White Tigers Today – Are They Inbred ?

Ancestry charts indicate that today’s white tigers are genetically diverse and very far removed from the inbreeding of 50 years ago. The tigers have mixed ancestries, from numerous unrelated founder tigers, resulting from 5-6 generations of outcrossing. The proof is in the studbooks. Overall, white tigers today are not inbred and not descended from “only one tiger”.

This study is about the status of white Bengal tigers in India, but has parallels with populations in other countries.

Summary

  1. Condensed Ancestry Charts
    • Mohan-Begum chart, the Rewa line
    • Pradeep-Sikha chart, the Orissa line
  2. So where is all the inbreeding ?
  3. How much inbreeding = “inbred” ?
  4. Significance of outcrossing – adding new founders
    • Graph showing the gene pool increasing over time
  5. Genetic diversity of the founders
    • Map of geographic locations
  6. Are all descended from Mohan ?
  7. Conclusion
  8. A note about the accuracy of the charts
  9. Sources

Definition of Terms
hz = heterozygous for the white gene, ie an orange tiger that carries the white gene

IC = inbreeding coefficient

Outcrossing = mating with an unrelated Bengal tiger

 

1. Condensed Ancestry Charts

The International Bengal Tiger Studbook and the White Tiger Datset contain records of 100’s of tigers born since the 1950s but it is difficult to get a sense of the overall situation without spending many hours studying them. So below are 2 condensed ancestry charts, concerned only with the ancestry of living (or recently alive) white and hz tigers. They use color to show the breeding, outcrossing, and the multitude of different lines, at a glance:

  • Each color shading represents a different line of white/hz tigers with a different mix of founding ancestors – eg pale blue for the line descended from only Mohan and Begum;  light orange for Mohan/Begum/Tipu descendants; etc. Note that tigers of the same line (chart-color) are not identical, they each have a different mix of genes inherited from the various founders in their line.
  • A bright orange bar represents an unrelated outcross, resulting in a new line of tigers, shown by a new chart-color (or stripes). The outcross can be either with a new wild-born founder, or a cross between the Rewa and Orissa lines. Cubs of outcrossings have zero inbreeding.

The charts are read from left (1950s) to right (2016) and show tiger mating partners (in italics), and resulting cubs (in boxes) by generation (in columns). The cubs are grouped by parents, not by individual litter. Family lines that have died out, or that have only  possibly-hz descendants are omitted. For the most part, tigers on the left-hand half of the charts are deceased, those on the right still alive.

  • Left to right = 1950’s to 2016
  • LH side = deceased, RH side = alive
  • Chart-colors = different lines of tigers
  • Bright orange bar = unrelated outcross (IC=0)
  • Click charts to open in new window, click again to enlarge.

Mohan-Begum Chart – The Rewa Line

Rewa Line of Mohan and Begum – Ancestry of current white/hz tigers

The first chart starts from the Rewa line, tracing Mohan and Begum’s descendants.

Following the chart colors from left to right, it is easy to see that:

  • There are NONE of the original pure Rewa line (pale blue) left alive today ie tigers descended ONLY from Mohan and Begum (so clearly no tigers that got all of their genes from Mohan !)
  • There are numerous different lines of white/hz tigers with a kaleidoscope of different founders in their ancestry.
  • There was some inbreeding, but the early inbred lines were corrected by outcrossing to unrelated tigers – (orange bars on the chart).

 

Pradeep-Sikha Chart – The Orissa Line

WhiteTigerAncestryPSC

Orissa Line of Pradeep, Sikha, Rani – Ancestry of current white/hz tigers

The second chart starts from the Orissa line, following Pradeep, Sikha, and Rani’s descendants, and their crosses with the Rewa line. There is less early inbreeding than in the Rewa line, and there is a recent trend towards more outcrossing (eg Sara, Nandan, Jeevan, Cheri).

In the middle of the chart, where tigers of the same line have been paired, matings tend to be between distantly related tigers rather than siblings. For example, with a recent litter (Anini, Krishna, Snehashish, Subhransu) we have to go back 4 generations to find a common ancestor in the pedigree.

There does appear to be some sibling and mother-son inbreeding in recent times (eg Sunder-Tapsi, Sunder-Kamla), which is not good to see, but this involves a small number of zoos and does not affect the rest of the population (see below).

2. So where is all that inbreeding we were told about ?

There was indeed more inbreeding than is shown on these 2 charts, and it wasn’t restricted to white tigers. However, these charts show the ancestry of only the current population of white and hz. The full charts show more instances of inbred lines, and failed inbreeding attempts – but many of those inbred tigers of the past have no living descendants in the white tiger population today.

Past inbreeding of tigers that have no living descendants today has no effect on the current population.*

Inbreeding tends to be self-limiting – after 2-3 generations of continuous inbreeding, the inbred line dies out. (IC>.47). Not a good strategy !

This is likely to happen with the few current cases of inbreeding mentioned above – if not rescued by outcrossing, they will likely die out – hopefully these few zoos will change their practices in line with the majority, the aim should be to breed healthy tigers and avoid inbreeding.

* This is not saying that the inbreeding was OK – it is not necessary and not recommended. However, this article is not about judging the past, it is looking at the present and the status of white tigers today.

