classed as “unknown”.
Completeness and Accuracy of the Data
It is unlikely that any dataset of this type can claim to be entirely complete and accurate, but I have striven to make the data as complete, accurate and unbiased as possible.
An important point is that all of the difficulties listed below apply equally to white and orange tiger records, so the resulting dataset is therefore valid for a comparison of the different color types.
Inherent difficulties in building this type of dataset:
- Possibility that zoos may report only the positive events in some cases ie might only report the live births
- Differences in degree of reporting – some zoos’ records are more complete in the system than others
- Possibility that some stillborn cubs may have been eaten by the mother and therefore never seen by zoo staff
- Possibility of transcription and data entry errors
- For wild-caught tigers, birth date is often estimated or unknown.
With regard to a general suspicion from some quarters that white tiger breeders do not report dead cubs in a litter, I actively looked for possible examples of this. I found that while many litters were recorded in full, with both live and dead cubs, there was indeed some tendency not to record all dead cubs, but that this applied to both white and orange cubs. There were some examples of cub births and deaths reported in press releases where only the births appeared in the Inventory, (without the deaths), or the births did not appear at all. All such records found were added into the dataset.
The records for white tigers seemed to be relatively complete and well-recorded, but there were some gaps in the orange tiger records.
Records intentionally omitted from the dataset
- Tigers with unknown year of birth
- 48 births of Orange tigers from one zoo. From 1995-2005 inventory records showed 59 births. Only 11 of these (incl. 3 cub deaths) could be corroborated from other sources and were included. Of the 48 excluded, at least 23 appeared to die before 1yr, but this could not be verified .
- 12 births and 10 cub deaths of Orange tigers from one zoo. In the Inventory, this appeared to be a duplicate entry, recorded both as male/female births and again as unknown sex births, in the same year. For my dataset 12 births and 10 deaths were included, instead of the double figures of 24 and 20.
Purpose of the Dataset
The dataset is intended to be used to provide comparisons between white tigers and orange tigers, to ascertain if there are differences in mortality rates etc.
The figures obtained should be used as an indication only, not as absolute values.
The dataset has so far been used in the following analyses:
White Tiger Cub Mortality and Longevity – contains summary tables of age groups by decade of birth, for each colour type.