White Tigers – Inbreeding and Outcrossing

White Tigers – Inbreeding and Outcrossing

The early inbreeding in the captive White Tiger population is well-known and often referred to – but it is only half the story. The subsequent corrections by outcrossing are often ignored or misunderstood, resulting in myths such as “all white tigers are inbred”. Here we explain how outcrossing works, not just in theory, but using five real life examples from white tiger captive breeding, in India and America. As a result, several myths are proven false.

In this article:

  • Some myths about white tigers and inbreeding
  • What is inbreeding, and outcrossing ?
  • “But outcrossing doesn’t work…”
  • Examples 1-3 – White tigers with zero or very low inbreeding
  • Examples 4-5 – American white tigers with zero or very low inbreeding
  • More evidence of outcrossing in American white tigers
  • Putting it into perspective
  • Notes and sources

White Bengal  tigress paired with orange Bengal tiger at  Chattbir Zoo.
By Weryah11 (Own work) [CC BY-SA 4.0]

Some myths about white tigers and inbreeding

Inbreeding and outcrossing both occurred in the captive White Tiger population, but the outcrossing is often ignored, leading to white tigers being judged on only a small part of their pedigree. This skewed view leads to misconceptions such as:

  • “All white tigers are descended from just one tiger”
  • “White tigers are only produced by parent to child, brother to sister etc mating”
  • “The early white tigers were inbred, therefore all their descendants must be also”
  • “If a cub’s parents are inbred, the cub must also be inbred”
  • “If it’s a white tiger, it must be inbred, simply because it is a white tiger”
  • “Outcrossing doesn’t work because you still have to backcross to related tigers”

Examples 1-5 below, taken from the actual breeding history, disprove all the above myths.

What are inbreeding and outcrossing ?

Inbreeding is the breeding of closely-related tigers together, eg Father to Daughter, or Brother to Sister. Inbreeding is not “either/or” – it is a matter of degree, which can be measured by the Inbreeding Coefficient (IC). This is the probability that 2 gene alleles in a pair will be identical by descent ie that both alleles in a pair are inherited from the same ancestor. Breeders try to keep the IC as low as possible, with an average of 10-15% generally acceptable (opinions vary, but maintaining an IC of zero is impractical in any captive tiger population).

Outcrossing is the breeding of completely unrelated tigers, [note i] ie the opposite of inbreeding. Outcrossing results in an IC of zero because there is zero probability that any 2 gene alleles in a pair will be identical by descent. Outcrossing introduces new genes from new unrelated tigers (ie founders) into the gene pool, increasing genetic diversity.

Outcrossing a white tiger to an unrelated orange tiger produces heterozygous cubs – ie orange tigers who carry the white gene. These orange tigers may have white cubs when paired with another heterozygous tiger. See White Tiger Genetics – The Basics for details.

“But outcrossing doesn’t work”…

Some critics claim that outcrossing “does not work because you still have to backcross the heterozygous tigers to closely-related tigers in order to produce the white tiger” which again involves inbreeding.  Is this true? If there was only ever one outcross mating possible, this would be true. But in the white tiger population there have been many different outcrosses, resulting in white tigers with zero or low IC, showing that outcrossing does indeed “work”.

Let’s take a look at a few examples and see the effect on the Inbreeding Coefficient [1]. Note that these are not just theoretical examples, but matings that actually happened.

Example 1: Multiple outcrosses producing white tigers with very low IC

Chart 1

Chart 1 – click to enlarge

In Chart 1 the inbreeding % is shown in the blue column. You can see how this increased over the first 20 years of inbreeding (1955-1977) until the 4th generation tigress Seema had a high IC of 50%. But then methods changed. Three new unrelated tigers were bred with the white tigers, ie three outcrosses, and when the distant cousins were backcrossed in 1996 the white tigress Aishwarya was born, with a very low IC of  4.69%. [2]

So far we have been using only Rewa-line tigers, but it gets even better when tigers from the Orissa line were introduced. Aishwarya’s brother Gaurav was bred to Nidhi – a tigress with mixed Rewa-Orissa+outcross ancestry. Their cubs included white tigress Mahasweta, with only 2.44% inbreeding.

Thus Aishwarya is descended from FIVE unique founder tigers (not just one!) – Dhittoo, Moti, Sheroo, Begum and Mohan, while Mahasweta is descended from NINE – the same five, plus Pradeep, Sikha, Rani and Tipu (not shown on chart).

Example 2: Crossing 2 unrelated lines producing white tigers with zero inbreeding

Chart 2

Chart 2 – click to enlarge

In 1980 a new line of white tigers was discovered in the Nandankanan zoo in Orissa. This line traced back to a wild-born heterozygous orange tiger with no relation to the Rewa line. Chart 2 shows what happened when these 2 family lines were crossed: even though the parent tigers were moderately inbred (Debabrata) and heavily inbred (Diana), their white cubs were entirely non-inbred, with IC=0.

Example 3: Crossing two family lines plus one other outcross

Chart 3

Chart 3 – click to enlarge

Chart 3 shows just one outcross in 1981, producing the non-inbred heterozygous tiger Sundar. When he was backcrossed to Rewa-line Shanti, their cubs (including Kiko) had IC of 22.66% – not as low as we’d like, but still far below that of the Rewa line mother. But then in 2001 Laxman, a white tiger born from the Rewa-Orissa line crosses, was bred with Kiko. The resulting white tiger cubs had a low and quite acceptable IC of 4.54%.

American White Tigers

The three examples above are from white tiger breeding in India, but the same principles apply to white tigers anywhere in the world – including the American white tiger population. [3]

Full details of the American pedigree are not publicly available, as much of the breeding has been by private breeders (which begs the question, without knowing the pedigrees how can it be claimed that they are all inbred? ) However, we have some evidence of multiple family lines and outcrossing, and we have more evidence from a recent genetic study.