3. How much inbreeding = “inbred” ?

Inbreeding is a matter of degree, measured by the Inbreeding Coefficient (IC) on a scale of 0 to 1. For example:

  • IC of 0 = zero inbreeding.
  • IC of 0.25 = 25% inbreeding (eg from a full siblings mating).
  • IC of 0.01 = 1% inbreeding.

At what point do we say an animal, or population, is “inbred” ? If we use the IC>0 criteria, then we’d probably have to say that most white tigers today are inbred BUT we would also then have to say that most captive tigers are inbred – and quite possibly most wild tigers also. (Inbreeding has been observed in wild tigers – and lions – and increases with population fragmentation).

The charts show that there are white/hz tigers with zero inbreeding, and more with a very low level (as their parents were only distantly related).

“Inbred” is popularly understood to mean severely inbred, suffering or close to inbreeding depression. White tigers were at this point 40-50 years ago. Today they are not – they are very far from it.

4. Significance of Outcrossing – Adding new Founders

White tigers are not considered to be a separate subspecies. This puts them in a very strong position: it is not necessary to breed one white tiger to another white tiger. They can be bred to unrelated orange Bengal tigers, adding new founders into the population, and producing hz cubs.

We are 5 to 6 generations onwards from the Rewa line of Mohan-Begum, and the number of founders has increased dramatically from 2 to 19. Each new founder brings his or her unique set of genes into the pool, increasing the genetic variety available to the next generation. The process is ongoing.

  • Each tiger cub gets half of its genes from its mother, and half from its father. When mother and father are unrelated, as in an out-cross, there is zero probability of any of the genes being “identical by descent”, and therefore there is zero inbreeding. This is how inbreeding is corrected.

The graph shows the founders joining the India white tiger gene pool (in blue), 24 in all but 5 have left. (They join the pool when their first white/hz descendant is born, and leave it when their last known white/hz descendant died.)

Look at how that gene pool increased in size!

But numbers are not everything – a small number of founders can hold a large genetic variety if they come from a very diverse population or far-flung locations. Conversely, a large number of founders might have little variation if they all came from one small inbred population.

5. Genetic Diversity of the Founders

WhiteTigerFounderMapThe map shows approximate locations for the capture of the wild-born founders, where known. They are from a wide range of geographic locations and therefore very likely to be a genetically diverse bunch. Add to this that they were collected over a 60 year time period, making it even more likely that they represent very good diversity.

6. Are all descended from Mohan ?

All white tigers in India today are descended from Mohan – AND from Begum, Pradeep, Sikha, and Rani. All 5 of these founder tigers feature in the ancestry of most white/hz tigers. The other 14 founders also feature but in lesser proportions. This is not uncommon:

“In an ideal situation, each founder would be equally represented in the living SSP population. This is seldom the case—typically, a large proportion of the animals have descended from a few prolific founders who have many more living descendants than other founders and are thus more fully represented in the population’s gene pool. ”

from Management and Conservation of Captive Tigers – Ch 8 Intro to a Species Survival Plan (SSP) R. Tilson, K. Traylor-Holzer and G. Brady

After all the outcrossing and generations since Mohan, it is even possible that there are white tigers today that have few of Mohan’s original genes, even though he is an ancestor. Even the white gene itself could have come from the Orissa line (Sikha).

Having Mohan as an ancestor is not a bad thing at all though – he was a fine, big long-lived white tiger (19 years), a fitting patriarch. Begum, Pradeep, Sikha and Rani also lived to good old ages of 16 or 17.

7. Conclusion

Are today’s white tigers “inbred”?

Their ancestry indicates that they are genetically diverse. 5 to 6 generations of outcrossing has increased the gene-pool to that of 19 founders from widely-separated geographic locations, resulting in a mixture of lines and ancestors. Much of the past inbreeding has no effect on the current population (as there are no descendants). While some individual tigers may be inbred today, inbreeding is now the exception not the norm. Taking all this into account, it is reasonable to state that white tigers today are not inbred.

The breeding history is not perfect, and more work would be needed to make the population “text book perfect” – but it is impressive how well the Indian zoos have done over all with the white tiger breeding pool.

8. A note about the accuracy of the charts

The ancestry of some tigers is not clear, and if it looks like there could have been inbreeding, I assume the worst, ie that there was inbreeding. My apologies to the zoos concerned if it is incorrect. I will gladly correct any errors when informed.

The charts are a work in progress and may be updated from time to time.

9. Sources

Studbook of White Tigers in Indian Zoos in ZOO ZEN Vol IV Issue XI June 1989.

International Studbook of Bengal Tigers 2012

Management and Conservation of Captive Tigers – Ch 8 Intro to a Species Survival Plan (SSP) R. Tilson, K. Traylor-Holzer and G. Brady

The White Tiger Dataset

Inbreeding in White Tigers – A K Roychoudhury and K S Sankhala. Proc. Indian Acad. Sci Vol 88B, Part 1, Number 5, Oct 1979.

Genetic Status of White Tigers at Nandankanan Biological park, Orissa – A K Roychoudhury and L N Acharjyo. in JBNHS V.79 1982.

 

 

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