Example 4: Multiple outcrosses producing white tigers with low IC

Chart 4

Chart 4 – click to enlarge

Similar to Example 1, chart 4 shows part of the early pedigree of the Rewa line white tigers in USA – Mohini, her cubs Ramana and Kesari, and “grand-cubs” Ranjit and Bharat. Three different outcrosses in the 1980s eventually led to white tigress Kitra and her siblings born with low IC of  6.45%.

Note: I do not know Hylton’s ancestry. If he was related to Jack, say his brother, Cherry’s IC would then be 12.5%, but there would be negligible change to Kitra’s IC.

Example 5: Crossing 2 unrelated lines producing white tigers with zero inbreeding

Chart 5

Chart 5 – click to enlarge

In USA there were at least 2 (probably more) unrelated family lines of white tigers – the Rewa line, via Mohini from India, and the Sioux Falls/Circus line via Susie, a heterozygous orange tigress imported from India. [note ii]

Similar to example 2, chart 5 shows how the IC dropped down to zero when Rewa-line Kesari was crossed with Circus line Tony at the Cincinnati zoo  – all their white cubs were non-inbred.

Later, siblings Bhim and Sumita were mated, and their cubs’ IC would be increased. More outcrossing would be needed to lower the IC again. Pedigree charts show that more outcrossing did indeed occur, and genetic analysis gives us even more evidence….

More evidence of outcrossing in American white tigers

A genetic study in 2013 [4] on a group of orange and white tigers found an average heterozygosity (genetic variation) score of 76.1% for white tigers and 77.2% for orange.

This finding has been dismissed by some, who claim that the tigers were all related so of course they will have similar heterozygosity, meaning they are all as inbred as each other. But this claim misses the important point – that a score of 76% means a high level of genetic variation for a tiger, regardless of what colour the tiger is – 76% variation does not indicate an inbred tiger. We should say, they were all as NON-inbred as each other!

Here are a few other heterozygosity scores for comparison, from a different genetic study in 2004. [5] The tigers used in this study were wild-born, or traceable to wild parents.

 

 

Putting it into perspective

The 5 examples plus genetic evidence prove that the myth that “all white tigers are inbred” is false – both in India and America.

They do not prove the opposite, and are not intended to.

The examples do also disprove the myth that “the only way to breed white tigers is to inbreed father to daughter……etc”, and all the other myths listed at the beginning of this article.

There have been many more outcrosses than shown in the examples, and more low-IC and zero-IC white tigers born, and there has been inbreeding too, both of white tigers AND orange tigers (with no white gene). This happens for various reasons  – lack of space for a zoo to keep enough suitable partners; lack of coordination of breeding efforts; larger zoos breeding within their own gene-pool; tigers not cooperating; tendency to obtain brother-sister pairs who already get along well; “accidental” matings, where bro-sis are kept together a little too long (eg Kanpur and Lucknow at MetroZoo Miami); reluctance to risk moving tigers from one zoo to another; maybe ignorance in some cases – let’s hope that it’s not because some actually believe the irresponsible myth that the only way to produce white tigers is to inbreed !

In hindsight, there have been missed opportunities; and there have been cases of attempted breeding that looked good on paper failing because the tigers would not cooperate.

The trend of outcrossing has continued in recent years, creating a new generation of non-inbred heterozygous tigers and new possibilities for healthy mating choices. The future options are beyond the scope of this article, but the examples above show how outcrossing and backcrossing do indeed work to produce white tigers with acceptable low IC, ie non-inbred. These are techniques that continue to be used in the white tiger population.

Notes and Sources

[Note i] Pedigrees can usually only be traced back to the wild-caught ancestors, ie the founders, and we assume that these founders are unrelated. This is not always the case – for example the Amur tiger founders mostly came from a very small population and were quite possibly related, especially as sibling pairs were often captured. In contrast, the founders of the white tiger population came from many different areas of a large population and are not so likely to be related. Please see ..30 founders and ..are they inbred? for further details of the founders.

[Note ii] Susie was a Bengal tigress whose origin is not fully known. It is sometimes assumed that she “must” be related to Mohan, but this assumption seems to be based on the belief that Mohan was the only white tiger, which is incorrect. White tigers occurred in the wild in many districts of India, and where there were white tigers there would also be heterozygous orange tigers – some of which could easily have found their way into captivity in America. Furthermore, as Susie seems to be the source of the gene for ghost-striping, it is unlikely that she was from Mohan’s family. See Golden Tabby and Snow-white Tigers for more information.

Sources

[1] Inbreeding Coefficients were calculated using Wright’s formula to 10 generations using Breed Mate Pedigree Software. (Some were also checked – laboriously – by hand).

[2] Pedigree information for tigers in India, examples 1-3,  is from the International Studbook of Bengal Tigers (2012); Studbook of White Tigers in Indian Zoos 1989; Indian National Studbook of Bengal Tiger 2011, Srivastav, A et al.; and The White Tiger Dataset

[3] Pedigree information for tigers in America, examples 4-5,  is from “Breeding White Tigers” by Sara Iverson, in Zoogoer Vol II No 1, Jan-Feb 1982; “White Tigers and their Conservation” by AK Roychoudhury, in Tigers of the World, Tilson & Seal Ed, Noyes Publications 1987; and pedigree charts from Messybeast.com.

[4] Genetic Diversity of White Tigers and Genetic Factors Related to Coat Color by Sara Elizabeth Carney, May 2013 (PDF).

[5] Phylogeography and Genetic Ancestry of Tigers (Panthera Tigris) by Luo et al, PLoS Bio, Dec 2004 v2 Iss 12 e442

 

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